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10:11). Sentinel Mission: Finding an Asteroid Headed for Earth: https://www.youtube.com/watch?v=efz8c3ijD_A. Public lecture by astronaut Ed Lu (1:08:57). COLLABORATIVE GROUP ACTIVITIES A. If we can predict that lots of ground movement takes place along subduction zones and faults, then why do so many people live there? Should we try to do anything to discourage people from living in these areas? What inducement would your group offer people to move? Who would pay for the relocation? (Note that two of the original authors of this book live quite close to the San Andreas and Hayward faults. If they wrote this chapter and haven’t moved, what are the chances others living in these kinds of areas will move?) This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 8 Earth as a Planet 299 B. After your group reads the feature box on Alfred Wegener: Catching the Drift of Plate Tectonics, discuss some reasons his idea did not catch on right away among scientists. From your studies in this course and in other science courses (in college and before), can you cite other scientific ideas that we now accept but that had controversial beginnings? Can you think of any scientific theories that are still controversial today? If your group comes up with some, discuss ways scientists could decide whether each theory on your list is right. C. Suppose we knew that a large chunk of rock or ice (about the same size as the one that hit 65 million years ago) will impact Earth in about 5 years. What could or should we do about it? (The film Deep Impact dealt with this theme.) Does your group think that the world as a whole should spend more money to find and predict the orbits of cosmic debris near Earth? D. Carl Sagan pointed out that any defensive weapon that we might come up with to deflect an asteroid away from Earth could be used as an offensive weapon by an unstable dictator in the future to cause an asteroid not heading our way to come toward Earth. The history of human behavior, he noted, has shown that most weapons that are built (even with the best of motives) seem to wind up being used. Bearing this in mind, does your group think we should be building weapons to protect Earth from asteroid or comet impact? Can we afford not to build them? How can we safeguard against these collisions? |
E. Is there evidence of climate change in your area over the past century? How would you distinguish a true climate change from the random variations in weather that take place from one year to the next? EXERCISES Review Questions 1. What is the thickest interior layer of Earth? The thinnest? 2. What are Earth’s core and mantle made of? Explain how we know. 3. Describe the differences among primitive, igneous, sedimentary, and metamorphic rock, and relate these differences to their origins. 4. Explain briefly how the following phenomena happen on Earth, relating your answers to the theory of plate tectonics A. earthquakes B. continental drift C. mountain building D. volcanic eruptions E. creation of the Hawaiian island chain 5. What is the source of Earth’s magnetic field? 6. Why is the shape of the magnetosphere not spherical like the shape of Earth? 7. Although he did not present a mechanism, what were the key points of Alfred Wegener’s proposal for the concept of continental drift? 300 Chapter 8 Earth as a Planet 8. List the possible interactions between Earth’s crustal plates that can occur at their boundaries. 9. List, in order of decreasing altitude, the principle layers of Earth’s atmosphere. 10. In which atmospheric layer are almost all water-based clouds formed? 11. What is, by far, the most abundant component of Earth’s atmosphere? 12. In which domain of living things do you find humankind? 13. Describe three ways in which the presence of life has affected the composition of Earth’s atmosphere. 14. Briefly describe the greenhouse effect. 15. How do impacts by comets and asteroids influence Earth’s geology, its atmosphere, and the evolution of life? 16. Why are there so many impact craters on our neighbor world, the Moon, and so few on Earth? 17. Detail some of the anthropogenic changes to Earth’s climate and their potential impact on life. Thought Questions 18. If you wanted to live where the chances of a destructive earthquake were small, would you pick a location near a fault zone, near a mid ocean ridge, near a subduction zone, or on a volcanic island such as Hawaii? What are the relative risks of earthquakes at each of these locations? 19. Which type of object would likely cause more damage if it struck near an urban area: a small metallic object or a large stony |
/icy one? 20. If all life were destroyed on Earth by a large impact, would new life eventually form to take its place? Explain how conditions would have to change for life to start again on our planet. 21. Why is a decrease in Earth’s ozone harmful to life? 22. Why are we concerned about the increases in CO2 and other gases that cause the greenhouse effect in Earth’s atmosphere? What steps can we take in the future to reduce the levels of CO2 in our atmosphere? What factors stand in the way of taking the steps you suggest? (You may include technological, economic, and political factors in your answer.) 23. Do you think scientists should make plans to defend Earth from future asteroid impacts? Is it right to intervene in the same evolutionary process that made the development of mammals (including us) possible after the big impact 65 million years ago? Figuring For Yourself 24. Europe and North America are moving apart by about 5 m per century. As the continents separate, new ocean floor is created along the mid-Atlantic Rift. If the rift is 5000 km long, what is the total area of new ocean floor created in the Atlantic each century? (Remember that 1 km = 1000 m.) 25. Over the entire Earth, there are 60,000 km of active rift zones, with average separation rates of 5 m/ century. How much area of new ocean crust is created each year over the entire planet? (This area is approximately equal to the amount of ocean crust that is subducted since the total area of the oceans remains about the same.) This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 8 Earth as a Planet 301 26. With the information from Exercise 8.25, you can calculate the average age of the ocean floor. First, find the total area of the ocean floor (equal to about 60% of the surface area of Earth). Then compare this with the area created (or destroyed) each year. The average lifetime is the ratio of these numbers: the total area of ocean crust compared to the amount created (or destroyed) each year. 27. What is the volume of new oceanic basalt added to Earth’s crust each year? Assume that the thickness of the new crust is 5 km, that there are 60,000 km of rifts, and that the average speed of plate motion is 4 cm/y. What fraction of Earth’s |
entire volume does this annual addition of new material represent? 28. Suppose a major impact that produces a mass extinction takes place on Earth once every 5 million years. Suppose further that if such an event occurred today, you and most other humans would be killed (this would be true even if the human species as a whole survived). Such impact events are random, and one could take place at any time. Calculate the probability that such an impact will occur within the next 50 years (within your lifetime). 29. How do the risks of dying from the impact of an asteroid or comet compare with other risks we are concerned about, such as dying in a car accident or from heart disease or some other natural cause? (Hint: To find the annual risk, go to the library or internet and look up the annual number of deaths from a particular cause in a particular country, and then divide by the population of that country.) 30. What fraction of Earth’s volume is taken up by the core? 31. Approximately what percentage of Earth’s radius is represented by the crust? 32. What is the drift rate of the Pacific plate over the Hawaiian hot spot? 33. What is the percent increase of atmospheric CO2 in the past 20 years? 34. Estimate the mass of the object that formed Meteor Crater in Arizona. 302 Chapter 8 Earth as a Planet This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 9 Cratered Worlds 303 9 CRATERED WORLDS Figure 9.1. Apollo 11 Astronaut Edwin “Buzz” Aldrin on the Surface of the Moon. Because there is no atmosphere, ocean, or geological activity on the Moon today, the footprints you see in the image will likely be preserved in the lunar soil for millions of years (credit: modification of work by NASA/ Neil A. Armstrong). Chapter Outline 9.1 General Properties of the Moon 9.2 The Lunar Surface 9.3 Impact Craters 9.4 The Origin of the Moon 9.5 Mercury Thinking Ahead The Moon is the only other world human beings have ever visited. What is it like to stand on the surface of our natural satellite? And what can we learn from going there and bringing home pieces of a different world? We begin our discussion of the planets as cratered worlds with two relatively simple objects: the Moon and Mercury. Unlike Earth, the Moon is geologically dead, a place that has exhausted its internal energy |
sources. Because its airless surface preserves events that happened long ago, the Moon provides a window on earlier epochs of solar system history. The planet Mercury is in many ways similar to the Moon, which is why the two are discussed together: both are relatively small, lacking in atmospheres, deficient in geological activity, and dominated by the effects of impact cratering. Still, the processes that have molded their surfaces are not unique to these two worlds. We shall see that they have acted on many other members of the planetary system as well. 9.1 GENERAL PROPERTIES OF THE MOON Learning Objectives By the end of this section, you will be able to: 304 Chapter 9 Cratered Worlds Discuss what has been learned from both manned and robotic lunar exploration Describe the composition and structure of the Moon The Moon has only one-eightieth the mass of Earth and about one-sixth Earth’s surface gravity—too low to retain an atmosphere (Figure 9.2). Moving molecules of a gas can escape from a planet just the way a rocket does, and the lower the gravity, the easier it is for the gas to leak away into space. While the Moon can acquire a temporary atmosphere from impacting comets, this atmosphere is quickly lost by freezing onto the surface or by escape to surrounding space. The Moon today is dramatically deficient in a wide range of volatiles, those elements and compounds that evaporate at relatively low temperatures. Some of the Moon’s properties are summarized in Table 9.1, along with comparative values for Mercury. Figure 9.2. Two Sides of the Moon. The left image shows part of the hemisphere that faces Earth; several dark maria are visible. The right image shows part of the hemisphere that faces away from Earth; it is dominated by highlands. The resolution of this image is several kilometers, similar to that of high-powered binoculars or a small telescope. (credit: modification of work by NASA/GSFC/Arizona State University) Properties of the Moon and Mercury Property Moon Mercury Mass (Earth = 1) Diameter (km) Density (g/cm3) Surface gravity (Earth = 1) Escape velocity (km/s) Rotation period (days) Table 9.1 This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 0.0123 3476 3.3 0.17 2.4 27.3 0.055 4878 5 |
.4 0.38 4.3 58.65 Chapter 9 Cratered Worlds 305 Properties of the Moon and Mercury Property Moon Mercury Surface area (Earth = 1) 0.27 0.38 Table 9.1 Exploration of the Moon Most of what we know about the Moon today derives from the US Apollo program, which sent nine piloted spacecraft to our satellite between 1968 and 1972, landing 12 astronauts on its surface (Figure 9.1). Before the era of spacecraft studies, astronomers had mapped the side of the Moon that faces Earth with telescopic resolution of about 1 kilometer, but lunar geology hardly existed as a scientific subject. All that changed beginning in the early 1960s. Initially, Russia took the lead in lunar exploration with Luna 3, which returned the first photos of the lunar far side in 1959, and then with Luna 9, which landed on the surface in 1966 and transmitted pictures and other data to Earth. However, these efforts were overshadowed on July 20, 1969, when the first American astronaut set foot on the Moon. Table 9.2 summarizes the nine Apollo flights: six that landed and three others that circled the Moon but did not land. The initial landings were on flat plains selected for safety reasons. But with increasing experience and confidence, NASA targeted the last three missions to more geologically interesting locales. The level of scientific exploration also increased with each mission, as the astronauts spent longer times on the Moon and carried more elaborate equipment. Finally, on the last Apollo landing, NASA included one scientist, geologist Jack Schmitt, among the astronauts (Figure 9.3). Apollo Flights to the Moon Flight Date Landing Site Main Accomplishment Apollo 8 Dec. 1968 — First humans to fly around the Moon Apollo 10 May 1969 — First spacecraft rendezvous in lunar orbit Apollo 11 July 1969 Mare Tranquillitatis First human landing on the Moon; 22 kilograms of samples returned Apollo 12 Nov. 1969 Oceanus Procellarum First Apollo Lunar Surface Experiment Package (ALSEP); visit to Surveyor 3 lander Apollo 13 Apr. 1970 — Landing aborted due to explosion in command module Apollo 14 Jan. 1971 Mare Nubium First “rickshaw” on the Moon Apollo 15 July 1971 Mare Imbrium/ Hadley First “rover;” visit to Hadley Rille; astronauts traveled 24 kilometers Table 9.2 306 Chapter 9 Cratered Worlds Apollo Flights to the Moon Flight Date Landing Site Main Accomplishment Apollo 16 Apr. 1972 Descartes First landing in highlands |
; 95 kilograms of samples returned Apollo 17 Dec. 1972 Taurus-Littrow highlands Geologist among the crew; 111 kilograms of samples returned Table 9.2 Figure 9.3. Scientist on the Moon. Geologist (and later US senator) Harrison “Jack” Schmitt in front of a large boulder in the Littrow Valley at the edge of the lunar highlands. Note how black the sky is on the airless Moon. No stars are visible because the surface is brightly lit by the Sun, and the exposure therefore is not long enough to reveal stars. In addition to landing on the lunar surface and studying it at close range, the Apollo missions accomplished three objectives of major importance for lunar science. First, the astronauts collected nearly 400 kilograms of samples for detailed laboratory analysis on Earth (Figure 9.4). These samples have revealed as much about the Moon and its history as all other lunar studies combined. Second, each Apollo landing after the first one deployed an Apollo Lunar Surface Experiment Package (ALSEP), which continued to operate for years after the astronauts departed. Third, the orbiting Apollo command modules carried a wide range of instruments to photograph and analyze the lunar surface from above. This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 9 Cratered Worlds 307 Figure 9.4. Handling Moon Rocks. Lunar samples collected in the Apollo Project are analyzed and stored in NASA facilities at the Johnson Space Center in Houston, Texas. Here, a technician examines a rock sample using gloves in a sealed environment to avoid contaminating the sample. (credit: NASA JSC) The last human left the Moon in December 1972, just a little more than three years after Neil Armstrong took his “giant leap for mankind.” The program of lunar exploration was cut off midstride due to political and economic pressures. It had cost just about $100 per American, spread over 10 years—the equivalent of one large pizza per person per year. Yet for many people, the Moon landings were one of the central events in twentieth-century history. The giant Apollo rockets built to travel to the Moon were left to rust on the lawns of NASA centers in Florida, Texas, and Alabama, although recently, some have at least been moved indoors to museums (Figure 9.5). Today, neither NASA nor Russia have plans to send astronauts to the Moon, and China appears to be the nation most likely to attempt this feat. (In a |
bizarre piece of irony, a few people even question whether we went to the Moon at all, proposing instead that the Apollo program was a fake, filmed on a Hollywood sound stage. See the Link to Learning box below for some scientists’ replies to such claims.) However, scientific interest in the Moon is stronger than ever, and more than half a dozen scientific spacecraft—sent from NASA, ESA, Japan, India, and China—have orbited or landed on our nearest neighbor during the past decade Read The Great Moon Hoax (https://openstax.org/l/30greatmoonhoax) about the claim that NASA never succeeded in putting people on the Moon. 308 Chapter 9 Cratered Worlds Figure 9.5. Moon Rocket on Display. One of the unused Saturn 5 rockets built to go to the Moon is now a tourist attraction at NASA’s Johnson Space Center in Houston, although it has been moved indoors since this photo was taken. (credit: modification of work by David Morrison) Lunar exploration has become an international enterprise with many robotic spacecraft focusing on lunar science. The USSR sent a number in the 1960s, including robot sample returns. Table 9.3 lists some of the most recent lunar missions. Some International Missions to the Moon Launch Year Spacecraft Type of Mission Agency 1994 1998 2003 2007 2007 2008 2009 2009 Table 9.3 Clementine Orbiter US (USAF/NASA) Lunar Prospector Orbiter US (NASA) SMART-1 SELENE 1 Chang’e 1 Chandrayaan-1 LRO LCROSS Orbiter Orbiter Orbiter Orbiter Orbiter Impactor Europe (ESA) Japan (JAXA) China (CNSA) India (ISRO) US (NASA) US (NASA) This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 9 Cratered Worlds 309 Some International Missions to the Moon Launch Year Spacecraft Type of Mission Agency 2010 2011 2013 2013 Table 9.3 Chang’e 2 Orbiter China (CNSA) GRAIL LADEE Twin orbiters US (NASA) Orbiter US (NASA) Chang’e 3 Lander/Rover China (CNSA) Composition and Structure of the Moon The composition of the Moon is not the same as that of Earth. With an average density of only 3.3 g/cm3, the Moon must be made almost entirely of silicate rock. Compared to Earth, it is depleted |
in iron and other metals. It is as if the Moon were composed of the same silicates as Earth’s mantle and crust, with the metals and the volatiles selectively removed. These differences in composition between Earth and Moon provide important clues about the origin of the Moon, a topic we will cover in detail later in this chapter. Studies of the Moon’s interior carried out with seismometers taken to the Moon as part of the Apollo program confirm the absence of a large metal core. The twin GRAIL spacecraft launched into lunar orbit in 2011 provided even more precise tracking of the interior structure. We also know from the study of lunar samples that water and other volatiles have been depleted from the lunar crust. The tiny amounts of water detected in these samples were originally attributed to small leaks in the container seal that admitted water vapor from Earth’s atmosphere. However, scientists have now concluded that some chemically bound water is present in the lunar rocks. Most dramatically, water ice has been detected in permanently shadowed craters near the lunar poles. In 2009, NASA crashed a small spacecraft called the Lunar Crater Observation and Sensing Satellite (LCROSS) into the crater Cabeus near the Moon’s south pole. The impact at 9,000 kilometers per hour released energy equivalent to 2 tons of dynamite, blasting a plume of water vapor and other chemicals high above the surface. This plume was visible to telescopes in orbit around the Moon, and the LCROSS spacecraft itself made measurements as it flew through the plume. A NASA spacecraft called the Lunar Reconnaissance Orbiter (LRO) also measured the very low temperatures inside several lunar craters, and its sensitive cameras were even able to image crater interiors by starlight. The total quantity of water ice in the Moon’s polar craters is estimated to be hundreds of billions of tons. As liquid, this would only be enough water to fill a lake 100 miles across, but compared with the rest of the dry lunar crust, so much water is remarkable. Presumably, this polar water was carried to the Moon by comets and asteroids that hit its surface. Some small fraction of the water froze in a few extremely cold regions (cold traps) where the Sun never shines, such as the bottom of deep craters at the Moon’s poles. One reason this discovery could be important is that it raises the possibility of future human habitation near the lunar poles, or even of a lunar base as a way-station |
on routes to Mars and the rest of the solar system. If the ice could be mined, it would yield both water and oxygen for human support, and it could be broken down into hydrogen and oxygen, a potent rocket fuel. 310 Chapter 9 Cratered Worlds 9.2 THE LUNAR SURFACE Learning Objectives By the end of this section, you will be able to: Differentiate between the major surface features of the Moon Describe the history of the lunar surface Describe the properties of the lunar “soil” General Appearance If you look at the Moon through a telescope, you can see that it is covered by impact craters of all sizes. The most conspicuous of the Moon’s surface features—those that can be seen with the unaided eye and that make up the feature often called “the man in the Moon”—are vast splotches of darker lava flows. Centuries ago, early lunar observers thought that the Moon had continents and oceans and that it was a possible abode of life. They called the dark areas “seas” (maria in Latin, or mare in the singular, pronounced “mah ray”). Their names, Mare Nubium (Sea of Clouds), Mare Tranquillitatis (Sea of Tranquility), and so on, are still in use today. In contrast, the “land” areas between the seas are not named. Thousands of individual craters have been named, however, mostly for great scientists and philosophers (Figure 9.6). Among the most prominent craters are those named for Plato, Copernicus, Tycho, and Kepler. Galileo only has a small crater, however, reflecting his low standing among the Vatican scientists who made some of the first lunar maps. We know today that the resemblance of lunar features to terrestrial ones is superficial. Even when they look somewhat similar, the origins of lunar features such as craters and mountains are very different from their terrestrial counterparts. The Moon’s relative lack of internal activity, together with the absence of air and water, make most of its geological history unlike anything we know on Earth. Figure 9.6. Sunrise on the Central Mountain Peaks of Tycho Crater, as Imaged by the NASA Lunar Reconnaissance Orbiter. Tycho, about 82 kilometers in diameter, is one of the youngest of the very large lunar craters. The central mountain rises 12 kilometers above the crater floor. (credit: modification of work by |
NASA/Goddard/Arizona State University) Lunar History To trace the detailed history of the Moon or of any planet, we must be able to estimate the ages of individual rocks. Once lunar samples were brought back by the Apollo astronauts, the radioactive dating techniques that had been developed for Earth were applied to them. The solidification ages of the samples ranged from about This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 9 Cratered Worlds 311 3.3 to 4.4 billion years old, substantially older than most of the rocks on Earth. For comparison, as we saw in the chapter on Earth, Moon, and Sky, both Earth and the Moon were formed between 4.5 and 4.6 billion years ago. Most of the crust of the Moon (83%) consists of silicate rocks called anorthosites; these regions are known as the lunar highlands. They are made of relatively low-density rock that solidified on the cooling Moon like slag floating on the top of a smelter. Because they formed so early in lunar history (between 4.1 and 4.4 billion years ago), the highlands are also extremely heavily cratered, bearing the scars of all those billions of years of impacts by interplanetary debris (Figure 9.7). Figure 9.7. Lunar Highlands. The old, heavily cratered lunar highlands make up 83% of the Moon’s surface. (credit: Apollo 11 Crew, NASA) Unlike the mountains on Earth, the Moon’s highlands do not have any sharp folds in their ranges. The highlands have low, rounded profiles that resemble the oldest, most eroded mountains on Earth (Figure 9.8). Because there is no atmosphere or water on the Moon, there has been no wind, water, or ice to carve them into cliffs and sharp peaks, the way we have seen them shaped on Earth. Their smooth features are attributed to gradual erosion, mostly due to impact cratering from meteorites. 312 Chapter 9 Cratered Worlds Figure 9.8. Lunar Mountain. This photo of Mt. Hadley on the edge of Mare Imbrium was taken by Dave Scott, one of the Apollo 15 astronauts. Note the smooth contours of the lunar mountains, which have not been sculpted by water or ice. (credit: NASA/Apollo Lunar Surface Journal) The maria are much less cratered than the highlands, and cover just 17% |
of the lunar surface, mostly on the side of the Moon that faces Earth (Figure 9.9). Figure 9.9. Lunar Maria. About 17% of the Moon’s surface consists of the maria—flat plains of basaltic lava. This view of Mare Imbrium also shows numerous secondary craters and evidence of material ejected from the large crater Copernicus on the upper horizon. Copernicus is an impact crater almost 100 kilometers in diameter that was formed long after the lava in Imbrium had already been deposited. (credit: NASA, Apollo 17) Today, we know that the maria consist mostly of dark-colored basalt (volcanic lava) laid down in volcanic eruptions billions of years ago. Eventually, these lava flows partly filled the huge depressions called impact basins, which had been produced by collisions of large chunks of material with the Moon relatively early in its history. The basalt on the Moon (Figure 9.10) is very similar in composition to the crust under the oceans of This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 9 Cratered Worlds 313 Earth or to the lavas erupted by many terrestrial volcanoes. The youngest of the lunar impact basins is Mare Orientale, shown in Figure 9.11. Figure 9.10. Rock from a Lunar Mare. In this sample of basalt from the mare surface, you can see the holes left by gas bubbles, which are characteristic of rock formed from lava. All lunar rocks are chemically distinct from terrestrial rocks, a fact that has allowed scientists to identify a few lunar samples among the thousands of meteorites that reach Earth. (credit: modification of work by NASA) Figure 9.11. Mare Orientale. The youngest of the large lunar impact basins is Orientale, formed 3.8 billion years ago. Its outer ring is about 1000 kilometers in diameter, roughly the distance between New York City and Detroit, Michigan. Unlike most of the other basins, Orientale has not been completely filled in with lava flows, so it retains its striking “bull’s-eye” appearance. It is located on the edge of the Moon as seen from Earth. (credit: NASA) Volcanic activity may have begun very early in the Moon’s history, although most evidence of the first half billion years is lost. What we do know is that the major mare volcanism, |
which involved the release of lava from hundreds of kilometers below the surface, ended about 3.3 billion years ago. After that, the Moon’s interior cooled, and volcanic activity was limited to a very few small areas. The primary forces altering the surface come from the outside, not the interior. 314 Chapter 9 Cratered Worlds On the Lunar Surface “The surface is fine and powdery. I can pick it up loosely with my toe. But I can see the footprints of my boots and the treads in the fine sandy particles.” —Neil Armstrong, Apollo 11 astronaut, immediately after stepping onto the Moon for the first time. The surface of the Moon is buried under a fine-grained soil of tiny, shattered rock fragments. The dark basaltic dust of the lunar maria was kicked up by every astronaut footstep, and thus eventually worked its way into all of the astronauts’ equipment. The upper layers of the surface are porous, consisting of loosely packed dust into which their boots sank several centimeters (Figure 9.12). This lunar dust, like so much else on the Moon, is the product of impacts. Each cratering event, large or small, breaks up the rock of the lunar surface and scatters the fragments. Ultimately, billions of years of impacts have reduced much of the surface layer to particles about the size of dust or sand. Figure 9.12. Footprint on Moon Dust. Apollo photo of an astronaut’s boot print in the lunar soil. (credit: NASA) In the absence of any air, the lunar surface experiences much greater temperature extremes than the surface of Earth, even though Earth is virtually the same distance from the Sun. Near local noon, when the Sun is highest in the sky, the temperature of the dark lunar soil rises above the boiling point of water. During the long lunar night (which, like the lunar day, lasts two Earth weeks[1]), the temperature drops to about 100 K (–173 °C). The extreme cooling is a result not only of the absence of air but also of the porous nature of the Moon’s dusty soil, which cools more rapidly than solid rock would Learn how the moon’s craters and maria were formed by watching a video produced by NASA’s Lunar Reconnaissance Orbiter (LRO) team (https://openstax.org/l/30mooncratersfo) about the evolution of the Moon, tracing it from its origin about 4.5 billion years ago |
to the Moon we see today. See a simulation of how the Moon’s craters and maria were formed through periods of impact, volcanic activity, and heavy bombardment. 1 You can see the cycle of day and night on the side of the Moon facing us in the form of the Moon’s phases. It takes about 14 days for the side of the Moon facing us to go from full moon (all lit up) to new moon (all dark). There is more on this in Chapter 4: Earth, Moon, and Sky. This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 9 Cratered Worlds 315 9.3 IMPACT CRATERS Learning Objectives By the end of this section, you will be able to: Compare and contrast ideas about how lunar craters form Explain the process of impact crater formation Discuss the use of crater counts to determine relative ages of lunar landforms The Moon provides an important benchmark for understanding the history of our planetary system. Most solid worlds show the effects of impacts, often extending back to the era when a great deal of debris from our system’s formation process was still present. On Earth, this long history has been erased by our active geology. On the Moon, in contrast, most of the impact history is preserved. If we can understand what has happened on the Moon, we may be able to apply this knowledge to other worlds. The Moon is especially interesting because it is not just any moon, but our Moon—a nearby world that has shared the history of Earth for more than 4 billion years and preserved a record that, for Earth, has been destroyed by our active geology. Volcanic Versus Impact Origin of Craters Until the middle of the twentieth century, scientists did not generally recognize that lunar craters were the result of impacts. Since impact craters are extremely rare on Earth, geologists did not expect them to be the major feature of lunar geology. They reasoned (perhaps unconsciously) that since the craters we have on Earth are volcanic, the lunar craters must have a similar origin. One of the first geologists to propose that lunar craters were the result of impacts was Grove K. Gilbert, a scientist with the US Geological Survey in the 1890s. He pointed out that the large lunar craters—mountain- rimmed, circular features with floors generally below the level of the surrounding plains—are larger and have different shapes from known volcanic craters |
on Earth. Terrestrial volcanic craters are smaller and deeper and almost always occur at the tops of volcanic mountains (Figure 9.13). The only alternative to explain the Moon’s craters was an impact origin. His careful reasoning, although not accepted at the time, laid the foundations for the modern science of lunar geology. Figure 9.13. Volcanic and Impact Craters. Profiles of a typical terrestrial volcanic crater and a typical lunar impact crater are quite different. Gilbert concluded that the lunar craters were produced by impacts, but he didn’t understand why all of them were circular and not oval. The reason lies in the escape velocity, the minimum speed that a body must reach to permanently break away from the gravity of another body; it is also the minimum speed that a projectile approaching Earth or the Moon will hit with. Attracted by the gravity of the larger body, the incoming chunk strikes with at least escape velocity, which is 11 kilometers per second for Earth and 2.4 kilometers per second (5400 miles per hour) for the Moon. To this escape velocity is added whatever speed the projectile already had with respect to Earth or Moon, typically 10 kilometers per second or more. At these speeds, the energy of impact produces a violent explosion that excavates a large volume of material in a symmetrical way. Photographs of bomb and shell craters on Earth confirm that explosion craters are always 316 Chapter 9 Cratered Worlds essentially circular. Only following World War I did scientists recognize the similarity between impact craters and explosion craters, but, sadly, Gilbert did not live to see his impact hypothesis widely accepted. The Cratering Process Let’s consider how an impact at these high speeds produces a crater. When such a fast projectile strikes a planet, it penetrates two or three times its own diameter before stopping. During these few seconds, its energy of motion is transferred into a shock wave (which spreads through the target body) and into heat (which vaporizes most of the projectile and some of the surrounding target). The shock wave fractures the rock of the target, while the expanding silicate vapor generates an explosion similar to that of a nuclear bomb detonated at ground level (Figure 9.14). The size of the excavated crater depends primarily on the speed of impact, but generally it is 10 to 15 times the diameter of the projectile. Figure 9.14. Stages in the Formation of an Impact Crater. (a) The impact occurs. (b) The projectile vapor |
izes and a shock wave spreads through the lunar rock. (c) Ejecta are thrown out of the crater. (d) Most of the ejected material falls back to fill the crater, forming an ejecta blanket. An impact explosion of the sort described above leads to a characteristic kind of crater, as shown in Figure 9.15. The central cavity is initially bowl-shaped (the word “crater” comes from the Greek word for “bowl”), but the rebound of the crust partially fills it in, producing a flat floor and sometimes creating a central peak. Around the rim, landslides create a series of terraces. Figure 9.15. Typical Impact Crater. King Crater on the far side of the Moon, a fairly recent lunar crater 75 kilometers in diameter, shows most of the features associated with large impact structures. (credit: NASA/JSC/Arizona State University) The rim of the crater is turned up by the force of the explosion, so it rises above both the floor and the adjacent terrain. Surrounding the rim is an ejecta blanket consisting of material thrown out by the explosion. This debris falls back to create a rough, hilly region, typically about as wide as the crater diameter. Additional, higher- This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 9 Cratered Worlds 317 speed ejecta fall at greater distances from the crater, often digging small secondary craters where they strike the surface (Figure 9.9). Some of these streams of ejecta can extend for hundreds or even thousands of kilometers from the crater, creating the bright crater rays that are prominent in lunar photos taken near full phase. The brightest lunar crater rays are associated with large young craters such as Kepler and Tycho Observing the Moon The Moon is one of the most beautiful sights in the sky, and it is the only object close enough to reveal its topography (surface features such as mountains and valleys) without a visit from a spacecraft. A fairly small amateur telescope easily shows craters and mountains on the Moon as small as a few kilometers across. Even as seen through a good pair of binoculars, we can observe that the appearance of the Moon’s surface changes dramatically with its phase. At full phase, it shows almost no topographic detail, and you must look closely to see more than a few craters. This is because sunlight illuminates the surface straight on, |
and in this flat lighting, no shadows are cast. Much more revealing is the view near first or third quarter, when sunlight streams in from the side, causing topographic features to cast sharp shadows. It is almost always more rewarding to study a planetary surface under such oblique lighting, when the maximum information about surface relief can be obtained. The flat lighting at full phase does, however, accentuate brightness contrasts on the Moon, such as those between the maria and highlands. Notice in Figure 9.16 that several of the large mare craters seem to be surrounded by white material and that the light streaks or rays that can stretch for hundreds of kilometers across the surface are clearly visible. These lighter features are ejecta, splashed out from the crater-forming impact. 318 Chapter 9 Cratered Worlds Figure 9.16. Appearance of the Moon at Different Phases. (a) Illumination from the side brings craters and other topographic features into sharp relief, as seen on the far left side. (b) At full phase, there are no shadows, and it is more difficult to see such features. However, the flat lighting at full phase brings out some surface features, such as the bright rays of ejecta that stretch out from a few large young craters. (credit: modification of work by Luc Viatour) By the way, there is no danger in looking at the Moon with binoculars or telescopes. The reflected sunlight is never bright enough to harm your eyes. In fact, the sunlit surface of the Moon has about the same brightness as a sunlit landscape of dark rock on Earth. Although the Moon looks bright in the night sky, its surface is, on average, much less reflective than Earth’s, with its atmosphere and white clouds. This difference is nicely illustrated by the photo of the Moon passing in front of Earth taken from the Deep Space Climate Observatory spacecraft (Figure 9.17). Since the spacecraft took the image from a position inside the orbit of Earth, we see both objects fully illuminated (full Moon and full Earth). By the way, you cannot see much detail on the Moon because the exposure has been set to give a bright image of Earth, not the Moon. Figure 9.17. The Moon Crossing the Face of Earth. In this 2015 image from the Deep Space Climate Observatory spacecraft, both objects are fully illuminated, but the Moon looks darker because it has a much lower average reflectivity than Earth. (credit: modification of work by NASA, DSCOV |
R EPIC team) This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 9 Cratered Worlds 319 One interesting thing about the Moon that you can see without binoculars or telescopes is popularly called “the new Moon in the old Moon’s arms.” Look at the Moon when it is a thin crescent, and you can often make out the faint circle of the entire lunar disk, even though the sunlight shines on only the crescent. The rest of the disk is illuminated not by sunlight but by earthlight—sunlight reflected from Earth. The light of the full Earth on the Moon is about 50 times brighter than that of the full Moon shining on Earth. Using Crater Counts If a world has had little erosion or internal activity, like the Moon during the past 3 billion years, it is possible to use the number of impact craters on its surface to estimate the age of that surface. By “age” here we mean the time since a major disturbance occurred on that surface (such as the volcanic eruptions that produced the lunar maria). We cannot directly measure the rate at which craters are being formed on Earth and the Moon, since the average interval between large crater-forming impacts is longer than the entire span of human history. Our best-known example of such a large crater, Meteor Crater in Arizona (Figure 9.18), is about 50,000 years old. However, the cratering rate can be estimated from the number of craters on the lunar maria or calculated from the number of potential “projectiles” (asteroids and comets) present in the solar system today. Both lines of reasoning lead to about the same estimations. Figure 9.18. Meteor Crater. This aerial photo of Meteor Crater in Arizona shows the simple form of a meteorite impact crater. The crater’s rim diameter is about 1.2 kilometers. (credit: Shane Torgerson) For the Moon, these calculations indicate that a crater 1 kilometer in diameter should be produced about every 200,000 years, a 10-kilometer crater every few million years, and one or two 100-kilometer craters every billion years. If the cratering rate has stayed the same, we can figure out how long it must have taken to make all the craters we see in the lunar maria. Our calculations show that it would have taken several billion years |
. This result is similar to the age determined for the maria from radioactive dating of returned samples—3.3 to 3.8 billion years old. The fact that these two calculations agree suggests that astronomers’ original assumption was right: comets and asteroids in approximately their current numbers have been impacting planetary surfaces for billions of years. Calculations carried out for other planets (and their moons) indicate that they also have been subject to about the same number of interplanetary impacts during this time. 320 Chapter 9 Cratered Worlds We have good reason to believe, however, that earlier than 3.8 billion years ago, the impact rates must have been a great deal higher. This becomes immediately evident when comparing the numbers of craters on the lunar highlands with those on the maria. Typically, there are 10 times more craters on the highlands than on a similar area of maria. Yet the radioactive dating of highland samples showed that they are only a little older than the maria, typically 4.2 billion years rather than 3.8 billion years. If the rate of impacts had been constant throughout the Moon’s history, the highlands would have had to be at least 10 times older. They would thus have had to form 38 billion years ago—long before the universe itself began. In science, when an assumption leads to an implausible conclusion, we must go back and re-examine that assumption—in this case, the constant impact rate. The contradiction is resolved if the impact rate varied over time, with a much heavier bombardment earlier than 3.8 billion years ago (Figure 9.19). This “heavy bombardment” produced most of the craters we see today in the highlands. Figure 9.19. Cratering Rates over Time. The number of craters being made on the Moon’s surface has varied with time over the past 4.3 billion years. This idea we have been exploring—that large impacts (especially during the early history of the solar system) played a major role in shaping the worlds we see—is not unique to our study of the Moon. As you read through the other chapters about the planets, you will see further indications that a number of the present-day characteristics of our system may be due to its violent past. 9.4 THE ORIGIN OF THE MOON Learning Objectives By the end of this section, you will be able to: Describe the top three early hypotheses of the formation of the Moon Summarize the |
current “giant impact” concept of how the Moon formed It is characteristic of modern science to ask how things originated. Understanding the origin of the Moon has proven to be challenging for planetary scientists, however. Part of the difficulty is simply that we know so much This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 9 Cratered Worlds 321 about the Moon (quite the opposite of our usual problem in astronomy). As we will see, one key problem is that the Moon is both tantalizingly similar to Earth and frustratingly different. Ideas for the Origin of the Moon Most of the earlier hypotheses for the Moon’s origin followed one of three general ideas: 1. The fission theory—the Moon was once part of Earth, but somehow separated from it early in their history. 2. The sister theory—the Moon formed together with (but independent of) Earth, as we believe many moons of the outer planets formed. 3. The capture theory—the Moon formed elsewhere in the solar system and was captured by Earth. Unfortunately, there seem to be fundamental problems with each of these ideas. Perhaps the easiest hypothesis to reject is the capture theory. Its primary drawback is that no one knows of any way that early Earth could have captured such a large moon from elsewhere. One body approaching another cannot go into orbit around it without a substantial loss of energy; this is the reason that spacecraft destined to orbit other planets are equipped with retro-rockets. Furthermore, if such a capture did take place, the captured object would go into a very eccentric orbit rather than the nearly circular orbit our Moon occupies today. Finally, there are too many compositional similarities between Earth and the Moon, particularly an identical fraction of the major isotopes[2] of oxygen, to justify seeking a completely independent origin. The fission hypothesis, which states that the Moon separated from Earth, was suggested in the late nineteenth century. Modern calculations have shown that this sort of spontaneous fission or splitting is impossible. Furthermore, it is difficult to understand how a Moon made out of terrestrial material in this way could have developed the many distinctive chemical differences now known to characterize our neighbor. Scientists were therefore left with the sister hypothesis—that the Moon formed alongside Earth—or with some modification of the fission hypothesis that can find a more acceptable way for the lunar material to have separated from Earth. But the more we learned about our Moon, the less these old ideas seem to fit the bill |
. The Giant Impact Hypothesis In an effort to resolve these apparent contradictions, scientists developed a fourth hypothesis for the origin of the Moon, one that involves a giant impact early in Earth’s history. There is increasing evidence that large chunks of material—objects of essentially planetary mass—were orbiting in the inner solar system at the time that the terrestrial planets formed. The giant impact hypothesis envisions Earth being struck obliquely by an object approximately one-tenth Earth’s mass—a “bullet” about the size of Mars. This is very nearly the largest impact Earth could experience without being shattered. Such an impact would disrupt much of Earth and eject a vast amount of material into space, releasing almost enough energy to break the planet apart. Computer simulations indicate that material totaling several percent of Earth’s mass could be ejected in such an impact. Most of this material would be from the stony mantles of Earth and the impacting body, not from their metal cores. This ejected rock vapor then cooled and formed a ring of material orbiting Earth. It was this ring that ultimately condensed into the Moon. While we do not have any current way of showing that the giant impact hypothesis is the correct model of the Moon’s origin, it does offer potential solutions to most of the major problems raised by the chemistry of the Moon. First, since the Moon’s raw material is derived from the mantles of Earth and the projectile, the absence of metals is easily understood. Second, most of the volatile elements would have been lost during the high- 2 Remember from the Radiation and Spectra chapter that the term isotope means a different “version” of an element. Specifically, different isotopes of the same element have equal numbers of protons but different numbers of neutrons (as in carbon-12 versus carbon-14.) 322 Chapter 9 Cratered Worlds temperature phase following the impact, explaining the lack of these materials on the Moon. Yet, by making the Moon primarily of terrestrial mantle material, it is also possible to understand similarities such as identical abundances of various oxygen isotopes. 9.5 MERCURY Learning Objectives By the end of this section, you will be able to: Characterize the orbit of Mercury around the Sun Describe Mercury’s structure and composition Explain the relationship between Mercury’s orbit and rotation Describe the topography and features of Mercury’s surface Summarize our ideas about the origin and evolution of Mercury The planet Mercury is |
similar to the Moon in many ways. Like the Moon, it has no atmosphere, and its surface is heavily cratered. As described later in this chapter, it also shares with the Moon the likelihood of a violent birth. Mercury’s Orbit Mercury is the nearest planet to the Sun, and, in accordance with Kepler’s third law, it has the shortest period of revolution about the Sun (88 of our days) and the highest average orbital speed (48 kilometers per second). It is appropriately named for the fleet-footed messenger god of the Romans. Because Mercury remains close to the Sun, it can be difficult to pick out in the sky. As you might expect, it’s best seen when its eccentric orbit takes it as far from the Sun as possible. The semimajor axis of Mercury’s orbit—that is, the planet’s average distance from the Sun—is 58 million kilometers, or 0.39 AU. However, because its orbit has the high eccentricity of 0.206, Mercury’s actual distance from the Sun varies from 46 million kilometers at perihelion to 70 million kilometers at aphelion (the ideas and terms that describe orbits were introduced in Orbits and Gravity). Composition and Structure Mercury’s mass is one-eighth that of Earth, making it the smallest terrestrial planet. Mercury is the smallest planet (except for the dwarf planets), having a diameter of 4878 kilometers, less than half that of Earth. Mercury’s density is 5.4 g/cm3, much greater than the density of the Moon, indicating that the composition of those two objects differs substantially. Mercury’s composition is one of the most interesting things about it and makes it unique among the planets. Mercury’s high density tells us that it must be composed largely of heavier materials such as metals. The most likely models for Mercury’s interior suggest a metallic iron-nickel core amounting to 60% of the total mass, with the rest of the planet made up primarily of silicates. The core has a diameter of 3500 kilometers and extends out to within 700 kilometers of the surface. We could think of Mercury as a metal ball the size of the Moon surrounded by a rocky crust 700 kilometers thick (Figure 9.20). Unlike the Moon, Mercury does have a weak magnetic field. The existence of this field is consistent with the presence of a large metal core, and it suggests that at least part of the core must be |
liquid in order to generate the observed magnetic field.[3] 3 Recall from the Radiation and Spectra chapter that magnetism is an effect of moving electric charges. In atoms of metals, the outer electrons are easier to dislodge and they can form a current when the metal is in liquid form and can flow. This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 9 Cratered Worlds 323 Figure 9.20. Mercury’s Internal Structure. The interior of Mercury is dominated by a metallic core about the same size as our Moon Densities of Worlds The average density of a body equals its mass divided by its volume. For a sphere, density is: density = mass 3πR3 4 Astronomers can measure both mass and radius accurately when a spacecraft flies by a body. Using the information in this chapter, we can calculate the approximate average density of the Moon. Solution For a sphere, density = mass 3πR3 = 4 7.35 × 1022 kg 4.2 × 5.2 × 1018 m3 = 3.4 × 103 kg/m3 Table 9.1 gives a value of 3.3 g/cm3, which is 3.3 × 103 kg/m3. Check Your Learning Using the information in this chapter, calculate the average density of Mercury. Show your work. Does your calculation agree with the figure we give in this chapter? Answer: density = mass 3πR3 = 4 3.3 × 1023 kg 4.2 × 1.45 × 1019 m3 = 5.4 × 103 kg/m3 That matches the value given in Table 9.1 when g/cm3 is converted into kg/m3. 324 Chapter 9 Cratered Worlds Mercury’s Strange Rotation Visual studies of Mercury’s indistinct surface markings were once thought to indicate that the planet kept one face to the Sun (as the Moon does to Earth). Thus, for many years, it was widely believed that Mercury’s rotation period was equal to its revolution period of 88 days, making one side perpetually hot while the other was always cold. Radar observations of Mercury in the mid-1960s, however, showed conclusively that Mercury does not keep one side fixed toward the Sun. If a planet is turning, one side seems to be approaching Earth while the other is moving away from it. The resulting Doppler shift spreads or broadens the precise transmitted radar-wave |
frequency into a range of frequencies in the reflected signal (Figure 9.21). The degree of broadening provides an exact measurement of the rotation rate of the planet. Figure 9.21. Doppler Radar Measures Rotation. When a radar beam is reflected from a rotating planet, the motion of one side of the planet’s disk toward us and the other side away from us causes Doppler shifts in the reflected signal. The effect is to cause both a redshift and a blueshift, widening the spread of frequencies in the radio beam. Mercury’s period of rotation (how long it takes to turn with respect to the distant stars) is 59 days, which is just two-thirds of the planet’s period of revolution. Subsequently, astronomers found that a situation where the spin and the orbit of a planet (its year) are in a 2:3 ratio turns out to be stable. (See What a Difference a Day Makes for more on the effects of having such a long day on Mercury.) Mercury, being close to the Sun, is very hot on its daylight side; but because it has no appreciable atmosphere, it gets surprisingly cold during the long nights. The temperature on the surface climbs to 700 K (430 °C) at noontime. After sunset, however, the temperature drops, reaching 100 K (–170 °C) just before dawn. (It is even colder in craters near the poles that receive no sunlight at all.) The range in temperature on Mercury is thus 600 K (or 600 °C), a greater difference than on any other planet. This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 9 Cratered Worlds 325 What a Difference a Day Makes Mercury rotates three times for each two orbits around the Sun. It is the only planet that exhibits this relationship between its spin and its orbit, and there are some interesting consequences for any observers who might someday be stationed on the surface of Mercury. Here on Earth, we take for granted that days are much shorter than years. Therefore, the two astronomical ways of defining the local “day”—how long the planet takes to rotate and how long the Sun takes to return to the same position in the sky—are the same on Earth for most practical purposes. But this is not the case on Mercury. While Mercury rotates (spins once) in 59 Earth days, the time for the Sun to return to the |
same place in Mercury’s sky turns out to be two Mercury years, or 176 Earth days. (Note that this result is not intuitively obvious, so don’t be upset if you didn’t come up with it.) Thus, if one day at noon a Mercury explorer suggests to her companion that they should meet at noon the next day, this could mean a very long time apart! To make things even more interesting, recall that Mercury has an eccentric orbit, meaning that its distance from the Sun varies significantly during each mercurian year. By Kepler’s law, the planet moves fastest in its orbit when closest to the Sun. Let’s examine how this affects the way we would see the Sun in the sky during one 176-Earth-day cycle. We’ll look at the situation as if we were standing on the surface of Mercury in the center of a giant basin that astronomers call Caloris (Figure 9.23). At the location of Caloris, Mercury is most distant from the Sun at sunrise; this means the rising Sun looks smaller in the sky (although still more than twice the size it appears from Earth). As the Sun rises higher and higher, it looks bigger and bigger; Mercury is now getting closer to the Sun in its eccentric orbit. At the same time, the apparent motion of the Sun slows down as Mercury’s faster motion in orbit begins to catch up with its rotation. At noon, the Sun is now three times larger than it looks from Earth and hangs almost motionless in the sky. As the afternoon wears on, the Sun appears smaller and smaller, and moves faster and faster in the sky. At sunset, a full Mercury year (or 88 Earth days after sunrise), the Sun is back to its smallest apparent size as it dips out of sight. Then it takes another Mercury year before the Sun rises again. (By the way, sunrises and sunsets are much more sudden on Mercury, since there is no atmosphere to bend or scatter the rays of sunlight.) Astronomers call locations like the Caloris Basin the “hot longitudes” on Mercury because the Sun is closest to the planet at noon, just when it is lingering overhead for many Earth days. This makes these areas the hottest places on Mercury. We bring all this up not because the exact details of this scenario are so important but to illustrate how many of the things we take for granted on Earth are not the same on other worlds. As we’ve mentioned |
before, one of the best things about taking an astronomy class should be ridding you forever of any “Earth chauvinism” you might have. The way things are on our planet is just one of the many ways nature can arrange reality. 326 Chapter 9 Cratered Worlds The Surface of Mercury The first close-up look at Mercury came in 1974, when the US spacecraft Mariner 10 passed 9500 kilometers from the surface of the planet and transmitted more than 2000 photographs to Earth, revealing details with a resolution down to 150 meters. Subsequently, the planet was mapped in great detail by the MESSENGER spacecraft, which was launched in 2004 and made multiple flybys of Earth, Venus, and Mercury before settling into orbit around Mercury in 2011. It ended its life in 2015, when it was commanded to crash into the surface of the planet. Mercury’s surface strongly resembles the Moon in appearance (Figure 9.22 and Figure 9.23). It is covered with thousands of craters and larger basins up to 1300 kilometers in diameter. Some of the brighter craters are rayed, like Tycho and Copernicus on the Moon, and many have central peaks. There are also scarps (cliffs) more than a kilometer high and hundreds of kilometers long, as well as ridges and plains. MESSENGER instruments measured the surface composition and mapped past volcanic activity. One of its most important discoveries was the verification of water ice (first detected by radar) in craters near the poles, similar to the situation on the Moon, and the unexpected discovery of organic (carbon-rich) compounds mixed with the water ice Scientists working with data from the MESSENGER mission (https://openstax.org/l/30MESSmercuryrt) put together a rotating globe of Mercury, in false color, showing some of the variations in the composition of the planet’s surface. You can watch it spin. Figure 9.22. Mercury’s Topography. The topography of Mercury’s northern hemisphere is mapped in great detail from MESSENGER data. The lowest regions are shown in purple and blue, and the highest regions are shown in red. The difference in elevation between the lowest and highest regions shown here is roughly 10 kilometers. The permanently shadowed low-lying craters near the north pole contain radar-bright water ice. (credit: modification of work by NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington |
) This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 9 Cratered Worlds 327 Figure 9.23. Caloris Basin. This partially flooded impact basin is the largest known structural feature on Mercury. The smooth plains in the interior of the basin have an area of almost two million square kilometers. Compare this photo with Figure 9.11, the Orientale Basin on the Moon. (credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington) Most of the mercurian features have been named in honor of artists, writers, composers, and other contributors to the arts and humanities, in contrast with the scientists commemorated on the Moon. Among the named craters are Bach, Shakespeare, Tolstoy, Van Gogh, and Scott Joplin. There is no evidence of plate tectonics on Mercury. However, the planet’s distinctive long scarps can sometimes be seen cutting across craters; this means the scarps must have formed later than the craters (Figure 9.24). These long, curved cliffs appear to have their origin in the slight compression of Mercury’s crust. Apparently, at some point in its history, the planet shrank, wrinkling the crust, and it must have done so after most of the craters on its surface had already formed. If the standard cratering chronology applies to Mercury, this shrinkage must have taken place during the last 4 billion years and not during the solar system’s early period of heavy bombardment. Figure 9.24. Discovery Scarp on Mercury. This long cliff, nearly 1 kilometer high and more than 100 kilometers long, cuts across several craters. Astronomers conclude that the compression that made “wrinkles” like this in the plank’s surface must have taken place after the craters were formed. (credit: modification of work by NASA/JPL/Northwestern University) 328 Chapter 9 Cratered Worlds The Origin of Mercury The problem with understanding how Mercury formed is the reverse of the problem posed by the composition of the Moon. We have seen that, unlike the Moon, Mercury is composed mostly of metal. However, astronomers think that Mercury should have formed with roughly the same ratio of metal to silicate as that found on Earth or Venus. How did it lose so much of its rocky material? The most probable explanation for Mercury’s silicate loss may be similar to the explanation |
for the Moon’s lack of a metal core. Mercury is likely to have experienced several giant impacts very early in its youth, and one or more of these may have torn away a fraction of its mantle and crust, leaving a body dominated by its iron core You can follow some of NASA’s latest research on Mercury (https://openstax.org/l/ 30NASAresmercu) and see some helpful animations on the MESSENGER web page. Today, astronomers recognize that the early solar system was a chaotic place, with the final stages of planet formation characterized by impacts of great violence. Some objects of planetary mass have been destroyed, whereas others could have fragmented and then re-formed, perhaps more than once. Both the Moon and Mercury, with their strange compositions, bear testimony to the catastrophes that must have characterized the solar system during its youth. This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 9 Cratered Worlds 329 CHAPTER 9 REVIEW KEY TERMS highlands the lighter, heavily cratered regions of the Moon, which are generally several kilometers higher than the maria mare (plural: maria) Latin for “sea;” the name applied to the dark, relatively smooth features that cover 17% of the Moon’s surface SUMMARY 9.1 General Properties of the Moon Most of what we know about the Moon derives from the Apollo program, including 400 kilograms of lunar samples still being intensively studied. The Moon has one-eightieth the mass of Earth and is severely depleted in both metals and volatile materials. It is made almost entirely of silicates like those in Earth’s mantle and crust. However, more recent spacecraft have found evidence of a small amount of water near the lunar poles, most likely deposited by comet and asteroid impacts. 9.2 The Lunar Surface The Moon, like Earth, was formed about 4.5 billion year ago. The Moon’s heavily cratered highlands are made of rocks more than 4 billion years old. The darker volcanic plains of the maria were erupted primarily between 3.3 and 3.8 billion years ago. Generally, the surface is dominated by impacts, including continuing small impacts that produce its fine-grained soil. 9.3 Impact Craters A century ago, Grove Gilbert suggested that the lunar craters were caused by impacts, but the cratering process was not well understood until more recently. High |
-speed impacts produce explosions and excavate craters 10 to 15 times the size of the impactor with raised rims, ejecta blankets, and often central peaks. Cratering rates have been roughly constant for the past 3 billion years but earlier were much greater. Crater counts can be used to derive approximate ages for geological features on the Moon and other worlds with solid surfaces. 9.4 The Origin of the Moon The three standard hypotheses for the origin of the Moon were the fission hypothesis, the sister hypothesis, and the capture hypothesis. All have problems, and they have been supplanted by the giant impact hypothesis, which ascribes the origin of the Moon to the impact of a Mars-sized projectile with Earth 4.5 billion years ago. The debris from the impact made a ring around Earth which condensed and formed the Moon. 9.5 Mercury Mercury is the nearest planet to the Sun and the fastest moving. Mercury is similar to the Moon in having a heavily cratered surface and no atmosphere, but it differs in having a very large metal core. Early in its evolution, it apparently lost part of its silicate mantle, probably due to one or more giant impacts. Long scarps on its surface testify to a global compression of Mercury’s crust during the past 4 billion years. 330 Chapter 9 Cratered Worlds FOR FURTHER EXPLORATION Articles The Moon Bakich, Michael. “Asia’s New Assault on the Moon.” Astronomy (August 2009): 50. The Japanese Selene and Chinese Chang’e 1 missions. Beatty, J. “NASA Slams the Moon.” Sky & Telescope (February 2010): 28. The impact of the LCROSS mission on the Moon and what we learned from it. Bell, T. “Warning: Dust Ahead.” Astronomy (March 2006): 46. What we know about lunar dust and the problems it can cause. Dorminey, B. “Secrets beneath the Moon’s Surface.” Astronomy (March 2011): 24. A nice timeline of the Moon’s evolution and the story of how we are finding out more about its internal structure. Jayawardhana, R. “Deconstructing the Moon.” Astronomy (September 1998): 40. An update on the giant impact hypothesis for forming the Moon. Register, B. “The Fate of the Moon Rocks.” Astronomy (December 1985): 15 |
. What was done with the rocks the astronauts brought back from the Moon. Schmitt, H. “Exploring Taurus–Littrow: Apollo 17.” National Geographic (September 1973). First-person account given by the only scientist to walk on the Moon. Schmitt, H. “From the Moon to Mars.” Scientific American (July 2009): 36. The only scientist to walk on the Moon reflects on the science from Apollo and future missions to Mars. Schultz, P. “New Clues to the Moon’s Distant Past.” Astronomy (December 2011): 34. Summary of results and ideas from the LCROSS and LRO missions. Shirao, M. “Kayuga’s High Def Highlights.” Sky & Telescope (February 2010): 20. Results from the Japanese mission to the Moon, with high definition TV cameras. Wadhwa, M. “What Are We Learning from the Moon Rocks?” Astronomy (June 2013): 54. Very nice discussion of how the rocks tell us about Moon’s composition, age, and origin. Wood, Charles. “The Moon’s Far Side: Nearly a New World.” Sky & Telescope (January 2007): 48. This article compares what we know about the two sides and why they are different. Zimmerman, R. “How Much Water is on the Moon?” Astronomy (January 2014): 50. Results from the LRO’s instruments and good overview of issue. Mercury Beatty, J. “Mercury Gets a Second Look.” Sky & Telescope (March 2009): 26. The October 2008 MESSENGER mission flyby. Beatty, J. “Reunion with Mercury.” Sky & Telescope (May 2008): 24. The January 2008 MESSENGER encounter with Mercury. “Mercury: Meet the Planet Nearest the Sun.” Sky & Telescope (March 2014): 39. Four-page pictorial introduction, including the new MESSENGER probe full map of the planet provided. Oberg, J. “Torrid Mercury’s Icy Poles.” Astronomy (December 2013): 30. A nice overview of results from MESSENGER mission, including the ice in polar craters. This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 9 |
Cratered Worlds 331 Sheehan, W., and Dobbins, T. “Mesmerized by Mercury.” Sky & Telescope (June 2000): 109. History of Mercury observations and how amateur astronomers can contribute. Talcott, R. “Surprises from MESSENGER’s Historic Mercury Fly-by.” Astronomy (March 2009): 28. Talcott, R. “Mercury Reveals its Hidden Side.” Astronomy (May 2008): 26. Results and image from the MESSENGER mission flyby of January 2008. Websites The Moon Apollo Lunar Surface Journal: http://www.hq.nasa.gov/office/pao/History/alsj/. Information, interviews, maps, photos, video and audio clips, and much more on each of the Apollo landing missions. Lunar & Planetary Institute: http://www.lpi.usra.edu/lunar/missions/. Lunar Science and Exploration web pages. Lunar Reconnaissance Orbiter Mission Page: http://lro.gsfc.nasa.gov/. NASA’s Guide to Moon Missions and Information: http://nssdc.gsfc.nasa.gov/planetary/planets/ moonpage.html. Origin of the Moon: http://www.psi.edu/projects/moon/moon.html. By William Hartmann, who, with a colleague, first suggested the giant impact hypothesis for how the Moon formed, in 1975. Sky & Telescope magazine’s observing guides and articles about the Moon: http://www.skyandtelescope.com/ observing/celestial-objects-to-watch/moon/. To the Moon: http://www.pbs.org/wgbh/nova/tothemoon/. PBS program on the Apollo landings. We Choose the Moon: http://wechoosethemoon.org/. A recreation of the Apollo 11 mission. Mercury Mercury Unveiled by G. Jeffrey Taylor (summarizing the Mariner 10 Mission): http://www.psrd.hawaii.edu/ Jan97/MercuryUnveiled.html. MESSENGER Mission Website: http://messenger.jhuapl.edu/. NASA Planetary Data Center Mercury Page: http://nssdc.gsfc.nasa.gov/planetary/planets/mercurypage.html. Views of the Solar System Mercury Page: http |
://solarviews.com/eng/mercury.htm. COLLABORATIVE GROUP ACTIVITIES A. We mentioned that no nation on Earth now has the capability to send a human being to the Moon, even though the United States once sent 12 astronauts to land there. What does your group think about this? Should we continue the exploration of space with human beings? Should we put habitats on the Moon? Should we go to Mars? Does humanity have a “destiny in space?” Whatever your answer to these questions, make a list of the arguments and facts that support your position. B. When they hear about the giant impact hypothesis for the origin of the Moon, many students are intrigued and wonder why we can’t cite more evidence for it. In your group, make a list of reasons we cannot find any traces on Earth of the great impact that formed the Moon? 332 Chapter 9 Cratered Worlds C. We discussed that the ice (mixed into the soil) that is found on the Moon was most likely delivered by comets. Have your group make a list of all the reasons the Moon would not have any ice of its own left over from its early days. D. Can your group make a list of all the things that would be different if Earth had no Moon? Don’t restrict your answer to astronomy and geology. Think about our calendars and moonlit romantic strolls, for example. (You may want to review Earth, Moon, and Sky.) E. If, one day, humanity decides to establish a colony on the Moon, where should we put it? Make a list of the advantages and disadvantages of locating such a human habitat on the near side, the far side, or at the poles. What site would be best for doing visible-light and radio astronomy from observatories on the Moon? F. A member of the class (but luckily, not a member of your group) suggests that he has always dreamed of building a vacation home on the planet Mercury. Can your group make a list of all reasons such a house would be hard to build and keep in good repair? G. As you’ve read in this chapter, craters on the Moon are (mostly) named after scientists. (See the official list at: http://planetarynames.wr.usgs.gov/SearchResults?target=MOON&featureType=Crater,%20craters). The craters on Mercury, on the other hand, |
are named for writers, artists, composers, and others in the humanities. See the official list at: http://planetarynames.wr.usgs.gov/SearchResults? target=MERCURY&featureType=Crater,%20craters). Living persons are not eligible. Can each person in your group think of a scientist or someone in the arts whom they especially respect? Now check to see if they are listed. Are there scientists or people in the arts who should have their names on the Moon or Mercury and do not? H. Imagine that a distant relative, hearing you are taking an astronomy course, calls you up and tells you that NASA faked the Moon landings. His most significant argument is that all the photos of the Moon show black skies, but none of them have any stars showing. This proves that the photos were taken against a black backdrop in a studio and not on the Moon. Based on your reading in this chapter, what arguments can your group come up with to rebut this idea? EXERCISES Review Questions 1. What is the composition of the Moon, and how does it compare to the composition of Earth? Of Mercury? 2. Why does the Moon not have an atmosphere? 3. What are the principal features of the Moon observable with the unaided eye? 4. Frozen water exists on the lunar surface primarily in which location? Why? 5. Outline the main events in the Moon’s geological history. 6. What are the maria composed of? Is this material found elsewhere in the solar system? 7. The mountains on the Moon were formed by what process? 8. With no wind or water erosion of rocks, what is the mechanism for the creation of the lunar “soil?” This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 9 Cratered Worlds 333 9. What differences did Grove K. Gilbert note between volcanic craters on Earth and lunar craters? 10. Explain how high-speed impacts form circular craters. How can this explanation account for the various characteristic features of impact craters? 11. Explain the evidence for a period of heavy bombardment on the Moon about 4 billion years ago. 12. How did our exploration of the Moon differ from that of Mercury (and the other planets)? 13. Summarize the four main hypotheses for the origin of the Moon. 14. What are the difficulties with the capture hypothesis of the |
Moon’s origin? 15. What is the main consequence of Mercury’s orbit being so highly eccentric? 16. Describe the basic internal structure of Mercury. 17. How was the rotation rate of Mercury determined? 18. What is the relationship between Mercury’s rotational period and orbital period? 19. The features of Mercury are named in honor of famous people in which fields of endeavor? 20. What do our current ideas about the origins of the Moon and Mercury have in common? How do they differ? Thought Questions 21. One of the primary scientific objectives of the Apollo program was the return of lunar material. Why was this so important? What can be learned from samples? Are they still of value now? 22. Apollo astronaut David Scott dropped a hammer and a feather together on the Moon, and both reached the ground at the same time. What are the two distinct advantages that this experiment on the Moon had over the same kind of experiment as performed by Galileo on Earth? 23. Galileo thought the lunar maria might be seas of water. If you had no better telescope than the one he had, could you demonstrate that they are not composed of water? 24. Why did it take so long for geologists to recognize that the lunar craters had an impact origin rather than a volcanic one? 25. How might a crater made by the impact of a comet with the Moon differ from a crater made by the impact of an asteroid? 26. Why are the lunar mountains smoothly rounded rather than having sharp, pointed peaks (as they were almost always depicted in science-fiction illustrations and films before the first lunar landings)? 27. The lunar highlands have about ten times more craters in a given area than do the maria. Does this mean that the highlands are 10 times older? Explain your reasoning. 28. At the end of the section on the lunar surface, your authors say that lunar night and day each last about two Earth weeks. After looking over the information in Earth, Moon, and Sky and this chapter about the motions of the Moon, can you explain why? (It helps to draw a diagram for yourself.) 29. Give several reasons Mercury would be a particularly unpleasant place to build an astronomical observatory. 334 Chapter 9 Cratered Worlds 30. If, in the remote future, we establish a base on Mercury, keeping track of time will be a challenge. Discuss how to define a year on Mercury, and the two ways to define a day. Can you come up with ways that |
humans raised on Earth might deal with time cycles on Mercury? 31. The Moon has too little iron, Mercury too much. How can both of these anomalies be the result of giant impacts? Explain how the same process can yield such apparently contradictory results. Figuring For Yourself 32. In the future, astronomers discover a solid moon around a planet orbiting one of the nearest stars. This moon has a diameter of 1948 km and a mass of 1.6 × 1022 kg. What is its density? 33. The Moon was once closer to Earth than it is now. When it was at half its present distance, how long was its period of revolution? (See Orbits and Gravity for the formula to use.) 34. Astronomers believe that the deposit of lava in the giant mare basins did not happen in one flow but in many different eruptions spanning some time. Indeed, in any one mare, we find a variety of rock ages, typically spanning about 100 million years. The individual lava flows as seen in Hadley Rille by the Apollo 15 astronauts were about 4 m thick. Estimate the average time interval between the beginnings of successive lava flows if the total depth of the lava in the mare is 2 km. 35. The Moon requires about 1 month (0.08 year) to orbit Earth. Its distance from us is about 400,000 km (0.0027 AU). Use Kepler’s third law, as modified by Newton, to calculate the mass of Earth relative to the Sun. This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 10 Earthlike Planets: Venus and Mars 335 10 EARTHLIKE PLANETS: VENUS AND MARS Figure 10.1. Spirit Rover on Mars. This May 2004 image shows the tracks made by the Mars Exploration Spirit rover on the surface of the red planet. Spirit was active on Mars between 2004 and 2010, twenty times longer than its planners had expected. It “drove” over 7.73 kilometers in the process of examining the martian landscape. (credit: modification of work by NASA/JPL/Cornell) Chapter Outline 10.1 The Nearest Planets: An Overview 10.2 The Geology of Venus 10.3 The Massive Atmosphere of Venus 10.4 The Geology of Mars 10.5 Water and Life on Mars 10.6 Divergent Planetary Evolution Thinking Ahead The Moon and Mercury are geologically dead |
. In contrast, the larger terrestrial planets—Earth, Venus, and Mars—are more active and interesting worlds. We have already discussed Earth, and we now turn to Venus and Mars. These are the nearest planets and the most accessible to spacecraft. Not surprisingly, the greatest effort in planetary exploration has been devoted to these fascinating worlds. In the chapter, we discuss some of the results of more than four decades of scientific exploration of Mars and Venus. Mars is exceptionally interesting, with evidence that points to habitable conditions in the past. Even today, we are discovering things about Mars that make it the most likely place where humans might set up a habitat in the future. However, our robot explorers have clearly shown that neither Venus nor Mars has conditions similar to Earth. How did it happen that these three neighboring terrestrial planets have diverged so dramatically in their evolution? 10.1 THE NEAREST PLANETS: AN OVERVIEW Learning Objectives By the end of this section, you will be able to: 336 Chapter 10 Earthlike Planets: Venus and Mars Explain why it’s difficult to learn about Venus from Earth-based observation alone Describe the history of our interest in Mars before the Space Age Compare the basic physical properties of Earth, Mars, and Venus, including their orbits As you might expect from close neighbors, Mars and Venus are among the brightest objects in the night sky. The average distance of Mars from the Sun is 227 million kilometers (1.52 AU), or about half again as far from the Sun as Earth. Venus’ orbit is very nearly circular, at a distance of 108 million kilometers (0.72 AU) from the Sun. Like Mercury, Venus sometimes appears as an “evening star” and sometimes as a “morning star.” Venus approaches Earth more closely than does any other planet: at its nearest, it is only 40 million kilometers from us. The closest Mars ever gets to Earth is about 56 million kilometers. Appearance Venus appears very bright, and even a small telescope reveals that it goes through phases like the Moon. Galileo discovered that Venus displays a full range of phases, and he used this as an argument to show that Venus must circle the Sun and not Earth. The planet’s actual surface is not visible because it is shrouded by dense clouds that reflect about 70% of the sunlight that falls on them, frustrating efforts to study the underlying surface, even with cameras in orbit around the planet (Figure 10.2). Figure 10.2. Venus as Photographed by the |
Pioneer Venus Orbiter. This ultraviolet image shows an upper-atmosphere cloud structure that would be invisible at visible wavelengths. Note that there is not even a glimpse of the planet’s surface. (credit: modification of work by NASA) In contrast, Mars is more tantalizing as seen through a telescope (Figure 10.3). The planet is distinctly red, due (as we now know) to the presence of iron oxides in its soil. This color may account for its association with war (and blood) in the legends of early cultures. The best resolution obtainable from telescopes on the ground is about 100 kilometers, or about the same as what we can see on the Moon with the unaided eye. At this resolution, no hint of topographic structure can be detected: no mountains, no valleys, not even impact craters. On the other hand, bright polar ice caps can be seen easily, together with dusky surface markings that sometimes change in outline and intensity from season to season. This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 10 Earthlike Planets: Venus and Mars 337 Figure 10.3. Mars as Seen from Earth’s Surface. These are among the best Earth-based photos of Mars, taken in 1988 when the planet was exceptionally close to Earth. The polar caps and dark surface markings are evident, but not topographic features. (credit: modification of work by Steve Larson, Lunar and Planetary Laboratory, University of Arizona) For a few decades around the turn of the twentieth century, some astronomers believed that they saw evidence of an intelligent civilization on Mars. The controversy began in 1877, when Italian astronomer Giovanni Schiaparelli (1835–1910) announced that he could see long, faint, straight lines on Mars that he called canale, or channels. In English-speaking countries, the term was mistakenly translated as “canals,” implying an artificial origin. Even before Schiaparelli’s observations, astronomers had watched the bright polar caps change size with the seasons and had seen variations in the dark surface features. With a little imagination, it was not difficult to picture the canals as long fields of crops bordering irrigation ditches that brought water from the melting polar ice to the parched deserts of the red planet. (They assumed the polar caps were composed of water ice, which isn’t exactly true, as we |
will see shortly.) Until has death in 1916, the most effective proponent of intelligent life on Mars was Percival Lowell, a self-made American astronomer and member of the wealthy Lowell family of Boston (see the feature box on Percival Lowell: Dreaming of an Inhabited Mars). A skilled author and speaker, Lowell made what seemed to the public to be a convincing case for intelligent Martians, who had constructed the huge canals to preserve their civilization in the face of a deteriorating climate (Figure 10.4). 338 Chapter 10 Earthlike Planets: Venus and Mars Figure 10.4. Lowell’s Mars Globe. One of the remarkable globes of Mars prepared by Percival Lowell, showing a network of dozens of canals, oases, and triangular water reservoirs that he claimed were visible on the red planet. The argument for a race of intelligent Martians, however, hinged on the reality of the canals, a matter that remained in serious dispute among astronomers. The canal markings were always difficult to study, glimpsed only occasionally because atmospheric conditions caused the tiny image of Mars to shimmer in the telescope. Lowell saw canals everywhere (even a few on Venus), but many other observers could not see them at all and remained unconvinced of their existence. When telescopes larger than Lowell’s failed to confirm the presence of canals, the skeptics felt vindicated. Now it is generally accepted that the straight lines were an optical illusion, the result of the human mind’s tendency to see order in random features that are glimpsed dimly at the limits of the eye’s resolution. When we see small, dim dots of surface markings, our minds tend to connect those dots into straight lines Percival Lowell: Dreaming of an Inhabited Mars Percival Lowell was born into the well-to-do Massachusetts family about whom John Bossidy made the famous toast: And this is good old Boston, The home of the bean and the cod, Where the Lowells talk to the Cabots And the Cabots talk only to God. Percival’s brother Lawrence became president of Harvard University, and his sister, Amy, became a distinguished poet. Percival was already interested in astronomy as a boy: he made observations of Mars at age 13. His undergraduate thesis at Harvard dealt with the origin of the solar system, but he did not pursue this interest immediately. Instead, he entered the family business and traveled extensively in Asia. In 1892, |
however, he decided to dedicate himself to carrying on Schiaparelli’s work and solving the mysteries of the martian canals. This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 10 Earthlike Planets: Venus and Mars 339 In 1894, with the help of astronomers at Harvard but using his own funds, Lowell built an observatory on a high plateau in Flagstaff, Arizona, where he hoped the seeing would be clear enough to show him Mars in unprecedented detail. He and his assistants quickly accumulated a tremendous number of drawings and maps, purporting to show a vast network of martian canals (see Figure 10.4). He elaborated his ideas about the inhabitants of the red planet in several books, including Mars (1895) and Mars and Its Canals (1906), and in hundreds of articles and speeches. As Lowell put it, A mind of no mean order would seem to have presided over the system we see—a mind certainly of considerably more comprehensiveness than that which presides over the various departments of our own public works. Party politics, at all events, have had no part in them; for the system is planet-wide.... Certainly what we see hints at the existence of beings who are in advance of, not behind us, in the journey of life. Lowell’s views captured the public imagination and inspired many novels and stories, the most famous of which was H. G. Wells’ War of the Worlds (1897). In this famous “invasion” novel, the thirsty inhabitants of a dying planet Mars (based entirely on Lowell’s ideas) come to conquer Earth with advanced technology. Although the Lowell Observatory first became famous for its work on the martian canals, both Lowell and the observatory eventually turned to other projects as well. He became interested in the search for a ninth (and then undiscovered) planet in the solar system. In 1930, Pluto was found at the Lowell Observatory, and it is not a coincidence that the name selected for the new planet starts with Lowell’s initials. It was also at the Lowell Observatory that the first measurements were made of the great speed at which galaxies are moving away from us, observations that would ultimately lead to our modern view of an expanding universe. Lowell (Figure 10.5) continued to live at his observatory, marrying at age 53 and publishing extensively. He relished the debate |
his claims about Mars caused far more than the astronomers on the other side, who often complained that Lowell’s work was making planetary astronomy a less respectable field. At the same time, the public fascination with the planets fueled by Lowell’s work (and its interpreters) may, several generations later, have helped fan support for the space program and the many missions whose results grace the pages of our text. 340 Chapter 10 Earthlike Planets: Venus and Mars Figure 10.5. Percival Lowell (1855–1916). This 1914 photograph shows Percival Lowell observing Venus with his 24-inch telescope at Flagstaff, Arizona In October 1938, the Mercury Theater of the Air on radio dramatized The War of the Worlds as a series of radio news reports. This broadcast (https://openstax.org/l/30WarofWorlds) scared many people into thinking that Lowell’s Martians were really invading New Jersey, and caused something of a panic. You can listen to the original radio broadcast if you scroll down to “War of the Worlds.” Rotation of the Planets Astronomers have determined the rotation period of Mars with great accuracy by watching the motion of permanent surface markings; its sidereal day is 24 hours 37 minutes 23 seconds, just a little longer than the rotation period of Earth. This high precision is not obtained by watching Mars for a single rotation, but by noting how many turns it makes over a long period of time. Good observations of Mars date back more than 200 years, a period during which tens of thousands of martian days have passed. As a result, the rotation period can be calculated to within a few hundredths of a second. The rotational axis of Mars has a tilt of about 25°, similar to the tilt of Earth’s axis. Thus, Mars experiences seasons very much like those on Earth. Because of the longer martian year (almost two Earth years), however, each season there lasts about six of our months. The situation with Venus is different. Since no surface detail can be seen through Venus’ clouds, its rotation period can be found only by bouncing radar signals off the planet (as explained for Mercury in the Cratered Worlds chapter). The first radar observations of Venus’ rotation were made in the early 1960s. Later, topographical surface features were identified on the planet that showed up in the reflected radar signals. The rotation period of Venus, precisely determined from the motion of such “rad |
ar features” across its disk, is 243 days. Even more surprising than how long Venus takes to rotate is the fact that it spins in a backward or retrograde direction (east to west). This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 10 Earthlike Planets: Venus and Mars 341 Stop for a moment and think about how odd this slow rotation makes the calendar on Venus. The planet takes 225 Earth days to orbit the Sun and 243 Earth days to spin on its axis. So the day on Venus (as defined by its spinning once) is longer than the year! As a result, the time the Sun takes to return to the same place in Venus’ sky—another way we might define the meaning of a day—turns out to be 117 Earth days. (If you say “See you tomorrow” on Venus, you’ll have a long time to wait.) Although we do not know the reason for Venus’ slow backward rotation, we can guess that it may have suffered one or more extremely powerful collisions during the formation process of the solar system. Basic Properties of Venus and Mars Before discussing each planet individually, let us compare some of their basic properties with each other and with Earth (Table 10.1). Venus is in many ways Earth’s twin, with a mass 0.82 times the mass of Earth and an almost identical density. The average amount of geological activity has been also relatively high, almost as high as on Earth. On the other hand, with a surface pressure nearly 100 times greater than ours, Venus’ atmosphere is not at all like that of Earth. The surface of Venus is also remarkably hot, with a temperature of 730 K (over 850 °F), hotter than the self-cleaning cycle of your oven. One of the major challenges presented by Venus is to understand why the atmosphere and surface environment of this twin have diverged so sharply from those of our own planet. Properties of Earth, Venus, and Mars Property Earth Venus Mars Semimajor axis (AU) Period (year) Mass (Earth = 1) Diameter (km) Density (g/cm3) Surface gravity (Earth = 1) Escape velocity (km/s) 1.00 1.00 1.00 0.72 0.61 0.82 1.52 1.88 0.11 12,756 12,102 6,790 5.5 1.00 11 |
.2 5.3 0.91 10.4 3.9 0.38 5.0 Rotation period (hours or days) 23.9 h 243 d 24.6 h Surface area (Earth = 1) Atmospheric pressure (bar) Table 10.1 1.00 1.00 0.90 90 0.28 0.007 Mars, by contrast, is rather small, with a mass only 0.11 times the mass of Earth. It is larger than either the Moon or Mercury, however, and, unlike them, it retains a thin atmosphere. Mars is also large enough to have supported considerable geological activity in the distant past. But the most fascinating thing about Mars is that long ago it probably had a thick atmosphere and seas of liquid water—the conditions we associate with 342 Chapter 10 Earthlike Planets: Venus and Mars development of life. There is even a chance that some form of life persists today in protected environments below the martian surface. 10.2 THE GEOLOGY OF VENUS Learning Objectives By the end of this section, you will be able to: Describe the general features of the surface of Venus Explain what the study of craters on Venus tells us about the age of its surface Compare tectonic activity and volcanoes on Venus with those of Earth Explain why the surface of Venus is inhospitable to human life Since Venus has about the same size and composition as Earth, we might expect its geology to be similar. This is partly true, but Venus does not exhibit the same kind of plate tectonics as Earth, and we will see that its lack of erosion results in a very different surface appearance. Spacecraft Exploration of Venus Nearly 50 spacecraft have been launched to Venus, but only about half were successful. Although the 1962 US Mariner 2 flyby was the first, the Soviet Union launched most of the subsequent missions to Venus. In 1970, Venera 7 became the first probe to land and broadcast data from the surface of Venus. It operated for 23 minutes before succumbing to the high surface temperature. Additional Venera probes and landers followed, photographing the surface and analyzing the atmosphere and soil. To understand the geology of Venus, however, we needed to make a global study of its surface, a task made very difficult by the perpetual cloud layers surrounding the planet. The problem resembles the challenge facing air traffic controllers at an airport, when the weather is so cloudy or smoggy that they can’t locate the incoming planes visually. The solution is |
similar in both cases: use a radar instrument to probe through the obscuring layer. The first global radar map was made by the US Pioneer Venus orbiter in the late 1970s, followed by better maps from the twin Soviet Venera 15 and 16 radar orbiters in the early 1980s. However, most of our information on the geology of Venus is derived from the US Magellan spacecraft, which mapped Venus with a powerful imaging radar. Magellan produced images with a resolution of 100 meters, much better than that of previous missions, yielding our first detailed look at the surface of our sister planet (Figure 10.6). (The Magellan spacecraft returned more data to Earth than all previous planetary missions combined; each 100 minutes of data transmission from the spacecraft provided enough information, if translated into characters, to fill two 30-volume encyclopedias.) This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 10 Earthlike Planets: Venus and Mars 343 Figure 10.6. Radar Map of Venus. This composite image has a resolution of about 3 kilometers. Colors have been added to indicate elevation, with blue meaning low and brown and white high. The large continent Aphrodite stretches around the equator, where the bright (therefore rough) surface has been deformed by tectonic forces in the crust of Venus. (credit: modification of work by NASA/JPL/USGS) Consider for a moment how good Magellan’s resolution of 100 meters really is. It means the radar images from Venus can show anything on the surface larger than a football field. Suddenly, a whole host of topographic features on Venus became accessible to our view. As you look at the radar images throughout this chapter, bear in mind that these are constructed from radar reflections, not from visible-light photographs. For example, bright features on these radar images are an indication of rough terrain, whereas darker regions are smoother. Probing Through the Clouds of Venus The radar maps of Venus reveal a planet that looks much the way Earth might look if our planet’s surface were not constantly being changed by erosion and deposition of sediment. Because there is no water or ice on Venus and the surface wind speeds are low, almost nothing obscures or erases the complex geological features produced by the movements of Venus’ crust, by volcanic eruptions, and by impact craters. Having finally penetrated below the clouds of Venus, we find its surface to |
be naked, revealing the history of hundreds of millions of years of geological activity. About 75% of the surface of Venus consists of lowland lava plains. Superficially, these plains resemble the basaltic ocean basins of Earth, but they were not produced in quite the same way. There is no evidence of subduction zones on Venus, indicating that, unlike Earth, this planet never experienced plate tectonics. Although convection (the rising of hot materials) in its mantle generated great stresses in the crust of Venus, they did not start large continental plates moving. The formation of the lava plains of Venus more nearly resembles that of the lunar maria. Both were the result of widespread lava eruptions without the crustal spreading associated with plate tectonics. Rising above the lowland lava plains are two full-scale continents of mountainous terrain. The largest continent on Venus, called Aphrodite, is about the size of Africa (you can see it stand out in Figure 10.6). Aphrodite stretches along the equator for about one-third of the way around the planet. Next in size is the northern highland region Ishtar, which is about the size of Australia. Ishtar contains the highest region on the planet, the Maxwell Mountains, which rise 11 kilometers above the surrounding lowlands. (The Maxwell Mountains are the only feature on Venus named after a man. They commemorate James Clerk Maxwell, whose theory of 344 Chapter 10 Earthlike Planets: Venus and Mars electromagnetism led to the invention of radar. All other features are named for women, either from history or mythology.) Craters and the Age of the Venus Surface One of the first questions astronomers addressed with the high-resolution Magellan images was the age of the surface of Venus. Remember that the age of a planetary surface is rarely the age of the world it is on. A young age merely implies an active geology in that location. Such ages can be derived from counting impact craters. Figure 10.7 is an example of what these craters look like on the Venus radar images. The more densely cratered the surface, the greater its age. The largest crater on Venus (called Mead) is 275 kilometers in diameter, slightly larger than the largest known terrestrial crater (Chicxulub), but much smaller than the lunar impact basins. Figure 10.7. Impact Craters on Venus. (a) These large impact craters are in the Lavinia region of Venus. Because they are rough, the |
crater rims and ejecta appear brighter in these radar images than do the smoother surrounding lava plains. The largest of these craters has a diameter of 50 kilometers. (b) This small, complex crater is named after writer Gertrude Stein. The triple impact was caused by the breaking apart of the incoming asteroid during its passage through the thick atmosphere of Venus. The projectile had an initial diameter of between 1 and 2 kilometers. (credit a: modification of work by NASA/JPL; credit b: modification of work by NASA/JPL) You might think that the thick atmosphere of Venus would protect the surface from impacts, burning up the projectiles long before they could reach the surface. But this is the case for only smaller projectiles. Crater statistics show very few craters less than 10 kilometers in diameter, indicating that projectiles smaller than about 1 kilometer (the size that typically produces a 10-kilometer crater) were stopped by the atmosphere. Those craters with diameters from 10 to 30 kilometers are frequently distorted or multiple, apparently because the incoming projectile broke apart in the atmosphere before it could strike the ground as shown in the Stein crater in Figure 10.7. If we limit ourselves to impacts that produce craters with diameters of 30 kilometers or larger, however, then crater counts are as useful on Venus for measuring surface age as they are on airless bodies such as the Moon. The large craters in the venusian plains indicate an average surface age that is only between 300 and 600 million years. These results indicate that Venus is indeed a planet with persistent geological activity, intermediate between that of Earth’s ocean basins (which are younger and more active) and that of its continents (which are older and less active). This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 10 Earthlike Planets: Venus and Mars 345 Almost all of the large craters on Venus look fresh, with little degradation or filling in by either lava or windblown dust. This is one way we know that the rates of erosion or sediment deposition are very low. We have the impression that relatively little has happened since the venusian plains were last resurfaced by large-scale volcanic activity. Apparently Venus experienced some sort of planet-wide volcanic convulsion between 300 and 600 million years ago, a mysterious event that is unlike anything in terrestrial history. Volcanoes on Venus Like Earth, Venus is a planet that has experienced widespread volcanism. |
In the lowland plains, volcanic eruptions are the principal way the surface is renewed, with large flows of highly fluid lava destroying old craters and generating a fresh surface. In addition, numerous younger volcanic mountains and other structures are associated with surface hot spots—places where convection in the planet’s mantle transports the interior heat to the surface. The largest individual volcano on Venus, called Sif Mons, is about 500 kilometers across and 3 kilometers high—broader but lower than the Hawaiian volcano Mauna Loa. At its top is a volcanic crater, or caldera, about 40 kilometers across, and its slopes show individual lava flows up to 500 kilometers long. Thousands of smaller volcanoes dot the surface, down to the limit of visibility of the Magellan images, which correspond to cones or domes about the size of a shopping mall parking lot. Most of these seem similar to terrestrial volcanoes. Other volcanoes have unusual shapes, such as the “pancake domes” illustrated in Figure 10.8. Figure 10.8. Pancake-Shaped Volcanoes on Venus. These remarkable circular domes, each about 25 kilometers across and about 2 kilometers tall, are the result of eruptions of highly viscous (sludgy) lava that spreads out evenly in all directions. (credit: modification of work by NASA/JPL) All of the volcanism is the result of eruption of lava onto the surface of the planet. But the hot lava rising from the interior of a planet does not always make it to the surface. On both Earth and Venus, this upwelling lava can collect to produce bulges in the crust. Many of the granite mountain ranges on Earth, such as the Sierra Nevada in California, involve such subsurface volcanism. These bulges are common on Venus, where they produce large circular or oval features called coronae (singular: corona) (Figure 10.9). 346 Chapter 10 Earthlike Planets: Venus and Mars Figure 10.9. The “Miss Piggy” Corona. Fotla Corona is located in the plains to the south of Aphrodite Terra. Curved fracture patterns show where the material beneath has put stress on the surface. A number of pancake and dome volcanoes are also visible. Fotla was a Celtic fertility goddess. Some students see a resemblance between this corona and Miss Piggy of the Muppets (her left ear, at the top of the picture, is the |
pancake volcano in the upper center of the image). (credit: NASA/JPL) Tectonic Activity Convection currents of molten material in the mantle of Venus push and stretch the crust. Such forces are called tectonic, and the geological features that result from these forces are called tectonic features. On Venus’ lowland plains, tectonic forces have broken the lava surface to create remarkable patterns of ridges and cracks (Figure 10.10). In a few places, the crust has even torn apart to generate rift valleys. The circular features associated with coronae are tectonic ridges and cracks, and most of the mountains of Venus also owe their existence to tectonic forces. Figure 10.10. Ridges and Cracks. This region of the Lakshmi Plains on Venus has been fractured by tectonic forces to produce a cross-hatched grid of cracks and ridges. Be sure to notice the fainter linear features that run perpendicular to the brighter ones. As this is a radar image, the brightness of the ridges indicates their relative height. This image shows a region about 80 kilometers wide and 37 kilometers high. Lakshmi is a Hindu goddess of prosperity. (credit: modification of work by Magellan Team, JPL, NASA) The Ishtar continent, which has the highest elevations on Venus, is the most dramatic product of these tectonic forces. Ishtar and its tall Maxwell Mountains resemble the Tibetan Plateau and Himalayan Mountains on Earth. Both are the product of compression of the crust, and both are maintained by the continuing forces of mantle convection. This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 10 Earthlike Planets: Venus and Mars 347 On Venus’ Surface The successful Venera landers of the 1970s found themselves on an extraordinarily inhospitable planet, with a surface pressure of 90 bars and a temperature hot enough to melt lead and zinc. Despite these unpleasant conditions, the spacecraft were able to photograph their surroundings and collect surface samples for chemical analysis before their instruments gave out. The diffuse sunlight striking the surface was tinted red by the clouds, and the illumination level was equivalent to a heavy overcast on Earth. The probes found that the rock in the landing areas is igneous, primarily basalts. Examples of the Venera photographs are shown in Figure 10.11. Each picture shows a flat, desolate landscape with a variety |
of rocks, some of which may be ejecta from impacts. Other areas show flat, layered lava flows. There have been no further landings on Venus since the 1970s. Figure 10.11. Surface of Venus. These views of the surface of Venus are from the Venera 13 spacecraft. Everything is orange because the thick atmosphere of Venus absorbs the bluer colors of light. The horizon is visible in the upper corner of each image. (credit: NASA) 10.3 THE MASSIVE ATMOSPHERE OF VENUS Learning Objectives By the end of this section, you will be able to: Describe the general composition and structure of the atmosphere on Venus Explain how the greenhouse effect has led to high temperatures on Venus The thick atmosphere of Venus produces the high surface temperature and shrouds the surface in a perpetual red twilight. Sunlight does not penetrate directly through the heavy clouds, but the surface is fairly well lit by diffuse light (about the same as the light on Earth under a heavy overcast). The weather at the bottom of this deep atmosphere remains perpetually hot and dry, with calm winds. Because of the heavy blanket of clouds and atmosphere, one spot on the surface of Venus is similar to any other as far as weather is concerned. Composition and Structure of the Atmosphere The most abundant gas on Venus is carbon dioxide (CO2), which accounts for 96% of the atmosphere. The second most common gas is nitrogen. The predominance of carbon dioxide over nitrogen is not surprising when you recall that Earth’s atmosphere would also be mostly carbon dioxide if this gas were not locked up in marine sediments (see the discussion of Earth’s atmosphere in Earth as a Planet). Table 10.2 compares the compositions of the atmospheres of Venus, Mars, and Earth. Expressed in this way, as percentages, the proportions of the major gases are very similar for Venus and Mars, but in total quantity, their atmospheres are dramatically different. With its surface pressure of 90 bars, the venusian atmosphere is more than 10,000 times more massive than its martian counterpart. Overall, the atmosphere of Venus is very dry; the absence of water is one of the important ways that Venus differs from Earth. 348 Chapter 10 Earthlike Planets: Venus and Mars Atmospheric Composition of Earth, Venus, and Mars Gas Earth Venus Mars Carbon dioxide (CO2) Nitrogen (N2) Argon (Ar) Oxygen (O2) Neon (Ne) Table 10.2 0. |
03% 78.1% 0.93% 21.0% 0.002% 96% 3.5% 0.006% 0.003% 0.001% 95.3% 2.7% 1.6% 0.15% 0.0003% The atmosphere of Venus has a huge troposphere (region of convection) that extends up to at least 50 kilometers above the surface (Figure 10.12). Within the troposphere, the gas is heated from below and circulates slowly, rising near the equator and descending over the poles. Being at the base of the atmosphere of Venus is something like being a kilometer or more below the ocean surface on Earth. There, the mass of water evens out temperature variations and results in a uniform environment—the same effect the thick atmosphere has on Venus. Figure 10.12. Venus’ Atmosphere. The layers of the massive atmosphere of Venus shown here are based on data from the Pioneer and Venera entry probes. Height is measured along the left axis, the bottom scale shows temperature, and the red line allows you to read off the temperature at each height. Notice how steeply the temperature rises below the clouds, thanks to the planet’s huge greenhouse effect. In the upper troposphere, between 30 and 60 kilometers above the surface, a thick cloud layer is composed primarily of sulfuric acid droplets. Sulfuric acid (H2SO4) is formed from the chemical combination of sulfur dioxide (SO2) and water (H2O). In the atmosphere of Earth, sulfur dioxide is one of the primary gases emitted by volcanoes, but it is quickly diluted and washed out by rainfall. In the dry atmosphere of Venus, this unpleasant substance is apparently stable. Below 30 kilometers, the Venus atmosphere is clear of clouds. This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 10 Earthlike Planets: Venus and Mars 349 Surface Temperature on Venus The high surface temperature of Venus was discovered by radio astronomers in the late 1950s and confirmed by the Mariner and Venera probes. How can our neighbor planet be so hot? Although Venus is somewhat closer to the Sun than is Earth, its surface is hundreds of degrees hotter than you would expect from the extra sunlight it receives. Scientists wondered what could be heating the surface of Venus to a temperature above 700 K. The answer turned out to be the greenhouse effect. The greenhouse effect works on |
Venus just as it does on Earth, but since Venus has so much more CO2—almost a million times more—the effect is much stronger. The thick CO2 acts as a blanket, making it very difficult for the infrared (heat) radiation from the ground to get back into space. As a result, the surface heats up. The energy balance is only restored when the planet is radiating as much energy as it receives from the Sun, but this can happen only when the temperature of the lower atmosphere is very high. One way of thinking of greenhouse heating is that it must raise the surface temperature of Venus until this energy balance is achieved. Has Venus always had such a massive atmosphere and high surface temperature, or might it have evolved to such conditions from a climate that was once more nearly earthlike? The answer to this question is of particular interest to us as we look at the increasing levels of CO2 in Earth’s atmosphere. As the greenhouse effect becomes stronger on Earth, are we in any danger of transforming our own planet into a hellish place like Venus? Let us try to reconstruct the possible evolution of Venus from an earthlike beginning to its present state. Venus may once have had a climate similar to that of Earth, with moderate temperatures, water oceans, and much of its CO2 dissolved in the ocean or chemically combined with the surface rocks. Then we allow for modest additional heating—by gradual increase in the energy output of the Sun, for example. When we calculate how Venus’ atmosphere would respond to such effects, it turns out that even a small amount of extra heat can lead to increased evaporation of water from the oceans and the release of gas from surface rocks. This in turn means a further increase in the atmospheric CO2 and H2O, gases that would amplify the greenhouse effect in Venus’ atmosphere. That would lead to still more heat near Venus’ surface and the release of further CO2 and H2O. Unless some other processes intervene, the temperature thus continues to rise. Such a situation is called the runaway greenhouse effect. We want to emphasize that the runaway greenhouse effect is not just a large greenhouse effect; it is an evolutionary process. The atmosphere evolves from having a small greenhouse effect, such as on Earth, to a situation where greenhouse warming is a major factor, as we see today on Venus. Once the large greenhouse conditions develop, the planet establishes a new, much hotter equilibrium near its surface. Reversing the situation is difficult because of the role water plays. On |
Earth, most of the CO2 is either chemically bound in the rocks of our crust or dissolved by the water in our oceans. As Venus got hotter and hotter, its oceans evaporated, eliminating that safety valve. But the water vapor in the planet’s atmosphere will not last forever in the presence of ultraviolet light from the Sun. The light element hydrogen can escape from the atmosphere, leaving the oxygen behind to combine chemically with surface rock. The loss of water is therefore an irreversible process: once the water is gone, it cannot be restored. There is evidence that this is just what happened to the water once present on Venus. We don’t know if the same runaway greenhouse effect could one day happen on Earth. Although we are uncertain about the point at which a stable greenhouse effect breaks down and turns into a runaway greenhouse effect, Venus stands as clear testament to the fact that a planet cannot continue heating indefinitely without a major change in its oceans and atmosphere. It is a conclusion that we and our descendants will surely want to pay close attention to. 350 Chapter 10 Earthlike Planets: Venus and Mars 10.4 THE GEOLOGY OF MARS Learning Objectives By the end of this section, you will be able to: Discuss the main missions that have explored Mars Explain what we have learned from examination of meteorites from Mars Describe the various features found on the surface of Mars Compare the volcanoes and canyons on Mars with those of Earth Describe the general conditions on the surface of Mars Mars is more interesting to most people than Venus because it is more hospitable. Even from the distance of Earth, we can see surface features on Mars and follow the seasonal changes in its polar caps (Figure 10.13). Although the surface today is dry and cold, evidence collected by spacecraft suggests that Mars once had blue skies and lakes of liquid water. Even today, it is the sort of place we can imagine astronauts visiting and perhaps even setting up permanent bases. Figure 10.13. Mars Photographed by the Hubble Space Telescope. This is one of the best photos of Mars taken from our planet, obtained in June 2001 when Mars was only 68 million kilometers away. The resolution is about 20 kilometers—much better than can be obtained with ground-based telescopes but still insufficient to reveal the underlying geology of Mars. (credit: modification of work by NASA and the Hubble Heritage Team (STScI/AURA)) Spacecraft Exploration of Mars Mars has been intensively investigated by spacecraft. More than 50 spacecraft have been launched |
toward Mars, but only about half were fully successful. The first visitor was the US Mariner 4, which flew past Mars in 1965 and transmitted 22 photos to Earth. These pictures showed an apparently bleak planet with abundant impact craters. In those days, craters were unexpected; some people who were romantically inclined still hoped to see canals or something like them. In any case, newspaper headlines sadly announced that Mars was a “dead planet.” This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 10 Earthlike Planets: Venus and Mars 351 In 1971, NASA’s Mariner 9 became the first spacecraft to orbit another planet, mapping the entire surface of Mars at a resolution of about 1 kilometer and discovering a great variety of geological features, including volcanoes, huge canyons, intricate layers on the polar caps, and channels that appeared to have been cut by running water. Geologically, Mars didn’t look so dead after all. The twin Viking spacecraft of the 1970s were among the most ambitious and successful of all planetary missions. Two orbiters surveyed the planet and served to relay communications for two landers on the surface. After an exciting and sometimes frustrating search for a safe landing spot, the Viking 1 lander touched down on the surface of Chryse Planitia (the Plains of Gold) on July 20, 1976, exactly 7 years after Neil Armstrong’s historic first step on the Moon. Two months later, Viking 2 landed with equal success in another plain farther north, called Utopia. The landers photographed the surface with high resolution and carried out complex experiments searching for evidence of life, while the orbiters provided a global perspective on Mars geology. Mars languished unvisited for two decades after Viking. Two more spacecraft were launched toward Mars, by NASA and the Russian Space Agency, but both failed before reaching the planet. The situation changed in the 1990s as NASA began a new exploration program using spacecraft that were smaller and less expensive than Viking. The first of the new missions, appropriately called Pathfinder, landed the first wheeled, solar-powered rover on the martian surface on July 4, 1997 (Figure 10.14). An orbiter called Mars Global Surveyor (MGS) arrived a few months later and began high-resolution photography of the entire surface over more than one martian year. The most dramatic discovery by this spacecraft, which is still operating, was evidence |
of gullies apparently cut by surface water, as we will discuss later. These missions were followed in 2003 by the NASA Mars Odyssey orbiter, and the ESA Mars Express orbiter, both carrying high-resolution cameras. A gamma-ray spectrometer on Odyssey discovered a large amount of subsurface hydrogen (probably in the form of frozen water). Subsequent orbiters included the NASA Mars Reconnaissance Orbiter to evaluate future landing sites, MAVEN to study the upper atmosphere, and India’s Mangalayaan, also focused on study of Mars’ thin layers of air. Several of these orbiters are also equipped to communicate with landers and rovers on the surface and serve as data relays to Earth. Figure 10.14. Surface View from Mars Pathfinder. The scene from the Pathfinder lander shows a windswept plain, sculpted long ago when water flowed out of the martian highlands and into the depression where the spacecraft landed. The Sojourner rover, the first wheeled vehicle on Mars, is about the size of a microwave oven. Its flat top contains solar cells that provided electricity to run the vehicle. You can see the ramp from the lander and the path the rover took to the larger rock that the mission team nicknamed “Yogi.” (credit: NASA/JPL) In 2003, NASA began a series of highly successful Mars landers. Twin Mars Exploration Rovers (MER), named Spirit and Opportunity, have been successful far beyond their planned lifetimes. The design goal for the rovers was 600 meters of travel; in fact, they have traveled jointly more than 50 kilometers. After scouting around its 352 Chapter 10 Earthlike Planets: Venus and Mars rim, Opportunity drove down the steep walls into an impact crater called Victoria, then succeeded with some difficulty in climbing back out to resume its route (Figure 10.15). Dust covering the rovers’ solar cells caused a drop in power, but when a seasonal dust storm blew away the dust, the rovers resumed full operation. In order to survive winter, the rovers were positioned on slopes to maximize solar heating and power generation. In 2006, Spirit lost power on one of its wheels, and subsequently became stuck in the sand, where it continued operation as a fixed ground station. Meanwhile, in 2008, Phoenix (a spacecraft “reborn” of spare parts from a previous Mars mission that had failed) landed near the edge of the north polar cap, at latitude 68°, and |
directly measured water ice in the soil. Figure 10.15. Victoria Crater. (a) This crater in Meridiani Planum is 800 meters wide, making it slightly smaller than Meteor crater on Earth. Note the dune field in the interior. (b) This image shows the view from the Opportunity rover as it scouted the rim of Victoria crater looking for a safe route down into the interior. (credit a: modification of work by NASA/JPL-Caltech/University of Arizona/Cornell/Phio State University; credit b: modification of work by NASA/JPL/Cornell) In 2011, NASA launched its largest (and most expensive) Mars mission since Viking (see Figure 10.1). The 1-ton rover Curiosity, the size of a subcompact car, has plutonium-powered electrical generators, so that it is not dependent on sunlight for power. Curiosity made a pinpoint landing on the floor of Gale crater, a site selected for its complex geology and evidence that it had been submerged by water in the past. Previously, Mars landers had been sent to flat terrains with few hazards, as required by their lower targeting accuracy. The scientific goals of Curiosity include investigations of climate and geology, and assessment of the habitability of past and present Mars environments. It does not carry a specific life detection instrument, however. So far, scientists have not been able to devise a simple instrument that could distinguish living from nonliving materials on Mars The Curiosity rover required a remarkably complex landing sequence and NASA made a video (https://openstax.org/l/30Curiosityrove) about it called “7 Minutes of Terror” that went viral on the Internet. A dramatic video summary (https://openstax.org/l/30MarsSurface) of the first two years of Curiosity’s exploration of the martian surface can be viewed as well. This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 10 Earthlike Planets: Venus and Mars 353 Martian Samples Much of what we know of the Moon, including the circumstances of its origin, comes from studies of lunar samples, but spacecraft have not yet returned martian samples to Earth for laboratory analysis. It is with great interest, therefore, that scientists have discovered that samples of martian material are nevertheless already here on Earth, available for study. These are all members of a rare class of meteorites (Figure |
10.16)—rocks that have fallen from space. Figure 10.16. Martian Meteorite. This fragment of basalt, ejected from Mars in a crater-forming impact, eventually arrived on Earth’s surface. (credit: NASA) How would rocks have escaped from Mars? Many impacts have occurred on the red planet, as shown by its heavily cratered surface. Fragments blasted from large impacts can escape from Mars, whose surface gravity is only 38% of Earth’s. A long time later (typically a few million years), a very small fraction of these fragments collide with Earth and survive their passage through our atmosphere, just like other meteorites. (We’ll discuss meteorites in more detail in the chapter on Cosmic Samples and the Origin of the Solar System.) By the way, rocks from the Moon have also reached our planet as meteorites, although we were able to demonstrate their lunar origin only by comparison with samples returned by the Apollo missions Most of the martian meteorites are volcanic basalts; most of them are also relatively young—about 1.3 billion years old. We know from details of their composition that they are not from Earth or the Moon. Besides, there was no volcanic activity on the Moon to form them as recently as 1.3 billon years ago. It would be very difficult for ejecta from impacts on Venus to escape through its thick atmosphere. By the process of elimination, the only reasonable origin seems to be Mars, where the Tharsis volcanoes were active at that time. The martian origin of these meteorites was confirmed by the analysis of tiny gas bubbles trapped inside several of them. These bubbles match the atmospheric properties of Mars as first measured directly by Viking. It appears that some atmospheric gas was trapped in the rock by the shock of the impact that ejected it from Mars and started it on its way toward Earth. One of the most exciting results from analysis of these martian samples has been the discovery of both water and organic (carbon-based) compounds in them, which suggests that Mars may once have had oceans and perhaps even life on its surface. As we have already hinted, there is other evidence for the presence of flowing water on Mars in the remote past, and even extending to the present. In this and the following sections, we will summarize the picture of Mars as revealed by all these exploratory missions and by about 40 samples from Mars. Global Properties of Mars Mars has a diameter of 6790 kilometers, just over half the diameter of Earth |
, giving it a total surface area very nearly equal to the continental (land) area of our planet. Its overall density of 3.9 g/cm3 suggests a composition consisting primarily of silicates but with a small metal core. The planet has no global magnetic field, although 354 Chapter 10 Earthlike Planets: Venus and Mars there are areas of strong surface magnetization that indicate that there was a global field billions of years ago. Apparently, the red planet has no liquid material in its core today that would conduct electricity. Thanks to the Mars Global Surveyor, we have mapped the entire planet, as shown in Figure 10.17. A laser altimeter on board made millions of separate measurements of the surface topography to a precision of a few meters—good enough to show even the annual deposition and evaporation of the polar caps. Like Earth, the Moon, and Venus, the surface of Mars has continental or highland areas as well as widespread volcanic plains. The total range in elevation from the top of the highest mountain ( Olympus Mons) to the bottom of the deepest basin (Hellas) is 31 kilometers. Figure 10.17. Mars Map from Laser Ranging. These globes are highly precise topographic maps, reconstructed from millions of individual elevation measurements made with the Mars Global Surveyor. Color is used to indicate elevation. The hemisphere on the left includes the Tharsis bulge and Olympus Mons, the highest mountain on Mars; the hemisphere on the right includes the Hellas basin, which has the lowest elevation on Mars. (credit: modification of work by NASA/JPL) Approximately half the planet consists of heavily cratered highland terrain, found primarily in the southern hemisphere. The other half, which is mostly in the north, contains younger, lightly cratered volcanic plains at an average elevation about 5 kilometers lower than the highlands. Remember that we saw a similar pattern on Earth, the Moon, and Venus. A geological division into older highlands and younger lowland plains seems to be characteristic of all the terrestrial planets except Mercury. Lying across the north-south division of Mars is an uplifted continent the size of North America. This is the 10-kilometer-high Tharsis bulge, a volcanic region crowned by four great volcanoes that rise still higher into the martian sky. Volcanoes on Mars The lowland plains of Mars look very much like the lunar maria, and they have about the same density of impact craters. Like the lunar maria, they probably |
formed between 3 and 4 billion years ago. Apparently, Mars experienced extensive volcanic activity at about the same time the Moon did, producing similar basaltic lavas. The largest volcanic mountains of Mars are found in the Tharsis area (you can see them in Figure 10.17), although smaller volcanoes dot much of the surface. The most dramatic volcano on Mars is Olympus Mons (Mount Olympus), with a diameter larger than 500 kilometers and a summit that towers more than 20 This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 10 Earthlike Planets: Venus and Mars 355 kilometers above the surrounding plains—three times higher than the tallest mountain on Earth (Figure 10.18). The volume of this immense volcano is nearly 100 times greater than that of Mauna Loa in Hawaii. Placed on Earth’s surface, Olympus would more than cover the entire state of Missouri. Figure 10.18. Olympus Mons. The largest volcano on Mars, and probably the largest in the solar system, is Olympus Mons, illustrated in this computer-generated rendering based on data from the Mars Global Surveyor’s laser altimeter. Placed on Earth, the base of Olympus Mons would completely cover the state of Missouri; the caldera, the circular opening at the top, is 65 kilometers across, about the size of Los Angeles. (credit: NASA/Corbis) Images taken from orbit allow scientists to search for impact craters on the slopes of these volcanoes in order to estimate their age. Many of the volcanoes show a fair number of such craters, suggesting that they ceased activity a billion years or more ago. However, Olympus Mons has very, very few impact craters. Its present surface cannot be more than about 100 million years old; it may even be much younger. Some of the fresh- looking lava flows might have been formed a hundred years ago, or a thousand, or a million, but geologically speaking, they are quite young. This leads geologists to the conclusion that Olympus Mons possibly remains intermittently active today—something future Mars land developers may want to keep in mind. Martian Cracks and Canyons The Tharsis bulge has many interesting geological features in addition to its huge volcanoes. In this part of the planet, the surface itself has bulged upward, forced by great pressures from below, resulting in extensive tectonic cracking of the crust. Among the most spectacular tectonic features |
on Mars are the canyons called the Valles Marineris (or Mariner Valleys, named after Mariner 9, which first revealed them to us), which are shown in Figure 10.19. They extend for about 5000 kilometers (nearly a quarter of the way around Mars) along the slopes of the Tharsis bulge. If it were on Earth, this canyon system would stretch all the way from Los Angeles to Washington, DC. The main canyon is about 7 kilometers deep and up to 100 kilometers wide, large enough for the Grand Canyon of the Colorado River to fit comfortably into one of its side canyons. 356 Chapter 10 Earthlike Planets: Venus and Mars Figure 10.19. Heavily Eroded Canyonlands on Mars. This image shows the Valles Marineris canyon complex, which is 3000 kilometers wide and 8 kilometers deep. (credit: NASA/JPL/USGS An excellent 4-minute video tour (https://openstax.org/l/30VallesMariner) of Valles Marineris, narrated by planetary scientist Phil Christensen, is available for viewing. The term “canyon” is somewhat misleading here because the Valles Marineris canyons have no outlets and were not cut by running water. They are basically tectonic cracks, produced by the same crustal tensions that caused the Tharsis uplift. However, water has played a later role in shaping the canyons, primarily by seeping from deep springs and undercutting the cliffs. This undercutting led to landslides that gradually widened the original cracks into the great valleys we see today (Figure 10.20). Today, the primary form of erosion in the canyons is probably wind. This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 10 Earthlike Planets: Venus and Mars 357 Figure 10.20. Martian Landslides. This Viking orbiter image shows Ophir Chasma, one of the connected valleys of the Valles Marineris canyon system. Look carefully and you can see enormous landslides whose debris is piled up underneath the cliff wall, which tower up to 10 kilometers above the canyon floor. (credit: modification of work by NASA/JPL/USGS) While the Tharsis bulge and Valles Marineris are impressive, in general, we see fewer tectonic structures on Mars than on Venus. |
In part, this may reflect a lower general level of geological activity, as would be expected for a smaller planet. But it is also possible that evidence of widespread faulting has been buried by wind- deposited sediment over much of Mars. Like Earth, Mars may have hidden part of its geological history under a cloak of soil. The View on the Martian Surface The first spacecraft to land successfully on Mars were Vikings 1 and 2 and Mars Pathfinder. All sent back photos that showed a desolate but strangely beautiful landscape, including numerous angular rocks interspersed with dune like deposits of fine-grained, reddish soil (Figure 10.21). Figure 10.21. Three Martian Landing Sites. The Mars landers Viking 1 in Chryse, Pathfinder in Ares Valley, and Viking 2 in Utopia, all photographed their immediate surroundings. It is apparent from the similarity of these three photos that each spacecraft touched down on a flat, windswept plain littered with rocks ranging from tiny pebbles up to meter-size boulders. It is probable that most of Mars looks like this on the surface. (credit “Viking 1”: modification of work by Van der Hoorn/NASA; credit “Pathfinder”: modification of work by NASA; credit “Viking 2”: modification of work by NASA; credit Mars: modification of work by NASA/Goddard Space Flight Center) All three of these landers were targeted to relatively flat, lowland terrain. Instruments on the landers found that the soil consisted of clays and iron oxides, as had long been expected from the red color of the planet. All 358 Chapter 10 Earthlike Planets: Venus and Mars the rocks measured appeared to be of volcanic origin and roughly the same composition. Later landers were targeted to touch down in areas that apparently were flooded sometime in the past, where sedimentary rock layers, formed in the presence of water, are common. (Although we should note that nearly all the planet is blanketed in at least a thin layer of wind-blown dust). The Viking landers included weather stations that operated for several years, providing a perspective on martian weather. The temperatures they measured varied greatly with the seasons, due to the absence of moderating oceans and clouds. Typically, the summer maximum at Viking 1 was 240 K (–33 °C), dropping to 190 K (–83 °C) at the same location just before dawn. The lowest air temperatures, measured farther north by Viking 2, |
were about 173 K (–100 °C). During the winter, Viking 2 also photographed water frost deposits on the ground (Figure 10.22). We make a point of saying “water frost” here because at some locations on Mars, it gets cold enough for carbon dioxide (dry ice) to freeze out of the atmosphere as well. Figure 10.22. Water Frost in Utopia. This image of surface frost was photographed at the Viking 2 landing site during late winter. (credit: NASA/JPL) Most of the winds measured on Mars are only a few kilometers per hour. However, Mars is capable of great windstorms that can shroud the entire planet with windblown dust. Such high winds can strip the surface of some of its loose, fine dust, leaving the rock exposed. The later rovers found that each sunny afternoon the atmosphere became turbulent as heat rose off the surface. This turbulence generated dust devils, which play an important role in lifting the fine dust into the atmosphere. As the dust devils strip off the top layer of light dust and expose darker material underneath, they can produce fantastic patterns on the ground (Figure 10.23). Wind on Mars plays an important role in redistributing surface material. Figure 10.23 shows a beautiful area of dark sand dunes on top of lighter material. Much of the material stripped out of the martian canyons has been dumped in extensive dune fields like this, mostly at high latitudes. This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 10 Earthlike Planets: Venus and Mars 359 Figure 10.23. Dust Devil Tracks and Sand Dunes. (a) This high-resolution photo from the Mars Global Surveyor shows the dark tracks of several dust devils that have stripped away a thin coating of light-colored dust. This view is of an area about 3 kilometers across. Dust devils are one of the most important ways that dust gets redistributed by the martian winds. They may also help keep the solar panels of our rovers free of dust. (b) These windblown sand dunes on Mars overlay a lighter sandy surface. Each dune in this high-resolution view is about 1 kilometer across. (credit a: modification of work by NASA/JPL/University of Arizona; credit b: modification of work by NASA/JPL-Caltech/University of Arizona) 10.5 WATER AND LIFE ON MARS Learning Objectives By |
the end of this section, you will be able to: Describe the general composition of the atmosphere on Mars Explain what we know about the polar ice caps on Mars and how we know it Describe the evidence for the presence of water in the past history of Mars Summarize the evidence for and against the possibility of life on Mars Of all the planets and moons in the solar system, Mars seems to be the most promising place to look for life, both fossil microbes and (we hope) some forms of life deeper underground that still survive today. But where (and how) should we look for life? We know that the one requirement shared by all life on Earth is liquid water. Therefore, the guiding principle in assessing habitability on Mars and elsewhere has been to “follow the water.” That is the perspective we take in this section, to follow the water on the red planet and hope it will lead us to life. Atmosphere and Clouds on Mars The atmosphere of Mars today has an average surface pressure of only 0.007 bar, less than 1% that of Earth. (This is how thin the air is about 30 kilometers above Earth’s surface.) Martian air is composed primarily of carbon dioxide (95%), with about 3% nitrogen and 2% argon. The proportions of different gases are similar to those in the atmosphere of Venus (see Table 10.2), but a lot less of each gas is found in the thin air on Mars. While winds on Mars can reach high speeds, they exert much less force than wind of the same velocity would on Earth because the atmosphere is so thin. The wind is able, however, to loft very fine dust particles, which can sometimes develop planet-wide dust storms. It is this fine dust that coats almost all the surface, giving Mars its distinctive red color. In the absence of surface water, wind erosion plays a major role in sculpting the martian surface (Figure 10.24). 360 Chapter 10 Earthlike Planets: Venus and Mars Figure 10.24. Wind Erosion on Mars. These long straight ridges, called yardangs, are aligned with the dominant wind direction. This is a highresolution image from the Mars Reconnaissance Orbiter and is about 1 kilometer wide. (credit: NASA/JPL-Caltech/University of Arizona The issue of how strong the winds on Mars can be plays a big role in the 2015 hit movie The Martian (https://openstax.org/l/30TheMartian) in |
which the main character is stranded on Mars after being buried in the sand in a windstorm so great that his fellow astronauts have to leave the planet so their ship is not damaged. Astronomers have noted that the martian winds could not possibly be as forceful as depicted in the film. In most ways, however, the depiction of Mars in this movie is remarkably accurate. Although the atmosphere contains small amounts of water vapor and occasional clouds of water ice, liquid water is not stable under present conditions on Mars. Part of the problem is the low temperatures on the planet. But even if the temperature on a sunny summer day rises above the freezing point, the low pressure means that liquid water still cannot exist on the surface, except at the lowest elevations. At a pressure of less than 0.006 bar, the boiling point is as low or lower than the freezing point, and water changes directly from solid to vapor without an intermediate liquid state (as does “dry ice,” carbon dioxide, on Earth). However, salts dissolved in water lower its freezing point, as we know from the way salt is used to thaw roads after snow and ice forms during winter on Earth. Salty water is therefore sometimes able to exist in liquid form on the martian surface, under the right conditions. Several types of clouds can form in the martian atmosphere. First there are dust clouds, discussed above. Second are water-ice clouds similar to those on Earth. These often form around mountains, just as happens on our planet. Finally, the CO2 of the atmosphere can itself condense at high altitudes to form hazes of dry ice crystals. The CO2 clouds have no counterpart on Earth, since on our planet temperatures never drop low enough (down to about 150 K or about 125 °C) for this gas to condense. The Polar Caps Through a telescope, the most prominent surface features on Mars are the bright polar caps, which change with the seasons, similar to the seasonal snow cover on Earth. We do not usually think of the winter snow in northern latitudes as a part of our polar caps, but seen from space, the thin winter snow merges with Earth’s thick, permanent ice caps to create an impression much like that seen on Mars (Figure 10.25). This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 10 Earthlike Planets: Venus and Mars 361 Figure 10.25. Martian North Polar Cap. (a |
) This is a composite image of the north pole in summer, obtained in October 2006 by the Mars Reconnaissance Orbiter. It shows the mostly water-ice residual cap sitting atop light, tan-colored, layered sediments. Note that although the border of this photo is circular, it shows only a small part of the planet. (b) Here we see a small section of the layered terrain near the martian north pole. There is a mound about 40 meters high that is sticking out of a trough in the center of the picture. (credit a: modification of work by NASA/JPL/MSSS; credit b: modification of work by NASA/JPL-Caltech/University of Arizona) The seasonal caps on Mars are composed not of ordinary snow but of frozen CO2 (dry ice). These deposits condense directly from the atmosphere when the surface temperature drops below about 150 K. The caps develop during the cold martian winters and extend down to about 50° latitude by the start of spring. Quite distinct from these thin seasonal caps of CO2 are the permanent or residual caps that are always present near the poles. The southern permanent cap has a diameter of 350 kilometers and is composed of frozen CO2 deposits together with a great deal of water ice. Throughout the southern summer, it remains at the freezing point of CO2, 150 K, and this cold reservoir is thick enough to survive the summer heat intact. The northern permanent cap is different. It is much larger, never shrinking to a diameter less than 1000 kilometers, and is composed of water ice. Summer temperatures in the north are too high for the frozen CO2 to be retained. Measurements from the Mars Global Surveyor have established the exact elevations in the north polar region of Mars, showing that it is a large basin about the size of our own Arctic Ocean basin. The ice cap itself is about 3 kilometers thick, with a total volume of about 10 million km3 (similar to that of Earth’s Mediterranean Sea). If Mars ever had extensive liquid water, this north polar basin would have contained a shallow sea. There is some indication of ancient shorelines visible, but better images will be required to verify this suggestion. Images taken from orbit also show a distinctive type of terrain surrounding the permanent polar caps, as shown in Figure 10.25. At latitudes above 80° in both hemispheres, the surface consists of recent layered deposits that cover the older cratered ground below. Individual layers are typically ten to a few tens of meters thick |
, marked by alternating light and dark bands of sediment. Probably the material in the polar deposits includes dust carried by wind from the equatorial regions of Mars. What do these terraced layers tell us about Mars? Some cyclic process is depositing dust and ice over periods of time. The time scales represented by the polar layers are tens of thousands of years. Apparently the martian climate experiences periodic changes at intervals similar to those between ice ages on Earth. Calculations indicate that the causes are probably also similar: the gravitational pull of the other planets produces variations in Mars’ orbit and tilt as the great clockwork of the solar system goes through its paces. 362 Chapter 10 Earthlike Planets: Venus and Mars The Phoenix spacecraft landed near the north polar cap in summer (Figure 10.26). Controllers knew that is would not be able to survive a polar winter, but directly measuring the characteristics of the polar region was deemed important enough to send a dedicated mission. The most exciting discovery came when the spacecraft tried to dig a shallow trench under the spacecraft. When the overlying dust was stripped off, they saw bright white material, apparently some kind of ice. From the way this ice sublimated over the next few days, it was clear that it was frozen water. Figure 10.26. Evaporating Ice on Mars. We see a trench dug by the Phoenix lander in the north polar region four martian days apart in June 2008. If you look at the shadowed region in the bottom left of the trench, you can see three spots of ice in the left image which have sublimated away in the right image. (credit: modification of work by NASA/JPL-Caltech/University of Arizona/Texas A&M University Comparing the Amount of Water on Mars and Earth It is interesting to estimate the amount of water (in the form of ice) on Mars and to compare this with the amount of water on Earth. In each case, we can find the total volume of a layer on a sphere by multiplying the area of the sphere (4πR2) by the thickness of the layer. For Earth, the ocean water is equivalent to a layer 3 km thick spread over the entire planet, and the radius of Earth is 6.378 × 106 m (see Appendix F). For Mars, most of the water we are sure of is in the form of ice near the poles. We can calculate the amount of ice in one of the residual polar caps if it is (for example) |
2 km thick and has a radius of 400 km (the area of a circle is πR2). Solution The volume of Earth’s water is therefore the area 4πR2 ⎞ ⎛ ⎝6.378 × 106 m 4π ⎠ 2 = 5.1 × 1014 m2 This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 10 Earthlike Planets: Venus and Mars 363 multiplied by the thickness of 3000 m: 5.1 × 1014 m2 × 3000 m = 1.5 × 1018 m3 This gives 1.5 × 1018 m3 of water. Since water has a density of 1 ton per cubic meter (1000 kg/m3), we can calculate the mass: For Mars, the ice doesn’t cover the whole planet, only the caps; the polar cap area is 1.5 × 1018 m3 × 1 ton/m3 = 1.5 × 1018 tons πR2 = π(4 × 105 m)2 = 5 × 1011 m2 (Note that we converted kilometers to meters.) The volume = area × height, so we have: (2 × 103 m)(5 × 1011 m2) = 1 × 1015 m3 = 1015 m3 Therefore, the mass is: 1015 m3 × 1 ton/m3 = 1015 tons This is about 0.1% that of Earth’s oceans. Check Your Learning A better comparison might be to compare the amount of ice in the Mars polar ice caps to the amount of ice in the Greenland ice sheet on Earth, which has been estimated as 2.85 × 1015 m3. How does this compare with the ice on Mars? Answer: The Greenland ice sheet has about 2.85 times as much ice as in the polar ice caps on Mars. They are about the same to the nearest power of 10. Channels and Gullies on Mars Although no bodies of liquid water exist on Mars today, evidence has accumulated that rivers flowed on the red planet long ago. Two kinds of geological features appear to be remnants of ancient watercourses, while a third class—smaller gullies—suggests intermittent outbreaks of liquid water even today. We will examine each of these features in turn. In the highland equatorial plains, there are multitudes of small, sinuous (twisting) channels—typically |
a few meters deep, some tens of meters wide, and perhaps 10 or 20 kilometers long (Figure 10.27). They are called runoff channels because they look like what geologists would expect from the surface runoff of ancient rain storms. These runoff channels seem to be telling us that the planet had a very different climate long ago. To estimate the age of these channels, we look at the cratering record. Crater counts show that this part of the planet is more cratered than the lunar maria but less cratered than the lunar highlands. Thus, the runoff channels are probably older than the lunar maria, presumably about 4 billion years old. The second set of water-related features we see are outflow channels (Figure 10.27) are much larger than the runoff channels. The largest of these, which drain into the Chryse basin where Pathfinder landed, are 10 kilometers or more wide and hundreds of kilometers long. Many features of these outflow channels have 364 Chapter 10 Earthlike Planets: Venus and Mars convinced geologists that they were carved by huge volumes of running water, far too great to be produced by ordinary rainfall. Where could such floodwater have come from on Mars? Figure 10.27. Runoff and Outflow Channels. (a) These runoff channels in the old martian highlands are interpreted as the valleys of ancient rivers fed by either rain or underground springs. The width of this image is about 200 kilometers. (b) This intriguing channel, called Nanedi Valles, resembles Earth riverbeds in some (but not all) ways. The tight curves and terraces seen in the channel certainly suggest the sustained flow of a fluid like water. The channel is about 2.5 kilometers across. (credit a: modification of work by Jim Secosky/NASA; credit b: modification of work by Jim Secosky/NASA) As far we can tell, the regions where the outflow channels originate contained abundant water frozen in the soil as permafrost. Some local source of heating must have released this water, leading to a period of rapid and catastrophic flooding. Perhaps this heating was associated with the formation of the volcanic plains on Mars, which date back to roughly the same time as the outflow channels. Note that neither the runoff channels nor the outflow channels are wide enough to be visible from Earth, nor do they follow straight lines. They could not have been the “canals” Percival Lowell imagined seeing on the red planet. The third type |
of water feature, the smaller gullies, was discovered by the Mars Global Surveyor (Figure 10.28). The Mars Global Surveyor’s camera images achieved a resolution of a few meters, good enough to see something as small as a truck or bus on the surface. On the steep walls of valleys and craters at high latitudes, there are many erosional features that look like gullies carved by flowing water. These gullies are very young: not only are there no superimposed impact craters, but in some instances, the gullies seem to cut across recent wind- deposited dunes. Perhaps there is liquid water underground that can occasionally break out to produce short- lived surface flows before the water can freeze or evaporate. This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 10 Earthlike Planets: Venus and Mars 365 Figure 10.28. Gullies on the Wall of Garni Crater. This high-resolution image is from the Mars Reconnaissance Orbiter. The dark streaks, which are each several hundred meters long, change in a seasonal pattern that suggests they are caused by the temporary flow of surface water. (credit: NASA/JPL-Caltech/University of Arizona) The gullies also have the remarkable property of changing regularly with the martian seasons. Many of the dark streaks (visible in Figure 10.28) elongate within a period of a few days, indicating that something is flowing downhill—either water or dark sediment. If it is water, it requires a continuing source, either from the atmosphere or from springs that tap underground water layers (aquifers.) Underground water would be the most exciting possibility, but this explanation seems inconsistent with the fact that many of the dark streaks start at high elevations on the walls of craters. Additional evidence that the dark streaks (called by the scientists recurring slope lineae) are caused by water was found in 2015 when spectra were obtained of the dark streaks (Figure 10.29). These showed the presence of hydrated salts produced by the evaporation of salty water. If the water is salty, it could remain liquid long enough to flow downstream for distances of a hundred meters or more, before it either evaporates or soaks into the ground. However, this discovery still does not identify the ultimate source of the water. 366 Chapter 10 Earthlike Planets: Venus and Mars Figure 10.29. Evidence for Liquid Water on Mars |
. The dark streaks in Horowitz crater, which move downslope, have been called recurring slope lineae. The streaks in the center of the image go down the wall of the crater for about a distance of 100 meters. Spectra taken of this region indicate that these are locations where salty liquid water flows on or just below the surface of Mars. (The vertical dimension is exaggerated by a factor of 1.5 compared to horizontal dimensions.) (credit: NASA/JPL-Caltech/University of Arizona) Ancient Lakes The rovers (Spirit, Opportunity, and Curiosity) that have operated on the surface of Mars have been used to hunt for additional evidence of water. They could not reach the most interesting sites, such as the gullies, which are located on steep slopes. Instead, they explored sites that might be dried-out lake beds, dating back to a time when the climate on Mars was warmer and the atmosphere thicker—allowing water to be liquid on the surface. Spirit was specifically targeted to explore what looked like an ancient lake-bed in Gusev crater, with an outflow channel emptying into it. However, when the spacecraft landed, it found that the former lakebed had been covered by thin lava flows, blocking the rover from access to the sedimentary rocks it had hoped to find. However, Opportunity had better luck. Peering at the walls of a small crater, it detected layered sedimentary rock. These rocks contained chemical evidence of evaporation, suggesting there had been a shallow salty lake in that location. In these sedimentary rocks were also small spheres that were rich in the mineral hematite, which forms only in watery environments. Apparently this very large basin had once been underwater The small spherical rocks were nicknamed “blueberries” by the science team and the discovery of a whole “berry-bowl” of them was announced in this interesting news release (https://openstax.org/l/ 30berrybowl) from NASA. The Curiosity rover landed inside Gale crater, where photos taken from orbit also suggested past water erosion. It discovered numerous sedimentary rocks, some in the form of mudstones from an ancient lakebed; it also This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 10 Earthlike Planets: Venus and Mars 367 found indications of rocks formed by the action of shallow water at the time the sediment formed (Figure 10.30). Figure 10.30 |
. Gale Crater. (a) This scene, photographed by the Curiosity rover, shows an ancient lakebed of cracked mudstones. (b) Geologists working with the Curiosity rover interpret this image of cross-bedded sandstone in Gale crater as evidence of liquid water passing over a loose bed of sediment at the time this rock formed. (credit a: modification of work by NASA/JPL-Caltech/MSSS; credit b: modification of work by NASA/ JPL-Caltech/MSSS Astronomy and Pseudoscience: The “Face on Mars” People like human faces. We humans have developed great skill in recognizing people and interpreting facial expressions. We also have a tendency to see faces in many natural formations, from clouds to the man in the Moon. One of the curiosities that emerged from the Viking orbiters’ global mapping of Mars was the discovery of a strangely shaped mesa in the Cydonia region that resembled a human face. Despite later rumors of a cover-up, the “Face on Mars” was, in fact, recognized by Viking scientists and included in one of the early mission press releases. At the low resolution and oblique lighting under which the Viking image was obtained, the mile-wide mesa had something of a Sphinx-like appearance. Unfortunately, a small band of individuals decided that this formation was an artificial, carved sculpture of a human face placed on Mars by an ancient civilization that thrived there hundreds of thousands of years ago. A band of “true believers” grew around the face and tried to deduce the nature of the “sculptors” who made it. This group also linked the face to a variety of other pseudoscientific phenomena such as crop circles (patterns in fields of grain, mostly in Britain, now known to be the work of pranksters). Members of this group accused NASA of covering up evidence of intelligent life on Mars, and they received a great deal of help in publicizing their perspective from tabloid media. Some of the believers picketed the Jet Propulsion Laboratory at the time of the failure of the Mars Observer spacecraft, circulating stories that the “failure” of the Mars Observer was itself a fake, and that its true (secret) mission was to photograph the face. The high-resolution Mars Observer camera (MOC) was reflown on the Mars Global Surveyor mission, which arrived at Mars in 1997. On April 5, 1998 |
, in Orbit 220, the MOC obtained an oblique image of the face at a resolution of 4 meters per pixel, a factor-of-10 improvement in resolution over the Viking image. Another image in 2001 had even higher resolution. Immediately released by NASA, the new images showed a low mesa-like hill cut crossways by several roughly linear ridges and depressions, which were misidentified in 368 Chapter 10 Earthlike Planets: Venus and Mars the 1976 photo as the eyes and mouth of a face. Only with an enormous dose of imagination can any resemblance to a face be seen in the new images, demonstrating how dramatically our interpretation of geology can change with large improvements in resolution. The original and the higher resolution images can be seen in Figure 10.31. Figure 10.31. Face on Mars. The so-called “Face on Mars” is seen (a) in low resolution from Viking (the “face” is in the upper part of the picture) and (b) with 20 times better resolution from the Mars Global Surveyor. (credit a: modification of work NASA/JPL; credit b: modification of work by NASA/JPL/MSSS) After 20 years of promoting pseudoscientific interpretations and various conspiracy theories, can the “Face on Mars” believers now accept reality? Unfortunately, it does not seem so. They have accused NASA of faking the new picture. They also suggest that the secret mission of the Mars Observer included a nuclear bomb used to destroy the face before it could be photographed in greater detail by the Mars Global Surveyor. Space scientists find these suggestions incredible. NASA is spending increasing sums for research on life in the universe, and a major objective of current and upcoming Mars missions is to search for evidence of past microbial life on Mars. Conclusive evidence of extraterrestrial life would be one of the great discoveries of science and incidentally might well lead to increased funding for NASA. The idea that NASA or other government agencies would (or could) mount a conspiracy to suppress such welcome evidence is truly bizarre. Alas, the “Face on Mars” story is only one example of a whole series of conspiracy theories that are kept before the public by dedicated believers, by people out to make a fast buck, and by irresponsible media attention. Others include the “urban legend” that the Air Force has the bodies of extraterrestrials at a secret base, the widely circulated report that UFOs crashed near Roswell, New Mexico ( |
actually it was a balloon carrying scientific instruments to find evidence of Soviet nuclear tests), or the notion that alien astronauts helped build the Egyptian pyramids and many other ancient monuments because our ancestors were too stupid to do it alone. In response to the increase in publicity given to these “fiction science” ideas, a group of scientists, educators, scholars, and magicians (who know a good hoax when they see one) have formed the Committee for Skeptical Inquiry. Two of the original authors of your book are active on the committee. This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 10 Earthlike Planets: Venus and Mars 369 For more information about its work delving into the rational explanations for paranormal claims, see their excellent magazine, The Skeptical Inquirer, or check out their website at www.csicop.org/. Climate Change on Mars The evidence about ancient rivers and lakes of water on Mars discussed so far suggests that, billions of years ago, martian temperatures must have been warmer and the atmosphere must have been more substantial than it is today. But what could have changed the climate on Mars so dramatically? We presume that, like Earth and Venus, Mars probably formed with a higher surface temperature thanks to the greenhouse effect. But Mars is a smaller planet, and its lower gravity means that atmospheric gases could escape more easily than from Earth and Venus. As more and more of the atmosphere escaped into space, the temperature on the surface gradually fell. Eventually Mars became so cold that most of the water froze out of the atmosphere, further reducing its ability to retain heat. The planet experienced a sort of runaway refrigerator effect, just the opposite of the runaway greenhouse effect that occurred on Venus. Probably, this loss of atmosphere took place within less than a billion years after Mars formed. The result is the cold, dry Mars we see today. Conditions a few meters below the martian surface, however, may be much different. There, liquid water (especially salty water) might persist, kept warm by the internal heat of Mars or the insulating layers solid and rock. Even on the surface, there may be ways to change the martian atmosphere temporarily. Mars is likely to experience long-term climate cycles, which may be caused by the changing orbit and tilt of the planet. At times, one or both of the polar caps might melt, releasing a great deal of water vapor into the atmosphere. Perhaps an occasional impact by a comet might |
produce a temporary atmosphere that is thick enough to permit liquid water on the surface for a few weeks or months. Some have even suggested that future technology might allow us to terraform Mars—that is, to engineer its atmosphere and climate in ways that might make the planet more hospitable for long-term human habitation. The Search for Life on Mars If there was running water on Mars in the past, perhaps there was life as well. Could life, in some form, remain in the martian soil today? Testing this possibility, however unlikely, was one of the primary objectives of the Viking landers in 1976. These landers carried miniature biological laboratories to test for microorganisms in the martian soil. Martian soil was scooped up by the spacecraft’s long arm and placed into the experimental chambers, where it was isolated and incubated in contact with a variety of gases, radioactive isotopes, and nutrients to see what would happen. The experiments looked for evidence of respiration by living animals, absorption of nutrients offered to organisms that might be present, and an exchange of gases between the soil and its surroundings for any reason whatsoever. A fourth instrument pulverized the soil and analyzed it carefully to determine what organic (carbon-bearing) material it contained. The Viking experiments were so sensitive that, had one of the spacecraft landed anywhere on Earth (with the possible exception of Antarctica), it would easily have detected life. But, to the disappointment of many scientists and members of the public, no life was detected on Mars. The soil tests for absorption of nutrients and gas exchange did show some activity, but this was most likely caused by chemical reactions that began as water was added to the soil and had nothing to do with life. In fact, these experiments showed that martian soil seems much more chemically active than terrestrial soils because of its exposure to solar ultraviolet radiation (since Mars has no ozone layer). 370 Chapter 10 Earthlike Planets: Venus and Mars The organic chemistry experiment showed no trace of organic material, which is apparently destroyed on the martian surface by the sterilizing effect of this ultraviolet light. While the possibility of life on the surface has not been eliminated, most experts consider it negligible. Although Mars has the most earthlike environment of any planet in the solar system, the sad fact is that nobody seems to be home today, at least on the surface. However, there is no reason to think that life could not have begun on Mars about 4 billion years ago, at the same time it started on Earth. The |
two planets had very similar surface conditions then. Thus, the attention of scientists has shifted to the search for fossil life on Mars. One of the primary questions to be addressed by future spacecraft is whether Mars once supported its own life forms and, if so, how this martian life compared with that on our own planet. Future missions will include the return of martian samples selected from sedimentary rocks at sites that once held water and thus perhaps ancient life. The most powerful searches for martian life (past or present) will thus be carried out in our laboratories here on Earth Planetary Protection When scientists begin to search for life on another planet, they must make sure that we do not contaminate the other world with life carried from Earth. At the very beginning of spacecraft exploration on Mars, an international agreement specified that all landers were to be carefully sterilized to avoid accidentally transplanting terrestrial microbes to Mars. In the case of Viking, we know the sterilization was successful. Viking’s failure to detect martian organisms also implies that these experiments did not detect hitchhiking terrestrial microbes. As we have learned more about the harsh conditions on the martian surface, the sterilization requirements have been somewhat relaxed. It is evident that no terrestrial microbes could grow on the martian surface, with its low temperature, absence of water, and intense ultraviolet radiation. Microbes from Earth might survive in a dormant, dried state, but they cannot grow and proliferate on Mars. The problem of contaminating Mars will become more serious, however, as we begin to search for life below the surface, where temperatures are higher and no ultraviolet light penetrates. The situation will be even more daunting if we consider human flights to Mars. Any humans will carry with them a multitude of terrestrial microbes of all kinds, and it is hard to imagine how we can effectively keep the two biospheres isolated from each other if Mars has indigenous life. Perhaps the best situation could be one in which the two life-forms are so different that each is effectively invisible to the other—not recognized on a chemical level as living or as potential food. The most immediate issue of public concern is not with the contamination of Mars but with any dangers associated with returning Mars samples to Earth. NASA is committed to the complete biological isolation of returned samples until they are demonstrated to be safe. Even though the chances of contamination are extremely low, it is better to be safe than sorry. Most likely there is no danger, even if there is life on Mars and alien microbes hitch a ride to Earth |
inside some of the returned samples. In fact, Mars is sending samples to Earth all the time in the form of the Mars meteorites. Since some of these microbes (if they exist) could probably survive the trip to Earth inside their rocky home, we may have been exposed many times over to martian microbes. Either they do not interact with our terrestrial life, or in effect our planet has already been inoculated against such alien bugs. This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 10 Earthlike Planets: Venus and Mars 371 More than any other planet, Mars has inspired science fiction writers over the years. You can find scientifically reasonable stories about Mars in a subject index of such stories online. If you click on Mars (https://openstax.org/l/30MarsStories) as a topic, you will find stories by a number of space scientists, including William Hartmann, Geoffrey Landis, and Ludek Pesek. 10.6 DIVERGENT PLANETARY EVOLUTION Learning Objectives By the end of this section, you will be able to: Compare the planetary evolution of Venus, Earth, and Mars Venus, Mars, and our own planet Earth form a remarkably diverse triad of worlds. Although all three orbit in roughly the same inner zone around the Sun and all apparently started with about the same chemical mix of silicates and metals, their evolutionary paths have diverged. As a result, Venus became hot and dry, Mars became cold and dry, and only Earth ended up with what we consider a hospitable climate. We have discussed the runaway greenhouse effect on Venus and the runaway refrigerator effect on Mars, but we do not understand exactly what started these two planets down these separate evolutionary paths. Was Earth ever in danger of a similar fate? Or might it still be diverted onto one of these paths, perhaps due to stress on the atmosphere generated by human pollutants? One of the reasons for studying Venus and Mars is to seek insight into these questions. Some people have even suggested that if we understood the evolution of Mars and Venus better, we could possibly reverse their evolution and restore more earthlike environments. While it seems unlikely that humans could ever make either Mars or Venus into a replica of Earth, considering such possibilities is a useful part of our more general quest to understand the delicate environmental balance that distinguishes our planet from its two neighbors. In Cosmic Samples and the Origin of the Solar System, we return to the comparative |
study of the terrestrial planets and their divergent evolutionary histories. 372 Chapter 10 Earthlike Planets: Venus and Mars CHAPTER 10 REVIEW KEY TERMS runaway greenhouse effect the process by which the greenhouse effect, rather than remaining stable or being lessened through intervention, continues to grow at an increasing rate tectonic geological features that result from stresses and pressures in the crust of a planet; tectonic forces can lead to earthquakes and motion of the crust SUMMARY 10.1 The Nearest Planets: An Overview Venus, the nearest planet, is a great disappointment through the telescope because of its impenetrable cloud cover. Mars is more tantalizing, with dark markings and polar caps. Early in the twentieth century, it was widely believed that the “canals” of Mars indicated intelligent life there. Mars has only 11% the mass of Earth, but Venus is nearly our twin in size and mass. Mars rotates in 24 hours and has seasons like Earth; Venus has a retrograde rotation period of 243 days. Both planets have been extensively explored by spacecraft. 10.2 The Geology of Venus Venus has been mapped by radar, especially with the Magellan spacecraft. Its crust consists of 75% lowland lava plains, numerous volcanic features, and many large coronae, which are the expression of subsurface volcanism. The planet has been modified by widespread tectonics driven by mantle convection, forming complex patterns of ridges and cracks and building high continental regions such as Ishtar. The surface is extraordinarily inhospitable, with pressure of 90 bars and temperature of 730 K, but several Russian Venera landers investigated it successfully. 10.3 The Massive Atmosphere of Venus The atmosphere of Venus is 96% CO2. Thick clouds at altitudes of 30 to 60 kilometers are made of sulfuric acid, and a CO2 greenhouse effect maintains the high surface temperature. Venus presumably reached its current state from more earthlike initial conditions as a result of a runaway greenhouse effect, which included the loss of large quantities of water. 10.4 The Geology of Mars Most of what we know about Mars is derived from spacecraft: highly successful orbiters, landers, and rovers. We have also been able to study a few martian rocks that reached Earth as meteorites. Mars has heavily cratered highlands in its southern hemisphere, but younger, lower volcanic plains over much of its northern half. The Tharsis bulge, as big as North America, includes several huge volcan |
oes; Olympus Mons is more than 20 kilometers high and 500 kilometers in diameter. The Valles Marineris canyons are tectonic features widened by erosion. Early landers revealed only barren, windswept plains, but later missions have visited places with more geological (and scenic) variety. Landing sites have been selected in part to search for evidence of past water. 10.5 Water and Life on Mars The martian atmosphere has a surface pressure of less than 0.01 bar and is 95% CO2. It has dust clouds, water clouds, and carbon dioxide (dry ice) clouds. Liquid water on the surface is not possible today, but there is subsurface permafrost at high latitudes. Seasonal polar caps are made of dry ice; the northern residual cap This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 10 Earthlike Planets: Venus and Mars 373 is water ice, whereas the southern permanent ice cap is made predominantly of water ice with a covering of carbon dioxide ice. Evidence of a very different climate in the past is found in water erosion features: both runoff channels and outflow channels, the latter carved by catastrophic floods. Our rovers, exploring ancient lakebeds and places where sedimentary rock has formed, have found evidence for extensive surface water in the past. Even more exciting are the gullies that seem to show the presence of flowing salty water on the surface today, hinting at near-surface aquifers. The Viking landers searched for martian life in 1976, with negative results, but life might have flourished long ago. We have found evidence of water on Mars, but following the water has not yet led us to life on that planet. 10.6 Divergent Planetary Evolution Earth, Venus, and Mars have diverged in their evolution from what may have been similar beginnings. We need to understand why if we are to protect the environment of Earth. FOR FURTHER EXPLORATION Articles Venus Dorminey, B. “Cool Science on a Hot World.” Astronomy (February 2006): 46. Five-page overview of Venus and the Venus Express mission plans. Kargel, J. “Rivers of Venus.” Sky & Telescope (August 1997): 32. On lava channels. Robertson, D. “Parched Planet.” Sky & Telescope (April 2008): 26. Overview of our understanding of the planet |
. Robinson, C. “Magellan Reveals Venus.” Astronomy (February 1995): 32. Stofan, E. “The New Face of Venus.” Sky & Telescope (August 1993): 22. Zimmerman, R. “Taking Venus by Storm.” Astronomy (October 2008): 66. On results from the Venus Express mission. Mars Albee, A. “The Unearthly Landscapes of Mars.” Scientific American (June 2003): 44. Results from the Mars Global Surveyor and Mars Odyssey missions and an overview. Bell, J. “A Fresh Look at Mars.” Astronomy (August 2015): 28. Nice summary of recent spacecraft results and how they are revising our understanding of Mars. Bell, J. “Uncovering Mars’ Secret Past.” Sky & Telescope (July 2009): 22. How rovers and orbiters are helping us to understand Mars history and the role of water. Bell, J. “The Red Planet’s Watery Past.” Scientific American (December 2006): 62. Rovers are furnishing proof that ancient Mars was wet. Burnham, R. “Red Planet Rendezvous.” Astronomy (May 2006): 68. About Mariner Valley and a flyover film constructed from many still images. Christensen, P. “The Many Faces of Mars.” Scientific American (July 2005): 32. Results from the Rover mission; evidence that Mars was once wet in places. Lakdawalla, E. “The History of Water on Mars.” Sky & Telescope (September 2013): 16. Clear review of our current understanding of the role of water on Mars in different epochs. 374 Chapter 10 Earthlike Planets: Venus and Mars Malin, M. “Visions of Mars.” Sky & Telescope (April 1999): 42. A geological tour of the red planet, with new Mars Global Surveyor images. McEwen, A. “Mars in Motion.” Scientific American (May 2013): 58. On gullies and other surface changes. McKay, C. & Garcia, V. “How to Search for Life on Mars.” Scientific American (June 2014): 44. Experiments future probes could perform. Naeye, R. “Europe’s Eye on Mars.” Sky & Telescope (December 2005): 30. On the Mars Express mission and the |
remarkable close-up images it is sending. Talcott, R. “Seeking Ground Truth on Mars.” Astronomy (October 2009): 34. How rovers and orbiters are helping scientists understand the red planet’s surface. Websites European Space Agency Mars Express Page: http://www.esa.int/Our_Activities/Space_Science/Mars_Express. European Space Agency Venus Express Page: http://www.esa.int/Our_Activities/Space_Science/Venus_Express. High Resolution Imaging Science Experiment: http://hirise.lpl.arizona.edu/. Jet Propulsion Lab Mars Exploration Page: http://mars.jpl.nasa.gov/. Mars Globe HD app: https://itunes.apple.com/us/app/mars-globe-hd/id376020224?mt=8. Mars Rover 360° Panorama: http://www.360cities.net/image/curiosity-rover-martian-solar- day-2#171.10,26.50,70.0. Interactive. NASA Center for Mars Exploration: http://www.nasa.gov/mission_pages/mars/main/index.html. NASA Solar System Exploration Mars Page: http://solarsystem.nasa.gov/planets/mars. NASA Solar System Exploration Venus Page: http://solarsystem.nasa.gov/planets/venus. NASA’s apps about Mars for phones and tablets can be found at: http://mars.nasa.gov/mobile/info/. NASA’s Magellan Mission to Venus: http://www2.jpl.nasa.gov/magellan/. Russian (Soviet) Venus Missions and Images: http://mentallandscape.com/C_CatalogVenus.htm. Venus Atlas app: https://itunes.apple.com/us/app/venus-atlas/id317310503?mt=8. Venus Express Results Article: http://www.mpg.de/798302/F002_Focus_026-033.pdf. Videos 50 Years of Mars Exploration: http://www.jpl.nasa.gov/video/details.php?id=1395. NASA’s summary of all missions through MAVEN; good quick overview (4:08). Being a Mars Rover: |
What It’s Like to be an Interplanetary Explorer: https://www.youtube.com/ watch?v=nRpCOEsPD54. 2013 talk by Dr. Lori Fenton about what it’s like on the surface of Mars (1:07:24). Magellan Maps Venus: http://www.bbc.co.uk/science/space/solarsystem/space_missions/ magellan_probe#p005y07s. BBC clip with Dr. Ellen Stofan on the radar images of Venus and what they tell us (3:06). Our Curiosity: https://www.youtube.com/watch?v=XczKXWvokm4. Mars Curiosity rover 2-year anniversary video narrated by Neil deGrasse Tyson and Felicia Day (6:01). Planet Venus: The Deadliest Planet, Venus Surface and Atmosphere: https://www.youtube.com/ watch?v=HqFVxWfVtoo. Quick tour of Venus’ atmosphere and surface (2:04). This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 10 Earthlike Planets: Venus and Mars 375 Planetary Protection and Hitchhikers in the Solar System: The Danger of Mingling Microbes: https://www.youtube.com/watch?v=6iGC3uO7jBI. 2009 talk by Dr. Margaret Race on preventing contamination between worlds (1:28:50). COLLABORATIVE GROUP ACTIVITIES A. Your group has been asked by high NASA officials to start planning the first human colony on Mars. Begin by making a list of what sorts of things humans would need to bring along to be able to survive for years on the surface of the red planet. B. As a publicity stunt, the mayor of Venus, Texas (there really is such a town), proposes that NASA fund a mission to Venus with humans on board. Clearly, the good mayor neglected to take an astronomy course in college. Have your group assemble a list of as many reasons as possible why it is unlikely that humans will soon land on the surface of Venus. C. Even if humans would have trouble surviving on the surface of Venus, this does not mean we could not learn a lot more about our veiled sister planet. Have your group brainstorm a series of missions (pretend cost is no object) that would provide us with |
more detailed information about Venus’ atmosphere, surface, and interior. D. Sometime late in the twenty-first century, when travel to Mars has become somewhat routine, a very wealthy couple asks you to plan a honeymoon tour of Mars that includes the most spectacular sights on the red planet. Constitute your group as the Percival Lowell Memorial Tourist Agency, and come up with a list of not-to-be missed tourist stops on Mars. E. In the popular book and film, called The Martian, the drama really begins when our hero is knocked over and loses consciousness as he is half buried by an intense wind storm on Mars. Given what you have learned about Mars’ atmosphere in this chapter, have your group discuss how realistic that scenario is. (By the way, the author of the book has himself genially acknowledged in interviews and talks that this is a reasonable question to ask.) F. Astronomers have been puzzled and annoyed about the extensive media publicity that was given the small group of “true believers” who claimed the “Face on Mars” was not a natural formation (see the Astronomy and Pseudoscience: The “Face on Mars” feature box). Have your group make a list of the reasons many of the media were so enchanted by this story. What do you think astronomers could or should do to get the skeptical, scientific perspective about such issues before the public? G. Your group is a special committee of scientists set up by the United Nations to specify how any Mars samples should be returned to Earth so that possible martian microbes do not harm Earth life. What precautions would you recommend, starting at Mars and going all the way to the labs that analyze the martian samples back on Earth? H. Have your group brainstorm about Mars in popular culture. How many movies, songs or other music, and products can you think of connected with Mars? What are some reasons that Mars would be a popular theme for filmmakers, songwriters, and product designers? 376 Chapter 10 Earthlike Planets: Venus and Mars EXERCISES Review Questions 1. List several ways that Venus, Earth, and Mars are similar, and several ways they are different. 2. Compare the current atmospheres of Earth, Venus, and Mars in terms of composition, thickness (and pressure at the surface), and the greenhouse effect. 3. How might Venus’ atmosphere have evolved to its present state through a runaway greenhouse effect? 4. Describe the current atmosphere on Mars. What evidence |
suggests that it must have been different in the past? 5. Explain the runaway refrigerator effect and the role it may have played in the evolution of Mars. 6. What evidence do we have that there was running (liquid) water on Mars in the past? What evidence is there for water coming out of the ground even today? 7. What evidence is there that Venus was volcanically active about 300–600 million years ago? 8. Why is Mars red? 9. What is the composition of clouds on Mars? 10. What is the composition of the polar caps on Mars? 11. Describe two anomalous features of the rotation of Venus and what might account for them. 12. How was the Mars Odyssey spacecraft able to detect water on Mars without landing on it? Thought Questions 13. What are the advantages of using radar imaging rather than ordinary cameras to study the topography of Venus? What are the relative advantages of these two approaches to mapping Earth or Mars? 14. Venus and Earth are nearly the same size and distance from the Sun. What are the main differences in the geology of the two planets? What might be some of the reasons for these differences? 15. Why is there so much more carbon dioxide in the atmosphere of Venus than in that of Earth? Why so much more carbon dioxide than on Mars? 16. If the Viking missions were such a rich source of information about Mars, why have we sent the Pathfinder, Global Surveyor, and other more recent spacecraft to Mars? Make a list of questions about Mars that still puzzle astronomers. 17. Compare Mars with Mercury and the Moon in terms of overall properties. What are the main similarities and differences? 18. Contrast the mountains on Mars and Venus with those on Earth and the Moon. 19. We believe that all of the terrestrial planets had similar histories when it comes to impacts from space. Explain how this idea can be used to date the formation of the martian highlands, the martian basins, and the Tharsis volcanoes. How certain are the ages derived for these features (in other words, how do we check the ages we derive from this method)? This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 10 Earthlike Planets: Venus and Mars 377 20. Is it likely that life ever existed on either Venus or Mars? Justify your answer in each case. 21. Suppose that, decades from now, NASA is considering sending astronauts to Mars |
and Venus. In each case, describe what kind of protective gear they would have to carry, and what their chances for survival would be if their spacesuits ruptured. 22. We believe that Venus, Earth, and Mars all started with a significant supply of water. Explain where that water is now for each planet. 23. One source of information about Mars has been the analysis of meteorites from Mars. Since no samples from Mars have ever been returned to Earth from any of the missions we sent there, how do we know these meteorites are from Mars? What information have they revealed about Mars? 24. The runaway greenhouse effect and its inverse, the runaway refrigerator effect, have led to harsh, uninhabitable conditions on Venus and Mars. Does the greenhouse effect always cause climate changes leading to loss of water and life? Give a reason for your answer. 25. In what way is the high surface temperature of Venus relevant to concerns about global warming on Earth today? 26. What is a dust devil? Would you expect to feel more of a breeze from a dust devil on Mars or on Earth? Explain. 27. Near the martian equator, temperatures at the same spot can vary from an average of –135 °C at night to an average of 30 °C during the day. How can you explain such a wide difference in temperature compared to that on Earth? Figuring For Yourself 28. Estimate the amount of water there could be in a global (planet-wide) region of subsurface permafrost on Mars (do the calculations for two permafrost thicknesses, 1 and 10 km, and a concentration of ice in the permafrost of 10% by volume). Compare the two results you get with the amount of water in Earth’s oceans calculated in Example 10.1. 29. At its nearest, Venus comes within about 41 million km of Earth. How distant is it at its farthest? 30. If you weigh 150 lbs. on the surface of Earth, how much would you weigh on Venus? On Mars? 31. Calculate the relative land area—that is, the amount of the surface not covered by liquids—of Earth, the Moon, Venus, and Mars. (Assume that 70% of Earth is covered with water.) 32. The closest approach distance between Mars and Earth is about 56 million km. Assume you can travel in a spaceship at 58,000 km/h, which is the speed achieved by the New Horizons space probe that went to Pluto and |
is the fastest speed so far of any space vehicle launched from Earth. How long would it take to get to Mars at the time of closest approach? 378 Chapter 10 Earthlike Planets: Venus and Mars This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 11 The Giant Planets 379 11 THE GIANT PLANETS Figure 11.1. Giant Planets. The four giant planets in our solar system all have hydrogen atmospheres, but the warm gas giants, Jupiter and Saturn, have tan, beige, red, and white clouds that are thought to be composed of ammonia ice particles with various colorants called “chromophores.” The blue-tinted ice giants, Uranus and Neptune, are much colder and covered in methane ice clouds. (credit: modification of work by Lunar and Planetary Institute, NASA) Chapter Outline 11.1 Exploring the Outer Planets 11.2 The Giant Planets 11.3 Atmospheres of the Giant Planets Thinking Ahead “What do we learn about the Earth by studying the planets? Humility.”—Andrew Ingersoll discussing the results of the Voyager mission in 1986. Beyond Mars and the asteroid belt, we encounter a new region of the solar system: the realm of the giants. Temperatures here are lower, permitting water and other volatiles to condense as ice. The planets are much larger, distances between them are much greater, and each giant world is accompanied by an extensive system of moons and rings. From many perspectives, the outer solar system is where the action is, and the giant planets are the most important members of the Sun’s family. When compared to these outer giants, the little cinders of rock and metal that orbit closer to the Sun can seem insignificant. These four giant worlds—Jupiter, Saturn, Uranus, Neptune—are the subjects of this chapter. Their rings, moons, and the dwarf planet Pluto are discussed in a later chapter. 11.1 EXPLORING THE OUTER PLANETS Learning Objectives By the end of this section, you will be able to: 380 Chapter 11 The Giant Planets Provide an overview of the composition of the giant planets Chronicle the robotic exploration of the outer solar system Summarize the missions sent to orbit the gas giants The giant planets hold most of the mass in our planetary system. Jupiter alone exceeds the mass of all the other planets combined (Figure 11.2). The |
material available to build these planets can be divided into three classes by what they are made of: “gases,” “ices,” and “rocks” (see Table 11.1). The “gases” are primarily hydrogen and helium, the most abundant elements in the universe. The way it is used here, the term “ices” refers to composition only and not whether a substance is actually in a solid state. “Ices” means compounds that form from the next most abundant elements: oxygen, carbon, and nitrogen. Common ices are water, methane, and ammonia, but ices may also include carbon monoxide, carbon dioxide, and others. “Rocks” are even less abundant than ices, and include everything else: magnesium, silicon, iron, and so on. Figure 11.2. Jupiter. The Cassini spacecraft imaged Jupiter on its way to Saturn in 2012. The giant storm system called the Great Red Spot is visible to the lower right. The dark spot to the lower left is the shadow of Jupiter’s moon Europa. (credit: modification of work by NASA/JPL) Abundances in the Outer Solar System Type of Material Name Approximate % (by Mass) Gas Gas Ice Ice Ice Hydrogen (H2) Helium (He) Water (H2O) Methane (CH4) Ammonia (NH3) Rock Magnesium (Mg), iron (Fe), silicon (Si) Table 11.1 This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 75 24 0.6 0.4 0.1 0.3 Chapter 11 The Giant Planets 381 In the outer solar system, gases dominate the two largest planets, Jupiter and Saturn, hence their nickname “gas giants.” Uranus and Neptune are called “ice giants” because their interiors contain far more of the “ice” component than their larger cousins. The chemistry for all four giant planet atmospheres is dominated by hydrogen. This hydrogen caused the chemistry of the outer solar system to become reducing, meaning that other elements tend to combine with hydrogen first. In the early solar system, most of the oxygen combined with hydrogen to make H2O and was thus unavailable to form the kinds of oxidized compounds with other elements that are more familiar to us in the inner solar system (such |
as CO2). As a result, the compounds detected in the atmosphere of the giant planets are mostly hydrogen-based gases such as methane (CH4) and ammonia (NH3), or more complex hydrocarbons (combinations of hydrogen and carbon) such as ethane (C2H6) and acetylene (C2H2). Exploration of the Outer Solar System So Far Eight spacecraft, seven from the United States and one from Europe, have penetrated beyond the asteroid belt into the realm of the giants. Table 11.2 summarizes the spacecraft missions to the outer solar system. Missions to the Giant Planets Planet Spacecraft[1] Encounter Date Type Jupiter Pioneer 10 December 1973 Pioneer 11 December 1974 Voyager 1 March 1979 Voyager 2 July 1979 Flyby Flyby Flyby Flyby Ulysses February 1992 Flyby during gravity assist Galileo December 1995 Orbiter and probe Cassini December 2002 Flyby New Horizons February 2007 Flyby during gravity assist Juno July 2016 Saturn Pioneer 11 September 1979 Voyager 1 November 1980 Voyager 2 August 1981 Orbiter Flyby Flyby Flyby Cassini July 2004 (Saturn orbit injection 2000) Orbiter Table 11.2 1 Both the Ulysses and the New Horizons spacecraft (designed to study the Sun and Pluto, respectively) flew past Jupiter for a gravity boost (gaining energy by “stealing” a little bit from the giant planet’s rotation). 382 Chapter 11 The Giant Planets Missions to the Giant Planets Planet Spacecraft Encounter Date Type Uranus Voyager 2 January 1986 Neptune Voyager 2 August 1989 Flyby Flyby Table 11.2 The challenges of exploring so far away from Earth are considerable. Flight times to the giant planets are measured in years to decades, rather than the months required to reach Venus or Mars. Even at the speed of light, messages take hours to pass between Earth and the spacecraft. If a problem develops near Saturn, for example, a wait of hours for the alarm to reach Earth and for instructions to be routed back to the spacecraft could spell disaster. Spacecraft to the outer solar system must therefore be highly reliable and capable of a greater degree of independence and autonomy. Outer solar system missions also must carry their own power sources since the Sun is too far away to provide enough energy. Heaters are required to keep instruments at proper operating temperatures, and spacecraft must have radio transmitters powerful enough to send their data to receivers on distant Earth. The first spacecraft to investigate the regions past Mars were the NASA Pioneers 10 and 11, launched in 1972 and 1973 as |
pathfinders to Jupiter. One of their main objectives was simply to determine whether a spacecraft could actually navigate through the belt of asteroids that lies beyond Mars without getting destroyed by collisions with asteroidal dust. Another objective was to measure the radiation hazards in the magnetosphere (or zone of magnetic influence) of Jupiter. Both spacecraft passed through the asteroid belt without incident, but the energetic particles in Jupiter’s magnetic field nearly wiped out their electronics, providing information necessary for the safe design of subsequent missions. Pioneer 10 flew past Jupiter in 1973, after which it sped outward toward the limits of the solar system. Pioneer 11 undertook a more ambitious program, using the gravity of Jupiter to aim for Saturn, which it reached in 1979. The twin Voyager spacecraft launched the next wave of outer planet exploration in 1977. Voyagers 1 and 2 each carried 11 scientific instruments, including cameras and spectrometers, as well as devices to measure the characteristics of planetary magnetospheres. Since they kept going outward after their planetary encounters, these are now the most distant spacecraft ever launched by humanity. Voyager 1 reached Jupiter in 1979 and used a gravity assist from that planet to take it on to Saturn in 1980. Voyager 2 arrived at Jupiter four months later, but then followed a different path to visit all the outer planets, reaching Saturn in 1981, Uranus in 1986, and Neptune in 1989. This trajectory was made possible by the approximate alignment of the four giant planets on the same side of the Sun. About once every 175 years, these planets are in such a position, and it allows a single spacecraft to visit them all by using gravity-assisted flybys to adjust course for each subsequent encounter; such a maneuver has been nicknamed a “Grand Tour” by astronomers. This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 11 The Giant Planets 383 The Jet Propulsion Laboratory has a nice video called Voyager: The Grand Tour (https://openstax.org/ l/30JPLGrandT) that describes the Voyager mission and what it found Engineering and Space Science: Teaching an Old Spacecraft New Tricks By the time Voyager 2 arrived at Neptune in 1989, 12 years after its launch, the spacecraft was beginning to show signs of old age. The arm on which the camera and other instruments were located was “arthritic”: it could no longer move easily in all directions. The communications system was “hard of hearing”: part of its radio |
receiver had stopped working. The “brains” had significant “memory loss”: some of the onboard computer memory had failed. And the whole spacecraft was beginning to run out of energy: its generators had begun showing serious signs of wear. To make things even more of a challenge, Voyager’s mission at Neptune was in many ways the most difficult of all four flybys. For example, since sunlight at Neptune is 900 times weaker than at Earth, the onboard camera had to take much longer exposures in this light-starved environment. This was a nontrivial requirement, given that the spacecraft was hurtling by Neptune at ten times the speed of a rifle bullet. The solution was to swivel the camera backward at exactly the rate that would compensate for the forward motion of the spacecraft. Engineers had to preprogram the ship’s computer to execute an incredibly complex series of maneuvers for each image. The beautiful Voyager images of Neptune are a testament to the ingenuity of spacecraft engineers. The sheer distance of the craft from its controllers on Earth was yet another challenge. Voyager 2 received instructions and sent back its data via on-board radio transmitter. The distance from Earth to Neptune is about 4.8 billion kilometers. Over this vast distance, the power that reached us from Voyager 2 at Neptune was approximately10–16 watts, or 20 billion times less power than it takes to operate a digital watch. Thirty-eight different antennas on four continents were used by NASA to collect the faint signals from the spacecraft and decode the precious information about Neptune that they contained. Enter the Orbiters: Galileo and Cassini The Pioneer and Voyager missions were flybys of the giant planets: they each produced only quick looks before the spacecraft sped onward. For more detailed studies of these worlds, we require spacecraft that can go into orbit around a planet. For Jupiter and Saturn, these orbiters were the Galileo and Cassini spacecraft, respectively. To date, no orbiter missions have been started for Uranus and Neptune, although planetary scientists have expressed keen interest. The Galileo spacecraft was launched toward Jupiter in 1989 and arrived in 1995. Galileo began its investigations by deploying an entry probe into Jupiter, for the first direct studies of the planet’s outer atmospheric layers. 384 Chapter 11 The Giant Planets The probe plunged at a shallow angle into Jupiter’s atmosphere, traveling at a speed of 50 kilometers per second—that’s fast enough to fly from New York to San Francisco in 100 seconds! This was the highest speed at which |
any probe has so far entered the atmosphere of a planet, and it put great demands on the heat shield protecting it. The high entry speed was a result of acceleration by the strong gravitational attraction of Jupiter. Atmospheric friction slowed the probe within 2 minutes, producing temperatures at the front of its heat shield as high as 15,000 °C. As the probe’s speed dropped to 2500 kilometers per hour, the remains of the glowing heat shield were jettisoned, and a parachute was deployed to lower the instrumented probe spacecraft more gently into the atmosphere (Figure 11.3). The data from the probe instruments were relayed to Earth via the main Galileo spacecraft. Figure 11.3. Galileo Probe Falling into Jupiter. This artist’s depiction shows the Galileo probe descending into the clouds via parachute just after the protective heat shield separated. The probe made its measurements of Jupiter’s atmosphere on December 7, 1995. (credit: modification of work by NASA/Ames Research Center) The probe continued to operate for an hour, descending 200 kilometers into the atmosphere. A few minutes later the polyester parachute melted, and within a few hours the main aluminum and titanium structure of the probe vaporized to become a part of Jupiter itself. About 2 hours after receipt of the final probe data, the main spacecraft fired its retro-rockets so it could be captured into orbit around the planet, where its primary objectives were to study Jupiter’s large and often puzzling moons. The Cassini mission to Saturn (Figure 11.4), a cooperative venture between NASA and the European Space Agency, was similar to Galileo in its two-fold approach. Launched in 1997, Cassini arrived in 2004 and went into orbit around Saturn, beginning extensive studies of its rings and moons, as well as the planet itself. In January 2005, Cassini deployed an entry probe into the atmosphere of Saturn’s large moon, Titan, where it successfully landed on the surface. (We’ll discuss the probe and what it found in the chapter on Rings, Moons, and Pluto.) This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 11 The Giant Planets 385 Figure 11.4. Earth as Seen from Saturn. This popular Cassini image shows Earth as a tiny dot (marked with an arrow) seen below Saturn’s rings. It was taken in July 2013, when Saturn was 1.4 billion kilometers from Earth. (credit: modification of |
work by NASA/JPL-Caltech/Space Science Institute) 11.2 THE GIANT PLANETS Learning Objectives By the end of this section, you will be able to: Describe the basic physical characteristics, general appearance, and rotation of the giant planets Describe the composition and structure of Jupiter, Saturn, Uranus, and Neptune Compare and contrast the internal heat sources of the giant planets Describe the discovery and characteristics of the giant planets’ magnetic fields Let us now examine the four giant (or jovian) planets in more detail. Our approach is not just to catalog their characteristics, but to compare them with each other, noting their similarities and differences and attempting to relate their properties to their differing masses and distances from the Sun. Basic Characteristics The giant planets are very far from the Sun. Jupiter is more than five times farther from the Sun than Earth’s distance (5 AU), and takes just under 12 years to circle the Sun. Saturn is about twice as far away as Jupiter (almost 10 AU) and takes nearly 30 years to complete one orbit. Uranus orbits at 19 AU with a period of 84 years, while Neptune, at 30 AU, requires 165 years for each circuit of the Sun. These long timescales make it difficult for us short-lived humans to study seasonal change on the outer planets. Jupiter and Saturn have many similarities in composition and internal structure, although Jupiter is nearly four times more massive. Uranus and Neptune are smaller and differ in composition and internal structure from their large siblings. Some of the main properties of these four planets are summarized in Table 11.3. 386 Chapter 11 The Giant Planets Basic Properties of the Jovian Planets Planet Distance (AU) Period (years) Diameter (km) Mass (Earth = 1) Density (g/cm3) Rotation (hours) Jupiter Saturn 5.2 9.5 Uranus 19.2 11.9 29.5 84.1 142,800 318 120,540 51,200 95 14 17 1.3 0.7 1.3 1.6 9.9 10.7 17.2 16.1 Neptune 30.0 164.8 49,500 Table 11.3 Jupiter, the giant among giants, has enough mass to make 318 Earths. Its diameter is about 11 times that of Earth (and about one tenth that of the Sun). Jupiter’s average density is 1.3 g/cm3, much lower than that of any of the terrestrial |
planets. (Recall that water has a density of 1 g/cm3.) Jupiter’s material is spread out over a volume so large that more than 1400 Earths could fit within it. Saturn’s mass is 95 times that of Earth, and its average density is only 0.7 g/cm3—the lowest of any planet. Since this is less than the density of water, Saturn would be light enough to float. Uranus and Neptune each have a mass about 15 times that of Earth and, hence, are only 5% as massive as Jupiter. Their densities of 1.3 g/cm3 and 1.6 g/cm3, respectively, are much higher than that of Saturn. This is one piece of evidence that tells us that their composition must differ fundamentally from the gas giants. When astronomers began to discover other planetary systems (exoplanets), we found that planets the size of Uranus and Neptune are common, and that there are even more exoplanets intermediate in size between Earth and these ice giants, a type of planet not found in our solar system. Appearance and Rotation When we look at the planets, we see only their atmospheres, composed primarily of hydrogen and helium gas (see Figure 11.1). The uppermost clouds of Jupiter and Saturn, the part we see when looking down at these planets from above, are composed of ammonia crystals. On Neptune, the upper clouds are made of methane. On Uranus, we see no obvious cloud layer at all, but only a deep and featureless haze. Seen through a telescope, Jupiter is a colorful and dynamic planet. Distinct details in its cloud patterns allow us to determine the rotation rate of its atmosphere at the cloud level, although such atmosphere rotation may have little to do with the spin of the underlying planet. Much more fundamental is the rotation of the mantle and core; these can be determined by periodic variations in radio waves coming from Jupiter, which are controlled by its magnetic field. Since the magnetic field (which we will discuss below) originates deep inside the planet, it shares the rotation of the interior. The rotation period we measure in this way is 9 hours 56 minutes, which gives Jupiter the shortest “day” of any planet. In the same way, we can measure that the underlying rotation period of Saturn is 10 hours 40 minutes. Uranus and Neptune have slightly longer rotation periods of about 17 hours, also determined from the rotation of their magnetic fields. This OpenStax book is available for |
free at http://cnx.org/content/col11992/1.8 Chapter 11 The Giant Planets 387 brief video made from Hubble Space Telescope photos shows the rotation of Jupiter (https://openstax.org/l/30HSTJupRot) with its many atmospheric features. Remember that Earth and Mars have seasons because their spin axes, instead of “standing up straight,” are tilted relative to the orbital plane of the solar system. This means that as Earth revolves around the Sun, sometimes one hemisphere and sometimes the other “leans into” the Sun. What are the seasons like for the giant planets? The spin axis of Jupiter is tilted by only 3°, so there are no seasons to speak of. Saturn, however, does have seasons, since its spin axis is inclined at 27° to the perpendicular to its orbit. Neptune has about the same tilt as Saturn (29°); therefore, it experiences similar seasons (only more slowly). The strangest seasons of all are on Uranus, which has a spin axis tilted by 98° with respect to the north direction. Practically speaking, we can say that Uranus orbits on its side, and its ring and moon system follow along, orbiting about Uranus’ equator (Figure 11.5). Figure 11.5. Infrared Image of Uranus. The infrared camera on the Hubble Space Telescope took these false-color images of the planet Uranus, its ring system, and moons in 1997. The south pole of the planet (marked with a “+” on the right image) faces the Sun; its green color shows a strong local haze. The two images were taken 90 minutes apart, and during that time the five reddish clouds can be seen to rotate around the parallel to the equator. The rings (which are very faint in the visible light, but prominent in infrared) and eight moons can be seen around the equator. This was the “bull’s eye” arrangement that Voyager saw as it approached Uranus in 1986. (credit: modification of work by Erich Karkoschka (University of Arizona), and NASA/ESA) We don’t know what caused Uranus to be tipped over like this, but one possibility is a collision with a large planetary body when our system was first forming. Whatever the cause, this unusual tilt creates dramatic seasons. When Voyager 2 arrived at Uranus, its south pole was facing directly into the Sun. The |
southern hemisphere was experiencing a 21-year sunlit summer, while during that same period the northern hemisphere was plunged into darkness. For the next 21-year season, the Sun shines on Uranus’ equator, and both hemispheres go through cycles of light and dark as the planet rotates (Figure 11.6). Then there are 21 years of an illuminated northern hemisphere and a dark southern hemisphere. After that the pattern of alternating day and night repeats. 388 Chapter 11 The Giant Planets Just as on Earth, the seasons are even more extreme at the poles. If you were to install a floating platform at the south pole of Uranus, for example, it would experience 42 years of light and 42 years of darkness. Any future astronauts crazy enough to set up camp there could spend most of their lives without ever seeing the Sun. Figure 11.6. Strange Seasons on Uranus. (a) This diagram shows the orbit of Uranus as seen from above. At the time Voyager 2 arrived (position 1), the South Pole was facing the Sun. As we move counterclockwise in the diagram, we see the planet 21 years later at each step. (b) This graph compares the amount of sunlight seen at the poles and the equator of Uranus over the course of its 84-year revolution around the Sun. Composition and Structure Although we cannot see into these planets, astronomers are confident that the interiors of Jupiter and Saturn are composed primarily of hydrogen and helium. Of course, these gases have been measured only in their atmosphere, but calculations first carried out more than 50 years ago showed that these two light gases are the only possible materials out of which a planet with the observed masses and densities of Jupiter and Saturn could be constructed. The deep internal structures of these two planets are difficult to predict. This is mainly because these planets are so big that the hydrogen and helium in their centers become tremendously compressed and behave in ways that these gases can never behave on Earth. The best theoretical models we have of Jupiter’s structure predict a central pressure greater than 100 million bars and a central density of about 31 g/cm3. (By contrast, Earth’s core has a central pressure of 4 million bars and a central density of 17 g/cm3.) At the pressures inside the giant planets, familiar materials can take on strange forms. A few thousand kilometers below the visible clouds of Jupiter and Saturn, pressures become so great that hydrogen changes from a gaseous to a liquid state. Still deeper |
, this liquid hydrogen is further compressed and begins to act like a metal, something it never does on Earth. (In a metal, electrons are not firmly attached to their parent nuclei but can wander around. This is why metals are such good conductors of electricity.) On Jupiter, the greater part of the interior is liquid metallic hydrogen. Because Saturn is less massive, it has only a small volume of metallic hydrogen, but most of its interior is liquid. Uranus and Neptune are too small to reach internal pressures sufficient to liquefy hydrogen. We will return to the discussion of the metallic hydrogen layers when we examine the magnetic fields of the giant planets. Each of these planets has a core composed of heavier materials, as demonstrated by detailed analyses of their gravitational fields. Presumably these cores are the original rock-and-ice bodies that formed before the capture of gas from the surrounding nebula. The cores exist at pressures of tens of millions of bars. While scientists speak of the giant planet cores being composed of rock and ice, we can be sure that neither rock nor ice This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 11 The Giant Planets 389 assumes any familiar forms at such pressures and temperatures. Remember that what is really meant by “rock” is any material made up primarily of iron, silicon, and oxygen, while the term “ice” in this chapter denotes materials composed primarily of the elements carbon, nitrogen, and oxygen in combination with hydrogen. Figure 11.7 illustrates the likely interior structures of the four jovian planets. It appears that all four have similar cores of rock and ice. On Jupiter and Saturn, the cores constitute only a few percent of the total mass, consistent with the initial composition of raw materials shown in Table 11.1. However, most of the mass of Uranus and Neptune resides in these cores, demonstrating that the two outer planets were unable to attract massive quantities of hydrogen and helium when they were first forming. Figure 11.7. Internal Structures of the Jovian Planets. Jupiter and Saturn are composed primarily of hydrogen and helium (but hydrogen dominates), but Uranus and Neptune consist in large part of compounds of carbon, nitrogen, and oxygen. (The diagrams are drawn to scale; numbers show radii in thousands of kilometers.) Internal Heat Sources Because of their large sizes, all the giant planets were strongly heated during their formation by the collapse of surrounding material onto their cores. Jupiter |
, being the largest, was the hottest. Some of this primordial heat can still remain inside such large planets. In addition, it is possible for giant, largely gaseous planets to generate heat after formation by slowly contracting. (With so large a mass, even a minuscule amount of shrinking can generate significant heat.) The effect of these internal energy sources is to raise the temperatures in the interiors and atmospheres of the planets higher than we would expect from the heating effect of the Sun alone. Jupiter has the largest internal energy source, amounting to 4 × 1017 watts; that is, it is heated from inside with energy equivalent to 4 million billion 100-watt lightbulbs. This energy is about the same as the total solar energy absorbed by Jupiter. The atmosphere of Jupiter is therefore something of a cross between a normal planetary atmosphere (like Earth’s), which obtains most of its energy from the Sun, and the atmosphere of a star, which is entirely heated by an internal energy source. Most of the internal energy of Jupiter is primordial heat, left over from the formation of the planet 4.5 billon years ago. Saturn has an internal energy source about half as large as that of Jupiter, which means (since its mass is only about one quarter as great) that it is producing twice as much energy per kilogram of material as does Jupiter. Since Saturn is expected to have much less primordial heat, there must be another source at work generating most of this 2 × 1017 watts of power. This source is the separation of helium from hydrogen in 390 Chapter 11 The Giant Planets Saturn’s interior. In the liquid hydrogen mantle, the heavier helium forms droplets that sink toward the core, releasing gravitational energy. In effect, Saturn is still differentiating—letting lighter material rise and heavier material fall. Uranus and Neptune are different. Neptune has a small internal energy source, while Uranus does not emit a measurable amount of internal heat. As a result, these two planets have almost the same atmospheric temperature, in spite of Neptune’s greater distance from the Sun. No one knows why these two planets differ in their internal heat, but all this shows how nature can contrive to make each world a little bit different from its neighbors. Magnetic Fields Each of the giant planets has a strong magnetic field, generated by electric currents in its rapidly spinning interior. Associated with the magnetic fields are the planets’ magnetospheres, which are regions around the planet within which the |
planet’s own magnetic field dominates over the general interplanetary magnetic field. The magnetospheres of these planets are their largest features, extending millions of kilometers into space. In the late 1950s, astronomers discovered that Jupiter was a source of radio waves that got more intense at longer rather than at shorter wavelengths—just the reverse of what is expected from thermal radiation (radiation caused by the normal vibrations of particles within all matter). Such behavior is typical, however, of the radiation emitted when high-speed electrons are accelerated by a magnetic field. We call this synchrotron radiation because it was first observed on Earth in particle accelerators, called synchrotrons. This was our first hint that Jupiter must have a strong magnetic field. Later observations showed that the radio waves are coming from a region surrounding Jupiter with a diameter several times that of the planet itself (Figure 11.8). The evidence suggested that a vast number of charged atomic particles must be circulating around Jupiter, spiraling around the lines of force of a magnetic field associated with the planet. This is just what we observe happening, but on a smaller scale, in the Van Allen belt around Earth. The magnetic fields of Saturn, Uranus, and Neptune, discovered by the spacecraft that first passed close to these planets, work in a similar way, but are not as strong. Figure 11.8. Jupiter in Radio Waves. This false-color image of Jupiter was made with the Very Large Array (of radio telescopes) in New Mexico. We see part of the magnetosphere, brightest in the middle because the largest number of charged particles are in the equatorial zone of Jupiter. The planet itself is slightly smaller than the green oval in the center. Different colors are used to indicate different intensities of synchrotron radiation. (credit: modification of work by I. de Pater (UC Berkeley) NRAO, AUI, NSF) This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 11 The Giant Planets 391 Learn more about the magnetosphere of Jupiter (https://openstax.org/l/30NASAJupMag) and why we continue to be interested in it from this brief NASA video. Inside each magnetosphere, charged particles spiral around in the magnetic field; as a result, they can be accelerated to high energies. These charged particles can come from the Sun or from the neighborhood of the planet itself. In Jupiter’s |
case, Io, one of its moons, turns out to have volcanic eruptions that blast charged particles into space and right into the jovian magnetosphere. The axis of Jupiter’s magnetic field (the line that connects the magnetic north pole with the magnetic south pole) is not aligned exactly with the axis of rotation of the planet; rather, it is tipped by about 10°. Uranus and Neptune have even greater magnetic tilts, of 60° and 55°, respectively. Saturn’s field, on the other hand, is perfectly aligned with its rotation axis. Why different planets have such different magnetic tilts is not well understood. The physical processes around the jovian planets turn out to be milder versions of what astronomers find in many distant objects, from the remnants of dead stars to the puzzling distant powerhouses we call quasars. One reason to study the magnetospheres of the giant planets and Earth is that they provide nearby accessible analogues of more energetic and challenging cosmic processes. 11.3 ATMOSPHERES OF THE GIANT PLANETS Learning Objectives By the end of this section, you will be able to: Discuss the atmospheric composition of the giant planets Describe the cloud formation and atmospheric structure of the gas giants Characterize the giant planets’ wind and weather patterns Understand the scale and longevity of storms on the giant planets The atmospheres of the jovian planets are the parts we can observe or measure directly. Since these planets have no solid surfaces, their atmospheres are more representative of their general compositions than is the case with the terrestrial planets. These atmospheres also present us with some of the most dramatic examples of weather patterns in the solar system. As we will see, storms on these planets can grow bigger than the entire planet Earth. Atmospheric Composition When sunlight reflects from the atmospheres of the giant planets, the atmospheric gases leave their “fingerprints” in the spectrum of light. Spectroscopic observations of the jovian planets began in the nineteenth century, but for a long time, astronomers were not able to interpret the spectra they observed. As late as the 1930s, the most prominent features photographed in these spectra remained unidentified. Then better spectra revealed the presence of molecules of methane (CH4) and ammonia (NH3) in the atmospheres of Jupiter and Saturn. 392 Chapter 11 The Giant Planets At first astronomers thought that methane and ammonia might be the main constituents of these atmospheres, but now we know that hydrogen and helium are |
actually the dominant gases. The confusion arose because neither hydrogen nor helium possesses easily detected spectral features in the visible spectrum. It was not until the Voyager spacecraft measured the far-infrared spectra of Jupiter and Saturn that a reliable abundance for the elusive helium could be found. The compositions of the two atmospheres are generally similar, except that on Saturn there is less helium as the result of the precipitation of helium that contributes to Saturn’s internal energy source. The most precise measurements of composition were made on Jupiter by the Galileo entry probe in 1995; as a result, we know the abundances of some elements in the jovian atmosphere even better than we know those in the Sun James Van Allen: Several Planets under His Belt The career of physicist James Van Allen spanned the birth and growth of the space age, and he played a major role in its development. Born in Iowa in 1914, Van Allen received his PhD from the University of Iowa. He then worked for several research institutions and served in the Navy during World War II. After the war, Van Allen (Figure 11.9) was appointed Professor of Physics at the University of Iowa. He and his collaborators began using rockets to explore cosmic radiation in Earth’s outer atmosphere. To reach extremely high altitudes, Van Allen designed a technique in which a balloon lifts and then launches a small rocket (the rocket is nicknamed “the rockoon”). Figure 11.9. James Van Allen (1914–2006). In this 1950s photograph, Van Allen holds a “rockoon.” (credit: modification of work by Frederick W. Kent Collection, University of Iowa Archives) Over dinner one night in 1950, Van Allen and several colleagues came up with the idea of the International Geophysical Year (IGY), an opportunity for scientists around the world to coordinate their investigations of the physics of Earth, especially research done at high altitudes. In 1955, the United States and the Soviet Union each committed themselves to launching an Earth-orbiting satellite during IGY, a competition that began what came to be known as the space race. The IGY (stretched to 18 months) took place between July 1957 and December 1958. This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 11 The Giant Planets 393 The Soviet Union won the first lap of the race by launching Sputnik 1 in October 1957. The US government spurred its scientists and engineers to even greater efforts |
to get something into space to maintain the country’s prestige. However, the primary US satellite program, Vanguard, ran into difficulties: each of its early launches crashed or exploded. Simultaneously, a second team of rocket engineers and scientists had quietly been working on a military launch vehicle called Jupiter-C. Van Allen spearheaded the design of the instruments aboard a small satellite that this vehicle would carry. On January 31, 1958, Van Allen’s Explorer 1 became the first US satellite in space. Unlike Sputnik, Explorer 1 was equipped to make scientific measurements of high-energy charged particles above the atmosphere. Van Allen and his team discovered a belt of highly charged particles surrounding Earth, and these belts now bear his name. This first scientific discovery of the space program made Van Allen’s name known around the world. Van Allen and his colleagues continued to measure the magnetic and particle environment around planets with increasingly sophisticated spacecraft, including Pioneers 10 and 11, which made exploratory surveys of the environments of Jupiter and Saturn. Some scientists refer to the charged-particle zones around those planets as Van Allen belts as well. (Once, when Van Allen was giving a lecture at the University of Arizona, the graduate students in planetary science asked him if he would leave his belt at the school. It is now proudly displayed as the university’s “Van Allen belt.”) Van Allen was a strong supporter of space science and an eloquent senior spokesperson for the American scientific community, warning NASA not to put all its efforts into human spaceflight, but to also use robotic spacecraft as productive tools for space exploration. Clouds and Atmospheric Structure The clouds of Jupiter (Figure 11.10) are among the most spectacular sights in the solar system, much beloved by makers of science-fiction films. They range in color from white to orange to red to brown, swirling and twisting in a constantly changing kaleidoscope of patterns. Saturn shows similar but much more subdued cloud activity; instead of vivid colors, its clouds have a nearly uniform butterscotch hue (Figure 11.11). 394 Chapter 11 The Giant Planets Figure 11.10. Jupiter’s Colorful Clouds. The vibrant colors of the clouds on Jupiter present a puzzle to astronomers: given the cool temperatures and the composition of nearly 90% hydrogen, the atmosphere should be colorless. One hypothesis suggests that perhaps colorful hydrogen compounds rise from warm areas. The actual colors are a bit more muted, as shown in Figure 11.2. (credit: modification |
of work by Voyager Project, JPL, and NASA) Different gases freeze at different temperatures. At the temperatures and pressures of the upper atmospheres of Jupiter and Saturn, methane remains a gas, but ammonia can condense and freeze. (Similarly, water vapor condenses high in Earth’s atmosphere to produce clouds of ice crystals.) The primary clouds that we see around these planets, whether from a spacecraft or through a telescope, are composed of frozen ammonia crystals. The ammonia clouds mark the upper edge of the planets’ tropospheres; above that is the stratosphere, the coldest part of the atmosphere. (These layers were initially defined in Earth as a Planet.) Figure 11.11. Saturn over Five Years. These beautiful images of Saturn were recorded by the Hubble Space Telescope between 1996 and 2000. Since Saturn is tilted by 27°, we see the orientation of Saturn’s rings around its equator change as the planet moves along its orbit. Note the horizontal bands in the atmosphere. (credit: modification of work by NASA and The Hubble Heritage Team (STScI/AURA)) The diagrams in Figure 11.12 show the structure and clouds in the atmospheres of all four jovian planets. On both Jupiter and Saturn, the temperature near the cloud tops is about 140 K (only a little cooler than the polar caps of Mars). On Jupiter, this cloud level is at a pressure of about 0.1 bar (one tenth the atmospheric pressure at the surface of Earth), but on Saturn it occurs lower in the atmosphere, at about 1 bar. Because the ammonia This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 11 The Giant Planets 395 clouds lie so much deeper on Saturn, they are more difficult to see, and the overall appearance of the planet is much blander than is Jupiter’s appearance. Figure 11.12. Atmospheric Structure of the Jovian Planets. In each diagram, the yellow line shows how the temperature (see the scale on the bottom) changes with altitude (see the scale at the left). The location of the main layers on each planet is also shown. Within the tropospheres of these planets, the temperature and pressure both increase with depth. Through breaks in the ammonia clouds, we can see tantalizing glimpses of other cloud layers that can form in these deeper regions of the atmosphere—regions that were sampled directly for Jupiter by the Galileo probe that fell into the planet |
. As it descended to a pressure of 5 bars, the probe should have passed into a region of frozen water clouds, then below that into clouds of liquid water droplets, perhaps similar to the common clouds of the terrestrial troposphere. At least this is what scientists expected. But the probe saw no water clouds, and it measured a surprisingly low abundance of water vapor in the atmosphere. It soon became clear to the Galileo scientists that the probe happened to descend through an unusually dry, cloud-free region of the atmosphere—a giant downdraft of cool, dry gas. Andrew Ingersoll of Caltech, a member of the Galileo team, called this entry site the “desert” of Jupiter. It’s a pity that the probe did not enter a more representative region, but that’s the luck of the cosmic draw. The probe continued to make measurements to a pressure of 22 bars but found no other cloud layers before its instruments stopped working. It also detected lightning storms, but only at great distances, further suggesting that the probe itself was in a region of clear weather. Above the visible ammonia clouds in Jupiter’s atmosphere, we find the clear stratosphere, which reaches a minimum temperature near 120 K. At still higher altitudes, temperatures rise again, just as they do in the upper atmosphere of Earth, because here the molecules absorb ultraviolet light from the Sun. The cloud colors are due to impurities, the product of chemical reactions among the atmospheric gases in a process we call photochemistry. In Jupiter’s upper atmosphere, photochemical reactions create a variety of fairly complex compounds of hydrogen and carbon that form a thin layer of smog far above the visible clouds. We show this smog as a fuzzy orange region in Figure 11.12; however, this thin layer does not block our view of the clouds beneath it. The visible atmosphere of Saturn is composed of approximately 75% hydrogen and 25% helium, with trace amounts of methane, ethane, propane, and other hydrocarbons. The overall structure is similar to that of Jupiter. Temperatures are somewhat colder, however, and the atmosphere is more extended due to Saturn’s 396 Chapter 11 The Giant Planets lower surface gravity. Thus, the layers are stretched out over a longer distance, as you can see in Figure 11.12. Overall, though, the same atmospheric regions, condensation cloud, and photochemical reactions that we see on Jupiter should be present on Saturn (Figure 11.13). Figure 11. |
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