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Mars_water0 - :es 1ClS cal ids 1311 m ” Figure 9.26...

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Unformatted text preview: :es 1ClS cal ids 1311 m _ ...._: ”'..' Figure 9.26 Olympus Mons, the tallest volcano in the solar sys— tem, covers an area the size of Arizona and rises higher than Mount Everest on Earth. Note the tall cliff around its rim and the central volcanic crater from which lava erupted. meters across, large enough to cover an area the size of Arizona. Its peak stands about 26 kilometers above the av— erage Martian surface level, or some three times as high as Mount Everest stands above sea level on Earth. Much of Olympus Mons is rimmed by a cliff that in places is 6 kilo— meters high. Olympus Mons and several other large volcanoes are concentrated on or near the continent—size Tharsis Bulge (see Figure 9.25). Tharsis, as it is usually called, is some 4,000 kilometers across, and most of it rises several kilome— ters above the average Martian surface level. It was proba- bly created by a long-lived plume of rising mantle material that bulged the surface upward and provided the molten rock for the eruptions that built the giant volcanoes. Could volcanoes still be active on Mars? Until recently, we didn’t think so. We expect Mars to be much less volcani— cally active than Earth, because its smaller size has allowed its interior to cool much more, and Martian volcanoes show enough impact craters on their slopes to suggest that they have been inactive for at least tens of millions of years and possibly much longer. However, geologically speaking, tens of millions of years is not that long. In addition, radiometric dating of meteorites that appear to have come from Mars (so-called Martian meteorites [Section 24.3l) shows some of them to be made ofvolcanic rock that solidified from molten lava as little as 180 million years ago—quite recent in the 4.5—billion-year history of the solar system. Given this evidence of geologically recent volcanic eruptions, it is likely that Martian volcanoes will erupt again someday, though not necessarily in our lifetimes. Nevertheless, the Martian interior is presumably cooling and its lithosphere thickening, so that Mars will become as geologically dead as the Moon and Mercury within a few billion years. Tectonics and Valles Marineris Mars also has tectonic features, though none on a global scale like the plate tectonics of Earth. The most prominent tectonic feature is the long, deep system of valleys called Valles Marineris (Figure 9.27). Named for the Mariner 9 spacecraft that first imaged it, Valles Marineris extends almost a fifth of the way along the planet’s equator. It is as long as the United States is wide and almost four times as deep as Earth’s Grand Canyon. N 0 one knows exactly how Valles Marineris formed. Parts of the canyon are completely enclosed by high cliffs on all sides, so neither flowing lava nor water could have been responsible. Extensive cracks on its western end run up against the Tharsis Bulge (see Figure 9.25), suggesting a connection between the two features. Valles Marineris may have formed through tectonic stresses accompanying the uplift of material that created the Tharsis Bulge, cracking the surface and leaving the tall cliff walls of the valleys. 0 What geological evidence tells us that water once flowed on Mars? Impacts, volcanism, and tectonics explain most of the major geological features of Mars, but closer examination shows ample evidence of features of erosion. For example, Figure 9.28 looks much like dry riverbeds on Earth seen from above. These channels appear to have been carved by running water, though no one knows whether the water came from runoff after rainfall, from erosion by water—rich debris flows, or from an underground source. Regardless of the specific mechanism, water was almost certainly respon— sible, because it is the only substance that could have been liquid under past Martian conditions and that is sufficiently abundant to have created such extensive erosion features. Figure 9.27 Valles Marineris is a huge system of valleys on Mars created In part by tectonic stresses. 200 km chapter 9 0 Planetary Geology 269 Figure 9.28 This photo, taken by the Viking Orbiter, shows what appear to be dried—up riverbeds. Toward the top of the image we see many individual tributaries, which merge into the larger ”river” near the lower right. Counts of craters near the channels indicate that they formed more than 3 billion years ago. If you were to Visit Mars, however, the idea that parts of the surface were shaped by flowing water might seem quite strange. No liquid water exists anywhere on the surface of Mars today. We know this not only because we’ve studied most of the surface in reasonable detail, but also because the surface conditions would not allow liquid water to be present as lakes, rivers, or even puddles. In most places and at most times, Mars is so cold that any liquid water would immedi— ately freeze into ice. Even when the temperature rises above freezing, as it often does at midday near the equator, the air pressure is so low that liquid water would quickly evap— orate [Section 5.3]. In other words, liquid water is unstable on Mars today: If you donned a spacesuit and took a cup Figure 9.29 More orbital evidence of past water on Mars. a This photo shows a broad region ofthe southern highlands on Mars. The eroded rims of large craters and the lack of many small craters suggest erosion by rainfall. 270 p a rt lll ' Learning from Other Worlds b This close—up view shows the floor of the c a, This computergenerated perspective crater shown earlier in Figure 9,9c.The sculpted patterns appear to be layers of sedimentary rock that were laid down at a time when the crater was filled with water of water outside your pressurized spaceship, the water would either freeze or boil away (or both) almost immediately. Evidence for Ancient Water Flows The fact that we see water erosion features on Mars but that liquid water is absent today tells us that Mars must once have had very different surface conditions—conditions such as warmer tempera- tures and greater air pressure that would have allowed water to flow and rain to fall. This warmer and wetter pe— riod must have ended long ago, as you can see by looking again at Figure 9.28. Notice that a few impact craters lie on top of the channels. From counts of the craters in and near them, it appears that these channels are at least 2~3 billion years old, meaning that water has not flowed through them since that time. Other orbital evidence also argues that Mars had rain and surface water in the distant past. Figure 9.2% shows a broad region of the ancient, heavily cratered southern highlands. Notice the indistinct rims of many large craters and the relative lack of small craters. Both facts argue for ancient rainfall, which would have eroded crater rims and erased small craters altogether. Figure 9.2% shows a close- up of a crater floor with sculpted patterns that suggest it was once the site of a lake. Ancient rains may have filled the: crater, allowing sediments to build up from material that settled to the bottom. The sculpted patterns in the crater bottom may have been created as erosion by wind exposed- layer upon layer of sedimentary rock, much as erosion by the water in the Colorado River exposed the layers visible in the walls of the Grand Canyon on Earth. Figure 9.29c shows a three-dimensional perspective of the surface that suggests water once flowed between two ancient crater lakes. landing ite of mo k n .-. -- . . . l Spun IUUEEl view shows how a Martian valley forms a natural passage between two possible ancient lakes (shaded blue).\/ertical relief is exaggerated l4 times to reveal the topography In 2004, the Spirit rover landed in the crater (called Gusev) at the bottom ofthis picture, at kes. :tive lent aratecl l, the Isev) Surface studies have provided additional evidence for past water on Mars. In 2004, two robotic rovers named Spirit and Opportunity landed on nearly opposite sides of Mars. Spiritlanded in Gusev Crater, the site ofthe possible ancient lake shown in Figure 9.29c. Opportunity landed in the Meridiani Plains, where orbital spacecraft had detected spectroscopic hints of minerals that form in water. The twin rovers carried dozens of cameras, several instruments to identify rock composition, and a grinder to expose fresh rock for analysis. The rovers, designed for just 3 months of operation, were still going strong as this book was being written, more than a year and a half after their arrival. Both rovers found solid evidence that liquid water was once plentiful on Mars. Rocks at the Opportunity landing site contain tiny spheres—nicknamed “blueberries” al- though they’re neither blue nor as large as the berries we find in stores—and odd indentations suggesting that they formed in standing water, or possibly by groundwater per- colating through rocks (Figure 9.30). Compositional analy- sis showed that the abundant “blueberries” contained the iron—rich mineral hematite, and other rocks contained the sulfur-rich mineral jarosite; both minerals form in water. Detailed chemical analysis supports the case for formation in an acidic, salty environment—possibly a sea or ocean. Moreover, a close look at the layering ofthe sedi- mentary rocks suggests a changing environment of waves and/ or wind. Taken together, the orbital and surface stud— ies provide overwhelming evidence for abundant water in Mars’s past. Martian Water Today If water once flowed over large portions of Mars, where did it all go? As we’ll discuss in Chapter 10, much of the water was probably lost to space forever. How- ever, significant amounts of water apparently still remain, frozen at the polar caps and in the top meter or so of the surface soil around much of the rest of the planet (Fig- ure 9.31). If water ice exists on Mars this close to the surface, even more water probably lies deeper underground. If there is still volcanic heat on Mars, this water may sometimes melt and flow. Although we have found no geological evidence to suggest that any large-scale water flows have occurred on Mars in the past billion years, orbital photographs offer tantalizing hints of smaller—scale water flows in much more recent times. The strongest evidence for liquid water in recent times comes from photos of gullies on crater and channel walls. In Figure 9.32, for example, note the striking similarity to the gullies we see on almost any eroded slope on Earth. a This sequence zooms in on a rock outcropping near the rover's landing site.The outcrop stands about knee-highThe close—up shows a piece of the rock about 3 centimeters across (about the width ofyour big toe).The layered structure,the odd indentations, and the small sphere all support the idea that the rock formed from sediments in standing water. enter-w,r 'l by l or mntl b The wall of Endurance Crater, where the impact exposed deep bedrock layers profoundly altered by water. Close—ups show the durable hematite “blueberries" eroding out of the rock and tumbling downhill in piles. Changing tilts ofthe rock layers hint at changing wind or waves during formation. Figure 9.30 These photos were taken by the Opportunity rover, which landed on Mars in 2004. Figure 9.3“ This map, made with data from Mars Odyssey. rep, resents the hydrogen content ofthe Martian surface soil. The blue areas contain the most hydrogen. probably because they repre— sent regions in which the top meter or so of surface soil contains water ice. chapter9 - PlanetaryGeology 27l Figure 9.32 This photograph from the Mars Global Surveyor shows gullies on a crater wall. The gulf lies may have been formed by water melting under the protective cover of snowpack. One hypothesis suggests that the gullies form when snow accumulates on the crater walls in winter and melts away in spring. Because the gullies are relatively small (note the scale bar in Figure 9.32), they should be gradually covered over by blowing sand during Martian dust storms. Gullies that are still clearly Visible must be no more than a few million years old. Geologically speaking, this time is short enough to make it quite likely that water flows are still forming gul- lies today. Thus, while Mars’s era of abundant surface water is long gone, small amounts of liquid water may still flow on occasion. Even if water sometimes does still flow, Mars clearly was much warmer and wetter at times in the past than it is today. Ironically, Percival Lowell’s supposition that Mars was drying up has turned out to be basically correct, al— though in a very different way than he imagined. We’ll discuss the reasons for Mars’s dramatic climate change in Chapter 10. 9.5 Geology of Venus The surface of Venus is searing hot with brutal pressure [Section 7.2], making it seem quite unlike the “sister planet” to Earth it is sometimes called. However, beneath the sur- face, Venus must be quite similar to Earth. Venus is nearly the same size as Earth (see Figure 9.1), with a radius only about 5% smaller than Earth’s. Venus and Earth are also quite close together relative to the overall size of the solar system, so both should have been built from the same kinds of planetesimals. The very similar densities of Venus and Earth (see Table 7.1) offer further evidence that both plan— ets have the same overall composition. With the same basic size and the same basic composi— tion, we expect the interiors of Venus and Earth to be quite similar in structure and to retain about the same level of internal heat today. Nevertheless, we see ample evidence of “skin—deep” differences in surface geology. - What are the major geological features of Venus? Venus’s thick cloud cover prevents us from seeing through to its surface, but we can study its geological features with 272 part III - Learning from Other Worlds radar because radio waves can pass through clouds. Radar mapping bounces radio waves off the surface and uses the reflections to create three-dimensional images of the sur- face. From 1990 to 1993, the Magellan spacecraft used radar to map the surface of Venus, discerning features as small as 100 meters across. Scientists have named almost all the geo- logical features on Venus for goddesses and famous women. Figure 9.33a shows a global map of Venus based on the Magellan radar observations. Three large, elevated “conti- nents” (the three regions labeled “Terra” in Figure 9.33a) are the biggest features on the surface. Venus has some impact craters (Figure 9.33b), but far fewer than Mercury, the Moon, or Mars. Moreover, the rel- atively few impact craters are distributed fairly uniformly over the entire planet, suggesting that the surface is about the same age everywhere. Precise crater counts suggest that the surface of Venus is about 750 million years old. Thus, no craters from the heavy bombardment remain, and the only craters we see on Venus are those created by impacts during the past 750 million years. Older craters must have been erased by other geological processes. Geologists have also noted an absence of small craters on Venus—the very type of crater that is most common on other worlds. Ap- parently, Venus lacks small craters because small impactors completely burn up as they enter Venus’s thick atmosphere.- Other terrestrial worlds have smaller craters (as well as large ones), because their thinner atmospheres allow smaller objects to survive the plunge to the surface. Volcanism is clearly important on Venus. The surface shows abundant evidence of lava plains and many volcanic mountains. Some mountains are shield volcanoes (Fig- ure 9.33c), indicating eruptions in which the lava was about as runny as the lava that formed the Hawaiian Islands on Earth. A few volcanoes have steeper sides, indicating erup- tions of a higher—viscosity lava (Figure 9.33d). Volcanoes almost undoubtedly remain active on Venus, though we have not observed any eruptions. However, the composi— tion of Venus’s clouds suggests that volcanoes must still be active on geological timescales (erupting within the past 100 million years). The clouds contain sulfuric acid, which is made from sulfur dioxide (802) and water. Sulfur diox- ide enters the atmosphere through volcanic outgassing, but once in the atmosphere it is steadily removed by chemical reactions with surface rocks. Thus, the fact that sulfuric acid clouds still exist means that outgassing must continue to supply sulfur dioxide to the atmosphere. The most remarkable features on Venus are tectonic in origin. Its crust is quite contorted. In some regions, the surface appears to be fractured in a regular pattern (Figure 9.33e). Other tectonic features, called coronae (Fig- ure 9.33f), point directly to mantle convection on Venus. Coronae (Latin for “crowns”) were probably formed by hot. rising plumes in the mantle. The plumes push up on the crust, forming concentric tectonic stretch marks on the surface. The plumes also force lava to the surface, explain— ing why numerous volcanoes are found near coronae. The biggest difference between the geology of Venus and that of Earth is the lack of erosion on Venus. We might ...
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