Page Mars. Figure 9.17 Picture of Mars made by the Hubble Space Telescope.

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1 Page Mars Mars is named for the Roman god of war, presumably because of its distinctly reddish color. Compared with Mercury and Venus, Mars seems positively Earth like. Although its diameter is only about half and its mass about one tenth the Earth's, many other features are similar. The Martian day is just 39 minutes longer than an Earth day, and the tilt of its axis is almost the same as Earth's, so it experiences a similar sequence of seasons. On a warm day, the temperature at the Martian equator reaches about 50 F (10 C), and although winds sweep dust and patchy clouds of ice crystals through its sky, the Martian atmosphere generally allows astronomers on Earth to view its surface clearly. Such views show a world of familiar features. Polar caps of sparkling white contrast with the reddish color of most of the planet and are visible from Earth, as seen in figure Figure 9.17 Picture of Mars made by the Hubble Space Telescope. The similarities of Mars to Earth have excited interest in the planet, perhaps even as a place we might someday inhabit. A series of spacecraft sent to explore the planet Mariner, Vikings I and II, Mars Global Surveyor, Mars Odyssey, Mars Express, Mars Reconnaissance Orbiter, and many others have revealed the true marvels of the planet. Q What season is it in this picture? (North is at top of image.) answer The Surface of Mars Mars has some of the most dramatic surface features of any of the terrestrial planets. Along the equator runs a rift Valles Marineris that stretches 4000 kilometers (2500 miles) long, 100 kilometers (60 miles) wide, and 7 kilometers (4 miles) deep. This canyon, named for the Mariner spacecraft whose pictures led to its discovery, dwarfs the Grand Canyon and would span the continental United States. Planetary scientists used satellite data to reconstruct what Valles Marineris would look like from a high flying airplane (fig. 9.18).

2 Figure 9.18 A reconstructed view down Valles Marineris, the Grand Canyon of Mars. This image was constructed from hundreds of thousands of laser altimeter measurements made by the Mars Odyssey orbiter. This enormous gash may be a rift that began to split apart the Martian crust but failed to open farther. Mars At midlatitudes, a huge uplands called the Tharsis bulge (fig. 9.19) is dotted with enormous volcanic peaks. Another volcano at the edge of Tharsis, Olympus Mons, rises about 25 kilometers (about 16 miles) above its surroundings, nearly three times the height of Earth's highest peaks, and is illustrated in figure If ever interplanetary parks are established, Olympus Mons should lead the list. Figure 9.19 Topographic map of Mars showing its major features. The map is color coded according to elevations. Olympus Mons is the largest volcano in the Solar System, while Valles Marineris, the Grand Canyon of Mars, is an enormous gash in Mars's crust about 4000 kilometers (approximately 2500 miles) long. Were it on Earth, it would stretch from California to Florida.

3 Figure 9.20 (A) Olympus Mons, the largest known volcano (probably inactive) in the Solar System. (B) Profile of Olympus Mons; Mount Everest, the highest mountain above sea level on Earth; and the Hawaiian volcano Mauna Kea, rising from the sea floor. ANIMATION Mars Planetary geologists think that the Tharsis region formed as hot material rose from the deep interior of the planet and forced the surface upward as it reached the crust. The hot matter then erupted through the crust to form the volcanoes, some of which appear relatively young. For example, the small number of impact craters in its slopes implies that Olympus Mons is no older than 250 million years and that it may in fact have been active much more recently. Some planetary geologists think the Tharsis bulge may also have created the gigantic Valles Marineris, which lies to the southeast. According to this theory, Valles Marineris formed as the Tharsis region swelled, stretching and cracking the crust. Other planetary scientists think that this vast chasm is evidence for plate tectonic activity, like that of Earth, and that the Martian crust began to split, but the motion ceased as the planet aged and cooled. Page 237 Figure 9.21 shows the Martian polar caps. These frozen regions change in size during the cycle of the Martian seasons, a cycle resulting from the tilt of Mars's rotation axis in the same way that our cycle of seasons is caused by the tilt of Earth's rotation axis. The Martian seasons are more extreme than terrestrial ones because the Martian atmosphere is much less dense than Earth's, and therefore it does not retain heat as well.

4 Figure 9.21 Pictures of (A) the south Martian polar cap and (B) the north Martian polar cap. (C) Note the layered structure of the north polar cap visible in this view from the Mars Reconnaissance Orbiter. This highresolution image shows a portion of a chasm wall about 1.3 kilometers wide. Layers of dust and ice alternate in the chasm wall. Page 238 Because Mars's seasonal changes are so extreme, its polar caps vary greatly in size, shrinking during the Martian summer and growing again during the winter. Much of the visible part of the southern cap is frozen carbon dioxide dry ice and in winter its frost extends in a thin layer across a region some 5900 kilometers (about 3700 miles) in diameter, from the south pole to latitude 40, much as snow cover extends to middle latitudes such as New York in our winters. But because the frost is very thin over most of this vast cap, it shrinks in the summer to a diameter of about 350 kilometers (approximately 220 miles). The northern cap shrinks to a diameter of about 1000 kilometers (approximately 600 miles). Although the caps have a surface layer of CO 2, the bulk of the frozen material is ordinary water ice, as deduced from its temperature and radar studies. The northern cap consists of numerous separate layers, as can be seen in Figure 9.21C. These strata indicate that the Martian climate changes cyclically. Thus, Mars may have ice ages similar to those on Earth. Why do the Martian polar caps differ so? Altitude measurements made by the Mars Global Surveyor spacecraft show that Mars's south pole is considerably higher in elevation and is thus much colder than

5 its north pole. This creates a strong wind pattern that carries water vapor and carbon dioxide away from the south pole toward the north pole. There, it precipitates out, leading to a larger north polar cap. Satellites have been able to estimate the thickness of the ice caps using radar signals. From the depth of ice measured, there appears to be enough water in the caps to cover the entire surface of Mars with water to a depth of at least 10 meters (30 feet) The Martian poles are bordered by immense deserts with dunes blown into parallel ridges by the Martian winds, as illustrated in figure Huge dust storms blow the fine red dust over the entire surface of the planet, giving the planet its characteristic color. What makes it so red? The color comes from the iron minerals in its surface rocks. We know from everyday experience that a piece of iron will become rustcolored when exposed to air. Here on Earth, such rusting occurs because the iron combines with oxygen in our atmosphere to form iron oxides and other compounds. On Mars, even though there is little oxygen in its atmosphere, other chemical reactions with the iron in its surface minerals lead to the same effect. Figure 9.22 Mars Odyssey image of dunes that surround northern polar cap. Color tinting has been added ranging from blue for colder and yellow for warmer based on the thermal imaging system. Page 239 Water on Mars Over the last several decades, scientists have sent dozens of spacecraft to Mars. Six have successfully landed on the surface and sent back to Earth pictures and measurements of the Martian landscape and close up images of rocks. These missions have many goals, but one of the major ones is searching for evidence that liquid water was once present on Mars. Why is liquid water of such interest? The answer is simple: scientists who study the possibility of life elsewhere in the Universe think that liquid water is a critical ingredient for living organisms of almost any type. Thus, the search for water is a first step in the search for life. Perhaps the most intriguing features revealed by the two Viking orbiters in the 1970s were dry riverbeds, such as those seen in Figure 9.23A. We infer that water once flowed on Mars along these winding channels, which look so similar to tributaries converging to make a large river channel on Earth. Some of the channels appear to have once had major flows that carved teardrop shaped islands around crater rims (fig. 9.23B). In fact, many astronomers interpret the observed features to indicate that lakes and small oceans once existed on Mars. Evidence for these ancient bodies of water are seen in smooth terraces that

6 look like old beaches around the inner edges of craters and basins, as you can see in the crater lake in Figure 9.23C. Narrow canyons breach this crater's rim, showing where water flowed in from the south and drained out into lowland areas to the north, with the ancient shoreline still visible even though no surface liquid is present now. Figure 9.23 Images suggesting past water on Mars from the Viking orbiter. All three images have the same scale and are oriented with north at top. (A) Picture of channels probably carved by running water on Mars. (B) Teardrop shaped islands formed as water flowed (from bottom toward top of figure) around the rims of craters. (C) A Martian crater thought to have once been a crater lake. Note the inflow channel at the south end of the crater, and the outflow channel to the northeast. The smooth floor (apart from a few small craters) suggests that the crater bottom is covered with sediment left behind as the lake dried out. The images from the Viking orbiters provided strong evidence that liquid water was once present on Mars, but it raised even more questions. How long ago was the water present? Was it ever a long term feature of Mars, or did it occur in violent episodes of melting? For example, if there was ice buried under the soil, perhaps it was melted by an impact, with a sudden catastrophic flood. Furthermore, some features, such as the crater lake in Figure 9.23C, could also be interpreted as arising from lava flows, so were there ever standing bodies of water on Mars? And does much water remain today, perhaps frozen in the ground, or has it almost all evaporated and dissociated as on Venus, with the hydrogen lost into space? Page 240 The Viking landers provided our first view of the Martian surface in the 1970s, and in the 1990s the Mars Pathfinder mission provided a demonstration that we could operate a roving science vehicle across the surface of Mars. The next two NASA landers, Opportunity and Spirit, reached Mars in 2004 and carried out a remarkable set of explorations that far exceeded the original plan. These two missions were designed to explore Mars's surface, landing at sites that were chosen because pictures and spectral data taken from orbit suggested that water might have been present there long ago. Spirit landed in the center of the 150 km (90 mile) diameter Gusev Crater, a smooth floored crater at the end of a narrow Martian valley that appeared to have once been flooded. Figure 9.24 shows a panorama from the Spirit lander site. Opportunity landed on the flat plains along the Martian equator on the opposite side of Mars from Spirit. Both craft deployed rovers small, wheeled vehicles that can move away from the landing site and explore interesting features (fig. 9.25).

7 Figure 9.24 A panoramic view of the Bonneville Crater, lying in the floor of the much larger Gusev Crater on Mars. This spot is near where the Spirit spacecraft landed. Figure 9.25 A Martian rover. Cameras, a rock drill, and analysis instruments are powered by solar panels on the top of the rover. Both rovers were highly successful in their searches. For example, Opportunity took the pictures shown in figure 9.26, which both suggest processes that involved liquid water. Figure 9.26A shows small spheres (dubbed Martian blueberries ) that are made of hematite, which normally occurs from depositions of minerals in water. Figure 9.26B shows a rock outcropping thought to be material deposited

8 in an ancient, now dried up small sea. Examination of the rocks shows that they contain layers that are typical of sediment that sank to the bottom of a body of water and later was transformed into rock. This image also shows that the layers are wavy, similar to the ripple marks you see at the beach as water washes back and forth across the sand. Moreover, minerals in the rocks at the Opportunity site have a chemical makeup consistent with their having been deposited in a salty lake or small ocean. Half a world away Spirit found more layered rocks and other minerals that normally form in water. The evidence from the rovers has convinced many scientists that Mars once had large areas under liquid water. The original mission for the two rovers was planned for only 90 days, but the two rovers operated long past that, surviving cold winters and global dust storms. Spirit finally stopped responding in 2010, but Opportunity continues traveling over the Martian surface in Figure 9.26 (A) Small spherical blueberries most likely formed from iron depositions in standing water. The lighter circular area was swept off by the Opportunity rover to study the rock type. (B) A close up image of a rock outcropping at the landing site. The rocks show thin layers and contain minerals that suggest that they were formed on the bottom of a salty lake. Page 241 A larger rover, named Curiosity, reached Mars in August Curiosity has sophisticated instruments to analyze the geology and chemistry within Gale Crater, which possibly once contained a lake. Curiosity will climb and analyze the layered central mountain in this crater (fig. 9.27). Another important experiment was carried out by the Phoenix lander, which reached the northern polar region of Mars in Phoenix carried out experiments on the soil, showing that it is quite different from Earth's about as alkaline as baking soda with high levels of oxidizing chemicals. As the lander scooped up soil samples, it exposed ice below the surface (fig. 9.28). Figure 9.27 (A) The rover Curiosity landed in Gale crater, near the base of the central mountain, known as Mount Sharp, which it will climb and study. (B) A view in the direction of where Curiosity will begin its climb up Mount Sharp shows the layered structure.

9 Figure 9.28 The Phoenix lander scooped up soil samples, revealing ice under the polar dust. Meanwhile, a wide assortment of satellites have continued to send back detailed pictures of the surface of Mars with high resolution, allowing planetary scientists to build a more detailed understanding of Martian geology. The terraced layers in Figure 9.29A are at the bottom of one of Mars's great canyons. Such features on Earth are usually laid down deep underwater. A detailed examination of the scouring of features in Figure 9.29B implies that water carved these features with a flow exceeding that of the Mississippi River. Figure 9.29C shows a crater at a latitude of about 70 North that contains a large ice field, perhaps the remains of a lake. Figure 9.29

10 (A) Image from the Mars Global Surveyor of terraced features at the bottom of a Martian canyon. (B) Mars Odyssey image of former riverbed in which teardrop shaped islands formed behind craters, and the surface layers were scoured by the stream flow. (C) Mars Express image of a crater with a lake of ice in its interior. In addition to being able to make highly detailed images, each orbiter includes a variety of other detectors. Some instruments can measure the spectrum of light reflected from the ground, others collect radiation at other wavelengths. From these data, astronomers can deduce what minerals are present at different spots on the Martian surface. Matching the composition of those minerals with data on whether water is needed to produce those minerals gives additional evidence that Mars was once much wetter. Combining spectral information with the appearance of features can provide a much clearer picture of the geological origins of features, as illustrated in Extending Our Reach: Analyzing Martian Geology. Page 242 Extending our reach ANALYZING MARTIAN GEOLOGY Many features on Mars appear to be caused by water flow, but appearances can be deceiving. Similar looking features can often be produced by lava flows, for example. How then can we be sure? One way of deciding on the origin of a feature is to study the chemistry of the rocks left behind. We do not have to retrieve rock samples to do this. Visible and infrared spectroscopy can show what minerals are present. The image in Figure 9.30 is an example of an outflow into what is thought to have been a crater lake. An imaging spectrometer on the Mars Reconnaissance Orbiter has identified clays and minerals that indicate a body of water must have lasted for at least thousands of years.

11 Figure 9.30 Mars Reconnaissance Orbiter image of fanning outflow in a former crater lake. Spectral analysis of the surface features identified minerals normally found underwater, and different types of materials, such as clays, have been color coded in this image. Other instruments on the Mars Odyssey orbiter can detect hydrogen atoms locked up in water in the Martian soil by studying gamma rays. These measurements have revealed what appears to be huge amounts of water, probably ice, in the upper meter of the Martian surface (fig. 9.31). Astronomers are both excited and puzzled by the possibility that water was present on Mars. They are excited because of the belief that where there is water, there may be life even if only microbes. They are puzzled because Mars does not have conditions that allow liquid water to be present on the planet today. To understand why, we need to look at the properties of the Martian atmosphere.

12 Figure 9.31 Global map of the likely percentage of water in Martian surface layers. The Mars Odyssey can measure high energy radiation that is generated when cosmic rays from space strike the planet's surface. This radiation is reduced where there are hydrogen atoms, permitting scientists to estimate the amount of water that is present. Page 243 The Martian Atmosphere Clouds and wind blown dust are visible evidence that Mars has an atmosphere. Spectra and direct sampling by spacecraft landers confirm this and show that the atmosphere is mostly (95%) carbon dioxide with small amounts (3%) of nitrogen and traces of oxygen and water. From this, astronomers can determine the density of Mars's atmosphere, which turns out to be very low only about 1% the density of Earth's. This density is so low that, although the Martian atmosphere is mostly carbon dioxide, it creates only a very weak greenhouse effect. The consequent lack of heat trapping and Mars's greater distance from the Sun leaves the planet very cold. Temperatures at noon at the equator may reach a little above the freezing point of water, but at night they plummet to far below freezing. The resulting average temperature is a frigid 218 K ( 67 F). Thus, although water exists on Mars, it is frozen solid, locked up either below the surface in the form of permafrost or in the polar caps as solid water ice. Clouds of dry ice (frozen CO 2 ) and water ice crystals (H 2 O) drift through the Martian atmosphere carried by the Martian winds. These winds, like the large scale winds on Earth, arise because air that is warmed near the equator rises and moves toward the poles. This flow from equator to poles, however, is deflected by the Coriolis effect arising from the planet's rotation. The result is winds that blow around the planet approximately parallel to its equator, and small tornadoes or dust devils (fig. 9.32A). The Martian winds are generally gentle, but seasonally and near the poles they become gales, which sometimes pick up large amounts of dust from the surface. The resulting vast dust storms occasionally cover the planet completely and turn its sky pink.

13 Figure 9.32 (A) A dust devil (small tornado) spotted by the Mars Reconnaissance Orbiter. (B) Fog in Martian valleys seen by the Viking orbiter. (C) Frost on surface rocks near the Viking lander. No rain falls from the Martian sky, despite its clouds, because the atmosphere is too cold and contains too little water. In fact, there is so little water in the Martian atmosphere that even if all of it were to fall as rain, it would make a layer only about 12 micrometers deep (less than 1/2000 inch). For comparison, Earth's atmosphere holds enough water to make a layer a few centimeters (inches) deep. Despite such dryness, however, fog sometimes forms in Martian valleys (fig. 9.32B), and frost condenses on the ground on cold nights (fig. 9.32C). In addition, during the Martian winter, CO 2 snow falls on the Martian poles. Mars has not always been so dry, as we saw from the numerous channels in its highlands and other evidence from many Mars missions. But for a planet to have liquid water, it must have a temperature above freezing with a high enough pressure to keep the liquid from immediately boiling away. If the pressure on a liquid is very low, molecules can break free from its surface, evaporating easily because no external force restrains them. On the other hand, if the pressure is high, molecules in a liquid must be heated strongly to turn them into gas. For example, at normal atmospheric pressure, water boils at 100 Celsius. If the pressure is reduced, however, the boiling point drops, an effect used by food producers to make freeze dried foods, such as instant coffee. Coffee is brewed normally, then frozen and placed in a chamber from which the air is pumped out. The reduced pressure makes the liquid boil without heating and evaporate, leaving only a powder residue instant coffee. Similarly, on Mars, any liquid water on its surface today would evaporate. Page 244 The existence of channels carved by liquid water on Mars is therefore strong evidence that in its past, Mars was warmer and had a denser atmosphere. However, that milder climate must have ended billions of years ago. How do we know? The large number of impact craters on Mars shows that its surface has not been significantly eroded by rain or flowing water for about 3 billion years. Why did Mars dry out, and where have its water and atmosphere gone? Some water probably lies buried below the Martian surface as ice, as indicated by measurements from the Mars Odyssey orbiting spacecraft. If the Martian climate was once warmer and then cooled drastically, water would condense from its atmosphere and freeze, forming sheets of surface ice. Wind might then bury this ice under protective layers of dust, as happens in polar and high mountain regions of Earth. Figure 9.33 shows possible evidence that this buried ice continues to melt and trickle out of subsurface layers even today. The series of images made by the Mars Reconnaissance Orbiter shows dark lines growing down the inner wall of a crater over several months of the Martian summer. Scientists hypothesize that frozen groundwater is melting and seeping down the side of the crater.

14 Figure 9.33 Sequence of images of the wall of a crater from early to midsummer. As the crater warms, dark streaks extend down the crater wall, resembling wet patches. It is possible that a briny solution, melting and seeping out from underground layers, might produce these features. Pure water would evaporate too quickly to keep the soil moist. If Mars had a denser atmosphere in the past, as deduced from the higher pressure needed to allow liquid water to exist, then the greenhouse effect might have made the planet significantly warmer than it is now. The loss of such an atmosphere would have weakened the greenhouse effect and plunged the planet into a permanent ice age. Such a loss could happen in at least two ways. According to one idea, repeated asteroid impacts on Mars when it was young may have blasted its original atmosphere off into space. Such impacts, although rare now, did occur for hundreds of millions of years after the Solar System began to form. For example, some of the maria on the Moon date to impacts about 3.9 billion years ago. A less dramatic explanation for how Mars lost most of its atmosphere is that Mars's low gravity allowed gas molecules to escape over the first 1 to 2 billion years of the planet's history. Regardless of which explanation is correct, the loss of its atmosphere would have cooled the planet and locked up its remaining water as permafrost. But why have the Martian volcanoes not replenished its atmosphere, keeping the planet warm? Astronomers believe that the blame lies with Mars's low level of tectonic activity, a level set by conditions in its interior. Page 245 The Martian Interior Astronomers think that the interior of Mars is differentiated like the Earth's into a crust, a mantle, and an iron core (fig. 9.34). However, Mars is so small compared with the Earth that its interior is cooler. Mars's smaller mass supplies less heat, and its smaller radius allows the heat to escape more rapidly as for the Moon (Chapter 7). We have no direct confirmation of Mars's interior structure because no functioning seismic detectors have landed there yet. Thus, as is the case for Mercury and Venus, astronomers must rely on indirect evidence from its density and gravitational field to learn about the interior of Mars. Using the Mars Global Surveyor spacecraft in orbit around the planet, astronomers have measured Mars's magnetic field and internal structure. They concluded that Mars has a metallic core whose radius is approximately 1700 km, or about half of the overall radius of the planet. But Mars, unlike Earth, has no planetwide magnetic field, so its core is probably no longer molten.

15 Figure 9.34 Artist's sketch of the interior of Mars. Having a mass between that of dead Mercury and lively Earth and Venus implies that Mars should be intermediate in its tectonic activity. Such seems to be the case. Although it possesses numerous volcanic peaks and uplifted highlands, implying that it had an active crust, at least in the past, Mars bears no evidence of large scale crustal motion like the Earth's. For example, it has no folded mountain ranges. Astronomers therefore think that Mars has cooled and its crust thickened to perhaps twice the thickness of the Earth's crust. As a result, the now weak interior heat flow can no longer drive tectonic motions. A thick Martian crust may also explain why Mars has a small number of very large volcanoes, while the Earth has a large number of small ones. The volcanoes may have grown over hot spots in the core, and the crust did not shift to new positions. Mars's immense volcanoes are thus mute testimony to a more active past. Mars's current low level of tectonic activity is also demonstrated by the many impact craters that cover its older terrain, far more than are seen on either Earth or Venus. The number of those craters implies that Mars has been geologically quiet for billions of years. Mars is probably not dead, however, because some regions (for example, the slopes of Olympus Mons and other volcanoes) are essentially free of craters. Thus, these immense peaks may still occasionally erupt. They do not erupt often enough, however, to replace the gas lost to space because of the planet's low gravity. It appears that Mars has entered a phase of planetary old age. Recently, however, using Earth based telescopes, astronomers have detected methane in spectra of the Martian atmosphere. Because methane is rapidly destroyed in the Martian environment, this means that Mars must be producing it currently, indicating that there is still at least a low level of geological activity. The Martian Moons Mars has two tiny moons, Phobos and Deimos, which are named for the demigods of Fear and Panic (fig. 9.35). These bodies are only about 20 kilometers across and are probably captured asteroids. They are far too small for their gravity to have pulled them into spherical shapes. Both moons are cratered, implying bombardment by smaller objects. Phobos has cracks, suggesting that it may have been struck by a body large enough to split it nearly apart.

16 Figure 9.35 Picture of Phobos and Deimos, the moons of Mars. These tiny bodies are probably captured asteroids. Q. What does the irregular shape of these bodies tell you about the strength of their surface gravity? Is it likely these moons have any atmosphere of their own? answer Phobos and Deimos were discovered in 1877, but by chance they appeared in literature nearly two centuries earlier in Jonathan Swift's book Gulliver's Travels. Gulliver stops at the imaginary country Laputa whose inhabitants include numerous astronomers. Among the accomplishments of these people is the discovery of two tiny moons of Mars. Even earlier, Kepler guessed that Mars might have two moons because the Earth has one moon and Jupiter, at least in Kepler's time, was known to have four. Mars, lying between these two bodies should therefore (according to Kepler's mystic argument) have a number of moons lying between 1 and 4, and he chose 2 as the more likely case. Page 246 Life on Mars? Scientists have long wondered whether living organisms developed on Mars. Much of that interest grew from a misinterpretation of observations made in 1877 by the Italian astronomer Giovanni Schiaparelli. Schiaparelli saw what he took to be straight line features on Mars and called them canali, by which he meant channels. In English speaking countries, the Martian canali became canals, with the implication that Mars must be inhabited by intelligent beings who built them. The interest in these canals had become so great by 1894 that the wealthy Bostonian Percival Lowell built an observatory in northern Arizona to study Mars and search for signs of life there (fig. 9.36).

17 Figure 9.36 Drawing of Mars made by Percival Lowell around Lowell thought he could see straight line features that he believed were canals for irrigation or travel. Most astronomers could see no trace of the alleged canals, but they did note seasonal changes in the shape of dark regions, changes that some interpreted as the spread of plant life in the Martian spring. By the early 1970s, scientists were excited by satellite photographs of water carved canyons and old riverbeds because water at least on Earth is so important for life. Therefore, to further the search for life on Mars, the United States landed two Viking spacecraft on the planet in These craft carried instruments to search for signs of carbon chemistry in the soil and to look for metabolic activity in soil samples that were put in a nutrient broth carried on the lander. All tests either were negative or ambiguous. Then, in 1996, a group of American and English scientists reported possible signs of life in rocks from Mars. These were not samples returned to Earth by a spacecraft but samples of meteorites found in the Antarctic. They arrived here after being blasted off the surface of Mars, presumably by the impact of a small asteroid. Such impacts are not uncommon, but most fragments are scattered in space or fall back to Mars. Moreover, of those that are shot into space, only a tiny fraction have just the right combination of speed and direction to reach Earth. How can astronomers tell if a meteorite has come from Mars? One way is to sample the gas trapped in tiny bubbles in the meteorite and see if it matches the composition of Mars's atmosphere as measured by the Viking Mars landers. For the meteorite in question, the match was excellent, assuring that it came from Mars. Scientists have even been able to match mineralogical details of the meteorite to a probable origin in the Valles Marineris region. What was the evidence suggesting life? That turns out to be far more controversial. Microscopic

18 examination of samples from the interior of the meteorite revealed many tiny, rod shaped structures (fig. 9.37). These look very much like ancient terrestrial bacteria but are much smaller. Some scientists suggest they are fossilized primitive Martian life. The meteorite also contains traces of organic chemicals known as polycyclic aromatic hydrocarbons (PAHs, for short). Terrestrial bacteria make such chemicals when they die and decay, but PAHs can also form spontaneously, given the proper mix of chemicals. In fact, they have been found in a number of non Martian meteorites and have also been detected by their spectrum lines in the radio emission from interstellar gas and dust clouds. Other structures in the meteorite can also be interpreted as having a biological origin. But other scientists have shown that ordinary chemical weathering can form very similar structures. As a result, most scientists today are unconvinced that any meteorite yet studied shows evidence of Martian life. Figure 9.37 Fossils of ancient Martian life? The tiny rod shaped structures look similar to primitive fossils found in ancient rocks on Earth. However, some scientists think these structures formed chemically. A more immediate opportunity to search for evidence of past life on Mars is provided by the rover Curiosity, which began exploring Mars in August Curiosity landed in a crater that is thought to have once been a lake or sea that filled with sediments for more than a billion years. Later erosion exposed the central mountain, which shows terraced layers. The rover can climb the steep slopes of the mountain to investigate different geological layers, which may help reveal how the Marian climate changed over time. The rover carries a variety of scientific instruments to study the chemical composition of the rocks and soil and to examine samples microscopically. It will be able to assess whether Mars may once have been habitable, and perhaps it will discover clear signs that life attempted to arise when this cold dusty world was young.