NEWSFOCUS
Destination Mount Sharp. This sediment pile holds a history of water on early Mars for Curiosity to read.
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begins to climb the slopes of Mount Sharp? “That is the $2.5 billion question,” says planetary spectroscopist Ralph Milliken of Brown University. It could be a paleoclimatologist’s dream: a neat stack of sediment layers recording a changing but mostly wet climate, and maybe even containing the organic remains of ancient martian life. Or it could be a pile of sediments altered by ground water pooled far below a dry, lifeless surface. Or it could be a total surprise. “We’ll learn something useful and interesting,” says planetary geologist Jeffrey Moore of NASA’s Ames Research Center in Mountain View, California. “But it may not be what some people hoped it would be.” From wet to dry to wet Mars as 19th and early 20th century astronomers envisioned it was a world of deserts, lakes, and canals, not unlike the American West. All that turned to dust when the Mariner spacecraft of the 1960s flew past Mars and sent back the first close-ups of its surface, which seemed to show a parched, cratered terrain that looked more like the moon. A decade later, however, the image of Mars changed again with the two Viking orbiters, which revealed networks of river valleys in the martian tropics. Mars was warm and wet again—at least in its early history about 4 billion years ago. Planetary fluvial geologists, who study the shapes of erosion features to gauge how much water it took to carve them, concluded that early Mars must have had an atmosphere far thicker than today’s. By counting the number of craters that have
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accumulated on martian surfaces, geologists can infer their ages—roughly when valley networks were carved, for example, or a delta deposited. They estimated that the heyday of erosion was in the Noachian period— the first several hundred million years of the martian geologic record—when enough rain fell to cut valley networks, erase craters smaller than 4 kilometers in diameter, and erode hundreds of meters of rock off of larger craters. But there couldn’t have been a lot of water sloshing around, or the craters would have overflowed. “Early warm and wet Mars was never humid by terrestrial [Earth] standards,” says planetary fluvial geologist Rossman Irwin of the Smithsonian Institution’s National Air and Space Museum in Washington, D.C. But “it could have been semiarid. Mars in its heyday was like Utah or Nevada during the last ice age,” when Great Salt Lake was even greater and lakes abounded. Even in the warm and wet scenario, Mars went downhill, climatically speaking, after the few hundred million years of the Noachian. Erosion rates fell, although one or more periods of hydrological “reactivation” interrupted the long drying. In those clearly wet times, flowing water formed crater lakes, built river deltas in some lakes, and washed debris off crater rims to form half-cone-shaped alluvial fans. In fact, Curiosity landed on the toe of a kilometers-high fan nestled against the rim of Gale crater, which formed around the end of the Noachian about 3.7 billion years ago. Several hundred million years later, “Mars dies,” as Irwin’s Air and Space colleague Robert Craddock puts it. Researchers agree that for the 3 billion years since then, Mars
VOL 342 SCIENCE www.sciencemag.org Published by AAAS
CREDIT: NASA/JPL-CALTECH/MSSS
THE $2.5 BILLION CURIOSITY ROVER IS on a journey to a mountain in search of the truth about ancient Mars. In the midst of the 154-kilometer-wide Gale crater, where the rover landed in August 2012, rises a 5.5kilometer-high pile of sediment called Mount Sharp. Buried in its layers, Mars scientists believe, is a history of water on early Mars and a potential resolution to an emerging martian dispute. Ever since the two Viking spacecraft sent home images in the 1970s of water-cut valleys on Mars, most Mars scientists have believed that during its first billion years or so, the planet was shrouded in a thick, warm atmosphere capable of raining often enough to carve out those valleys. It was an environment seemingly hospitable to the origin and sustenance of life. But in the past decade, as the latest wave of Mars orbiters returned even sharper views of the surface, some planetary scientists have been advancing a far less life-friendly view. According to these cold-and-icy thinkers, early Mars never had a permanent atmosphere thick enough to drench the planet, even for brief intervals. There was water on the surface all right, but it was almost always tied up as ice. On this alternative Mars, the surface was icy, dry, and hostile for many millions of years at a time, and life would have struggled to gain a foothold there. What will Curiosity find next year, as it
Downloaded from www.sciencemag.org on March 18, 2015
Will the Curiosity rover discover an early Mars that was warm and wet and life-friendly or cold and icy and inhospitable?
CREDIT: © KEES VEENENBOS/SCIENCE SOURCE
NEWSFOCUS has remained icy and hyperarid (Science, 11 April 2003, p. 234). The picture of an early Mars of at least intermittent rain was reinforced 10 years ago when the Opportunity rover landed on the floor of what was taken to be an ancient shallow, salty sea. Its twin rover, Spirit, eventually discovered hot-spring deposits like Yellowstone’s. Convincing as the evidence was to geologists, however, planetary climate modelers weren’t at all sure. Their models of early Mars’s climate could not produce conditions warm enough for rain or flowing water. Half again as far as Earth from a young sun that was 25% dimmer than it is now, early Mars was a perpetual ice ball, at least in the models. Even a thick carbon dioxide atmosphere could not break the planet out of its deep freeze. That may be changing. Climate modelers had struggled to show how liquid water could have existed on early Earth, given the faintness of the newborn sun, but they have recently succeeded in producing a temperate early Earth. The key for some modelers: enriching the model planet’s atmosphere with hydrogen molecules, which—at least when they collide with lots of other molecules—can efficiently absorb radiation and act as a powerful greenhouse gas. Planetary climate scientists James Kasting, Ramses Mario Ramirez, and Michael Zugger of Pennsylvania State University, University Park, have followed that lead for early Mars. Their model tucks the planet up snugly in a thick carbon dioxide atmosphere that contains 10% hydrogen, the sort of atmosphere they believe likely enshrouded the planet back then. As the group reported at last December’s meeting of the American Geophysical Union, a hydrogen-rich atmosphere would have kept the martian surface at the time of Gale crater’s formation above freezing on average year-round. Icy Mars Planetary geologist James Head of Brown University doesn’t see the need for a hydrogen blanket because he doesn’t think Mars was ever permanently warm and even occasionally rainy. At last March’s Lunar and Planetary Science Conference, he cited a dozen reasons from his own work and that of others for arguing that Mars was always icy and that the water world envisioned a
decade ago should go the way of the 19th century martian canals. The eroded craters and water-cut valleys are real enough, Head says, but he thinks they could have formed during rare episodes of ice melting. That’s the way the water cycle works around the year in the Dry Valleys of Antarctica, where Head does fieldwork. It’s below freezing on average there, but enough ice melts in the daytime summer warmth to cut gullies in the bare earth nearby. In Head’s scheme—which he calls Noachian icy highlands—Mars had ice near its water-cut features. He and colleagues have deduced from lingering glacial features, like sediment ridges deposited in channels beneath glacial ice, that an ice sheet did indeed circle
similar climate modeling that puts snow and ice right where it was needed to supply meltwater to cut valley networks. To melt that snow and ice, Head calls on volcanoes or possibly large impacts. Giant impacts, of which there were lots on early Mars, scatter hot ejecta globally. That vaporizes ground ice, producing a hot, steamy atmosphere that could unleash heavy rains. Such sudden warmings had seemed too short-lived, but atmospheric scientist Teresa Segura of Space Systems/Loral LLC in Palo Alto, California, and her colleagues may have found a way around that. In the July 2012 issue of Icarus, they showed in a model how, under the right conditions, a giant impact could drive a frigid martian climate
A wet Mars for sure. While planetary scientists debate a warm and wet versus a cold and icy early Mars, they all agree that for at least a few geologically brief intervals, Mars was awash, as in this artist’s concept.
the south pole in the late Noachian and chill the southern hemisphere. Planetary climate modelers François Forget and Robin Wordsworth of Pierre and Marie Curie University in Paris, with Head and others, showed how the ice might have formed. They ran an early-Mars climate model with a moderately thick carbon dioxide atmosphere. It could not warm the planet above freezing, but like Earth’s atmosphere and unlike today’s wisp of a martian atmosphere, it cooled with increasing altitude. That meant that snow could fall at the high elevations of the southern hemisphere, compacting into ice. And in the 28 August issue of Geophysical Research Letters, Kathleen Scanlon of Brown, Head, and colleagues report on
www.sciencemag.org SCIENCE VOL 342 Published by AAAS
into a warm climate stable enough to get the required amount of erosion done. Volcanoes might work, too, warming climate by spewing greenhouse gases. The Viking orbiters discovered some of the solar system’s largest volcanoes on Mars, but they appeared to have erupted quietly, like the volcanoes on Hawaii, without releasing much gas. Now, in the 3 October issue of Nature, planetary geologist Joseph Michalski of the Planetary Science Institute in Tucson, Arizona, and Jacob Bleacher of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, report finding the lingering calderas of as many as seven explosive, gas-rich “supervolcanoes” that erupted on early Mars. They might have pumped enough carbon dioxide and other greenhouse gases into the atmosphere to trigger brief spring-
18 OCTOBER 2013
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NEWSFOCUS Valley network
Gale crater formation
Alluvial fan
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Mount Sharp’s spectral clay-sulfate transition
times on an otherwise frozen planet. With a scheme as grand as Head’s, there is no shortage of critics. But fluvial geologist Craddock focuses on one very small aspect: the raindrop. The icy highlands scenario has none, but Craddock sees them as essential to explain the observed crater erosion. Only falling raindrops hitting every square centimeter of crater rim and kicking up rock particles can create the observed smooth erosion, he says; meltwater flowing out from glaciers couldn’t do that. But to planetary spectroscopists, who analyze the spectral signatures of minerals from orbit, Head’s cold, dry Mars makes sense. A decade ago, the Opportunity rover encountered sulfate minerals that formed when salty, acidic water weathered rock. At first, rover team members attributed them to shallow, salty seas on early Mars. But subsequent rover investigation failed to turn up further signs of standing water. That and mineralogical evidence from the rover persuaded team members that the telltale minerals must have formed underground. Those supposed seas were just ephemeral, briny, and often acidic puddles oozing up from ground water. Researchers are also rethinking another class of minerals once hailed as supporting a warm, wet early Mars: the clays that
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Amazonian wind deposit on Mount Sharp River delta
the Mars Reconnaissance Orbiter and Mars Express probes identified in deposits around Mars. Rock weathering produces clays only after prolonged exposure to water. But in the 3 November 2011 issue of Nature, Bethany Ehlmann of the California Institute of Technology in Pasadena and several other leading planetary spectroscopists say that there is little need for much standing water on early Mars. The types of clay detected and the way they have been exposed by impacts “all point to ground water. … We don’t see highvolume flows of [surface] water for long durations,” Ehlmann says. In this picture of early Mars, “the proposed warmer and wetter early Mars was largely beneath the surface,” the group writes. They conclude that “frozen, arid conditions may have been hallmarks of the surface environment since the early-Noachian period”—that is, the entire known history of Mars. We just have to go That’s a disappointing verdict for anyone searching the martian surface for signs of ancient life. But NASA planners picked a good spot for determining whether it will stick. Gale crater is one of the lowest spots on
18 OCTOBER 2013
Early wetness, sometimes. Alluvial fans, valley networks, and river deltas all required at least momentary wetness. Wind deposits did not.
the planet, so if water was going to pool anywhere, that was the spot for it. Gale’s alluvial fans speak of rushing waters, at least momentarily. Curiosity spotted fine-grained, layered sediments on the crater floor that look for all the world like quiet lake sediments or at least puddle deposits. Stream gullies cut the slopes of Mount Sharp. And the layers in the lower slopes of the central mound preserve a transition from clays to sulfates. Curiosity has the tools to read the water story in Mount Sharp: how its sediments were laid down, how long water wetted them under what conditions, and—with luck— whether the water ever teemed with life. The tools include cameras with views from the panoramic to the microscopic that can scope out how sediments were deposited. One instrument package will analyze samples drilled from the rock. Another instrument will measure chemical composition from meters away by zapping rocks with its laser. All that could take years of roving, and the story will probably be convoluted. Mars, says planetary spectroscopist Milliken, is likely “no less complicated than Earth.”
VOL 342 SCIENCE www.sciencemag.org Published by AAAS
–RICHARD A. KERR
CREDITS (CLOCKWISE FROM TOP LEFT): ESA/DLR/FU BERLIN/G. NEUKUM; NASA/JPL-CALTECH/ASU/UA; NASA/JPL/UNIVERSITY OF ARIZONA; NASA/JPL-CALTECH/MSSS; ESA/DLR/FU BERLIN/G. NEUKUM; NASA/MRO/RALPH MILLIKEN/BROWN UNIVERSITY
4.6
Amazonian
Hesperian
Noachian
Destination Mount Sharp. This sediment pile holds a history of water on early Mars for Curiosity to read.
304
begins to climb the slopes of Mount Sharp? “That is the $2.5 billion question,” says planetary spectroscopist Ralph Milliken of Brown University. It could be a paleoclimatologist’s dream: a neat stack of sediment layers recording a changing but mostly wet climate, and maybe even containing the organic remains of ancient martian life. Or it could be a pile of sediments altered by ground water pooled far below a dry, lifeless surface. Or it could be a total surprise. “We’ll learn something useful and interesting,” says planetary geologist Jeffrey Moore of NASA’s Ames Research Center in Mountain View, California. “But it may not be what some people hoped it would be.” From wet to dry to wet Mars as 19th and early 20th century astronomers envisioned it was a world of deserts, lakes, and canals, not unlike the American West. All that turned to dust when the Mariner spacecraft of the 1960s flew past Mars and sent back the first close-ups of its surface, which seemed to show a parched, cratered terrain that looked more like the moon. A decade later, however, the image of Mars changed again with the two Viking orbiters, which revealed networks of river valleys in the martian tropics. Mars was warm and wet again—at least in its early history about 4 billion years ago. Planetary fluvial geologists, who study the shapes of erosion features to gauge how much water it took to carve them, concluded that early Mars must have had an atmosphere far thicker than today’s. By counting the number of craters that have
18 OCTOBER 2013
accumulated on martian surfaces, geologists can infer their ages—roughly when valley networks were carved, for example, or a delta deposited. They estimated that the heyday of erosion was in the Noachian period— the first several hundred million years of the martian geologic record—when enough rain fell to cut valley networks, erase craters smaller than 4 kilometers in diameter, and erode hundreds of meters of rock off of larger craters. But there couldn’t have been a lot of water sloshing around, or the craters would have overflowed. “Early warm and wet Mars was never humid by terrestrial [Earth] standards,” says planetary fluvial geologist Rossman Irwin of the Smithsonian Institution’s National Air and Space Museum in Washington, D.C. But “it could have been semiarid. Mars in its heyday was like Utah or Nevada during the last ice age,” when Great Salt Lake was even greater and lakes abounded. Even in the warm and wet scenario, Mars went downhill, climatically speaking, after the few hundred million years of the Noachian. Erosion rates fell, although one or more periods of hydrological “reactivation” interrupted the long drying. In those clearly wet times, flowing water formed crater lakes, built river deltas in some lakes, and washed debris off crater rims to form half-cone-shaped alluvial fans. In fact, Curiosity landed on the toe of a kilometers-high fan nestled against the rim of Gale crater, which formed around the end of the Noachian about 3.7 billion years ago. Several hundred million years later, “Mars dies,” as Irwin’s Air and Space colleague Robert Craddock puts it. Researchers agree that for the 3 billion years since then, Mars
VOL 342 SCIENCE www.sciencemag.org Published by AAAS
CREDIT: NASA/JPL-CALTECH/MSSS
THE $2.5 BILLION CURIOSITY ROVER IS on a journey to a mountain in search of the truth about ancient Mars. In the midst of the 154-kilometer-wide Gale crater, where the rover landed in August 2012, rises a 5.5kilometer-high pile of sediment called Mount Sharp. Buried in its layers, Mars scientists believe, is a history of water on early Mars and a potential resolution to an emerging martian dispute. Ever since the two Viking spacecraft sent home images in the 1970s of water-cut valleys on Mars, most Mars scientists have believed that during its first billion years or so, the planet was shrouded in a thick, warm atmosphere capable of raining often enough to carve out those valleys. It was an environment seemingly hospitable to the origin and sustenance of life. But in the past decade, as the latest wave of Mars orbiters returned even sharper views of the surface, some planetary scientists have been advancing a far less life-friendly view. According to these cold-and-icy thinkers, early Mars never had a permanent atmosphere thick enough to drench the planet, even for brief intervals. There was water on the surface all right, but it was almost always tied up as ice. On this alternative Mars, the surface was icy, dry, and hostile for many millions of years at a time, and life would have struggled to gain a foothold there. What will Curiosity find next year, as it
Downloaded from www.sciencemag.org on March 18, 2015
Will the Curiosity rover discover an early Mars that was warm and wet and life-friendly or cold and icy and inhospitable?
CREDIT: © KEES VEENENBOS/SCIENCE SOURCE
NEWSFOCUS has remained icy and hyperarid (Science, 11 April 2003, p. 234). The picture of an early Mars of at least intermittent rain was reinforced 10 years ago when the Opportunity rover landed on the floor of what was taken to be an ancient shallow, salty sea. Its twin rover, Spirit, eventually discovered hot-spring deposits like Yellowstone’s. Convincing as the evidence was to geologists, however, planetary climate modelers weren’t at all sure. Their models of early Mars’s climate could not produce conditions warm enough for rain or flowing water. Half again as far as Earth from a young sun that was 25% dimmer than it is now, early Mars was a perpetual ice ball, at least in the models. Even a thick carbon dioxide atmosphere could not break the planet out of its deep freeze. That may be changing. Climate modelers had struggled to show how liquid water could have existed on early Earth, given the faintness of the newborn sun, but they have recently succeeded in producing a temperate early Earth. The key for some modelers: enriching the model planet’s atmosphere with hydrogen molecules, which—at least when they collide with lots of other molecules—can efficiently absorb radiation and act as a powerful greenhouse gas. Planetary climate scientists James Kasting, Ramses Mario Ramirez, and Michael Zugger of Pennsylvania State University, University Park, have followed that lead for early Mars. Their model tucks the planet up snugly in a thick carbon dioxide atmosphere that contains 10% hydrogen, the sort of atmosphere they believe likely enshrouded the planet back then. As the group reported at last December’s meeting of the American Geophysical Union, a hydrogen-rich atmosphere would have kept the martian surface at the time of Gale crater’s formation above freezing on average year-round. Icy Mars Planetary geologist James Head of Brown University doesn’t see the need for a hydrogen blanket because he doesn’t think Mars was ever permanently warm and even occasionally rainy. At last March’s Lunar and Planetary Science Conference, he cited a dozen reasons from his own work and that of others for arguing that Mars was always icy and that the water world envisioned a
decade ago should go the way of the 19th century martian canals. The eroded craters and water-cut valleys are real enough, Head says, but he thinks they could have formed during rare episodes of ice melting. That’s the way the water cycle works around the year in the Dry Valleys of Antarctica, where Head does fieldwork. It’s below freezing on average there, but enough ice melts in the daytime summer warmth to cut gullies in the bare earth nearby. In Head’s scheme—which he calls Noachian icy highlands—Mars had ice near its water-cut features. He and colleagues have deduced from lingering glacial features, like sediment ridges deposited in channels beneath glacial ice, that an ice sheet did indeed circle
similar climate modeling that puts snow and ice right where it was needed to supply meltwater to cut valley networks. To melt that snow and ice, Head calls on volcanoes or possibly large impacts. Giant impacts, of which there were lots on early Mars, scatter hot ejecta globally. That vaporizes ground ice, producing a hot, steamy atmosphere that could unleash heavy rains. Such sudden warmings had seemed too short-lived, but atmospheric scientist Teresa Segura of Space Systems/Loral LLC in Palo Alto, California, and her colleagues may have found a way around that. In the July 2012 issue of Icarus, they showed in a model how, under the right conditions, a giant impact could drive a frigid martian climate
A wet Mars for sure. While planetary scientists debate a warm and wet versus a cold and icy early Mars, they all agree that for at least a few geologically brief intervals, Mars was awash, as in this artist’s concept.
the south pole in the late Noachian and chill the southern hemisphere. Planetary climate modelers François Forget and Robin Wordsworth of Pierre and Marie Curie University in Paris, with Head and others, showed how the ice might have formed. They ran an early-Mars climate model with a moderately thick carbon dioxide atmosphere. It could not warm the planet above freezing, but like Earth’s atmosphere and unlike today’s wisp of a martian atmosphere, it cooled with increasing altitude. That meant that snow could fall at the high elevations of the southern hemisphere, compacting into ice. And in the 28 August issue of Geophysical Research Letters, Kathleen Scanlon of Brown, Head, and colleagues report on
www.sciencemag.org SCIENCE VOL 342 Published by AAAS
into a warm climate stable enough to get the required amount of erosion done. Volcanoes might work, too, warming climate by spewing greenhouse gases. The Viking orbiters discovered some of the solar system’s largest volcanoes on Mars, but they appeared to have erupted quietly, like the volcanoes on Hawaii, without releasing much gas. Now, in the 3 October issue of Nature, planetary geologist Joseph Michalski of the Planetary Science Institute in Tucson, Arizona, and Jacob Bleacher of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, report finding the lingering calderas of as many as seven explosive, gas-rich “supervolcanoes” that erupted on early Mars. They might have pumped enough carbon dioxide and other greenhouse gases into the atmosphere to trigger brief spring-
18 OCTOBER 2013
305
NEWSFOCUS Valley network
Gale crater formation
Alluvial fan
4.0
3.9
3.8
3.7
3.5
3.5 3.4 Billions of years ago
3.3
Mount Sharp’s spectral clay-sulfate transition
times on an otherwise frozen planet. With a scheme as grand as Head’s, there is no shortage of critics. But fluvial geologist Craddock focuses on one very small aspect: the raindrop. The icy highlands scenario has none, but Craddock sees them as essential to explain the observed crater erosion. Only falling raindrops hitting every square centimeter of crater rim and kicking up rock particles can create the observed smooth erosion, he says; meltwater flowing out from glaciers couldn’t do that. But to planetary spectroscopists, who analyze the spectral signatures of minerals from orbit, Head’s cold, dry Mars makes sense. A decade ago, the Opportunity rover encountered sulfate minerals that formed when salty, acidic water weathered rock. At first, rover team members attributed them to shallow, salty seas on early Mars. But subsequent rover investigation failed to turn up further signs of standing water. That and mineralogical evidence from the rover persuaded team members that the telltale minerals must have formed underground. Those supposed seas were just ephemeral, briny, and often acidic puddles oozing up from ground water. Researchers are also rethinking another class of minerals once hailed as supporting a warm, wet early Mars: the clays that
306
3.2
3.1
3.0
0
Amazonian wind deposit on Mount Sharp River delta
the Mars Reconnaissance Orbiter and Mars Express probes identified in deposits around Mars. Rock weathering produces clays only after prolonged exposure to water. But in the 3 November 2011 issue of Nature, Bethany Ehlmann of the California Institute of Technology in Pasadena and several other leading planetary spectroscopists say that there is little need for much standing water on early Mars. The types of clay detected and the way they have been exposed by impacts “all point to ground water. … We don’t see highvolume flows of [surface] water for long durations,” Ehlmann says. In this picture of early Mars, “the proposed warmer and wetter early Mars was largely beneath the surface,” the group writes. They conclude that “frozen, arid conditions may have been hallmarks of the surface environment since the early-Noachian period”—that is, the entire known history of Mars. We just have to go That’s a disappointing verdict for anyone searching the martian surface for signs of ancient life. But NASA planners picked a good spot for determining whether it will stick. Gale crater is one of the lowest spots on
18 OCTOBER 2013
Early wetness, sometimes. Alluvial fans, valley networks, and river deltas all required at least momentary wetness. Wind deposits did not.
the planet, so if water was going to pool anywhere, that was the spot for it. Gale’s alluvial fans speak of rushing waters, at least momentarily. Curiosity spotted fine-grained, layered sediments on the crater floor that look for all the world like quiet lake sediments or at least puddle deposits. Stream gullies cut the slopes of Mount Sharp. And the layers in the lower slopes of the central mound preserve a transition from clays to sulfates. Curiosity has the tools to read the water story in Mount Sharp: how its sediments were laid down, how long water wetted them under what conditions, and—with luck— whether the water ever teemed with life. The tools include cameras with views from the panoramic to the microscopic that can scope out how sediments were deposited. One instrument package will analyze samples drilled from the rock. Another instrument will measure chemical composition from meters away by zapping rocks with its laser. All that could take years of roving, and the story will probably be convoluted. Mars, says planetary spectroscopist Milliken, is likely “no less complicated than Earth.”
VOL 342 SCIENCE www.sciencemag.org Published by AAAS
–RICHARD A. KERR
CREDITS (CLOCKWISE FROM TOP LEFT): ESA/DLR/FU BERLIN/G. NEUKUM; NASA/JPL-CALTECH/ASU/UA; NASA/JPL/UNIVERSITY OF ARIZONA; NASA/JPL-CALTECH/MSSS; ESA/DLR/FU BERLIN/G. NEUKUM; NASA/MRO/RALPH MILLIKEN/BROWN UNIVERSITY
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Hesperian
Noachian
