Geologists on Mars

For centuries, scientists have observed Mars from afar and wondered: How similar is it to Earth? Might water be found there? Does the planet harbor life? The advent of Mars-orbiting satellites in the 1960s dashed the most outlandish of these fantasies – visions of vast marshlands and seas, irrigation canals, and a race of intelligent (or at least shovel-wielding) Martians. In early 2004, however, two unlikely explorers traveled to the red planet and found strong evidence to confirm one important similarity to Earth: Water once existed on the surface of Mars, and in sufficient quantity to possibly have harbored life.

The explorers, two robotic rovers designed and built by NASA, were launched toward Mars in June and July of 2003. They landed in January 2004, on opposite sides of the planet: the rover Spirit in a surface feature called Gusev Crater, and the rover Opportunity in a large crater called Meridiani Planum. Both rovers are equipped with sophisticated tools for photographing, grinding, and analyzing Martian rocks. Basically, they're geologists on wheels. Their mission has been to probe the rocks of Mars for signs of past or current deposits of water. Within a month of landing, they'd succeeded.

"NASA launched the Mars Exploration Rover mission specifically to check whether at least one part of Mars ever had a persistently wet environment that could possibly have been hospitable to life," James Garvin, lead scientist for Mars and lunar exploration at NASA, announced at a press conference on March 2. "Today we have strong evidence for an exciting answer: Yes."

One early piece of evidence came from Opportunity's analysis of the chemistry of some nearby rocks. The rovers' load of scientific instruments includes at least three different kinds of spectrometers, which identify the chemical elements in a soil or rock sample by measuring the different energies, or spectra, of light given off. In the rocks of Meridiani Planum, Opportunity found abundant traces of various kinds of salts. On Earth, rocks that contain equally large amounts of these salts either formed in water or, after formation, were highly altered by long exposures to water.

A second clue came from the physical appearance of the rocks. Close-up photographs taken by Opportunity of an outcrop nicknamed "El Capitan" revealed that the rock was pocked with small indentations or crevices. Geologists call these features "vugs." On Earth, vugs represent places where crystals of salt minerals formed within rocks sitting in briny water. When the crystals later dissolve, either by wind erosion or the flow of less-salty water, only the vugs remain. The rover Spirit, roaming and probing the opposite side Mars, found similar crevices in a rock dubbed "Humphrey."

"Liquid water once flowed through these rocks," Steve Squyres, a scientist at Cornell University and the principal investigator for the science instruments on Opportunity and Spirit, told a roomful of reporters. "It changed their texture, and it changed their chemistry. We've been able to read the tell-tale clues the water left behind, giving us confidence in that conclusion."

A third line of evidence for the former presence of water on Mars came from the discovery of "blueberries," round mineral grains embedded in outcrops of Martian rocks. (Actually, these mineral deposits are only about the size of BB's and are gray, not blue.) At first, scientists thought the spherules might have been once-molten droplets of rock sent aloft by ancient volcanic eruptions or meteor impacts. Had that been the case, though, the droplets would occupy distinct layers within the rock and would have deformed the layers around them, like peas in a pile of mattresses.

Instead, Opportunity found the blueberries scattered indiscriminately throughout the rock layers. This suggests they were created not by an ancient volcano or a meteor impact, but from the gradually concretion of minerals within once-wet sediments. While examining the light spectra from a rock outcrop dotted with blueberries, Opportunity detected the presence of an iron-bearing mineral called gray hematite; on Earth, hematite containing crystalline grains of the size found in the Martian blueberries typically forms in the presence of water.

All of this evidence combined to indicate that Mars – or at least that particular region of Mars – had been soaked with water at some point in geologic history. This news alone is enough to bolster hopes of one day finding some sign of past life – most likely fossils of microbes – on Mars. "We know that wherever there is water on Earth, almost invariably there's some kind of microbial life," says Denton Ebel, a geoscientist at the American Museum of Natural History. "We suspect that there may once have been life on the planet Mars-but we don't know yet. So the mantra is, 'Follow the water.'"

Though exciting, the data didn't indicate whether Mars possessed large bodies of water or simply lots of damp soil. On March 23, scientists at NASA announced another set of major findings. The rover Opportunity had found sedimentary rocks that had been etched by ancient ripples – strong evidence that a large body of saltwater had persisted on the Martian surface for a significant period of time. "We think Opportunity is parked on what was once the shoreline of a salty sea on Mars," Steve Squyres told an assembled room of news reporters.

The telltale clue came from close-up photographs of Martian rocks. The rovers had already found copious evidence of sedimentary rocks – rocks made up of successive layers of sediment laid down by wind or, possibly, water. Closer examination revealed the subtle and distinctive signature of a former current: sinuous ripples of sediment similar to those found on Earth at the bottom of a shallow, moving body of water. To create such rock formations, the body of Martian water must have been relatively large and long-lived. NASA scientists tentatively compared it to a terrestrial salt flat, which is sometimes covered by shallow water and other times dry.

"I was astonished," said John Grotzinger, a scientist at the Massachusetts Institute of Technology and a member of NASA's rover science team. "We're seeing sedimentary structures just like we see on Earth."

Scientists still have much to learn about the ancient water on Mars. For example, the data doesn't reveal much about what the climate was like at the time; the water might have been covered by a layer of ice. In the coming weeks, the rovers will venture farther afield, probe other craters, and hopefully examine thicker slices of Martian rock in order to look deeper into Mars's geological past.

But even these most basic findings represent a significant scientific advance. "We see the beginnings of a new Mars," said Ed Weiler, NASA's associate administrator for space science. Earth is no longer the only planet known to have – or have had – water. "This has profound implications for astrobiology," Weiler added. "If you have an interest in searching for fossils on Mars, this is the place you want to go."

Neil deGrasse Tyson, director of the Hayden Planetarium at the American Museum of Natural History, is an astrophysicist and the author of numerous books and articles about space. In early 2004, he was appointed by President George W. Bush to serve on the President's Commission on Moon, Mars and Beyond, a nine-member panel designated to offer recommendations on how best to implement the president's vision for future space exploration. Science Bulletins caught up with Tyson recently in his office on the fifth floor of the Museum's Rose Center for Earth and Space.

Why go to Mars?

Mars has a 24-hour day. It has polar ice caps. Its axis is tilted compared with its orbit, just as Earth is tilted on its axis. That means Mars goes through seasons, just like Earth. Mars, as cold as it is, is not as oppressive an environment as almost any other place we can think of going in the Solar System. From a runaway greenhouse effect, Venus is 900 degrees Fahrenheit and would melt or vaporize most things you sent to its surface. Mercury is also very hot, being close to the Sun. So when you look at the nearby terrestrial planets, Mars is looking just right, in spite of the challenges.

While that alone might make a trip to Mars compelling, it's far from the most compelling reason. We learned in the 1960s that Mars's surface has features that, as far as we can tell, can only have been made in the presence of water: standing water, running water, deluging water. There are features that look like they're floodplains. There are riverbeds that are straight and riverbeds that meander. Combine all of this, and you consider how important water is to life on Earth, you can't help but speculate that Mars was once a really wet place, possibly even harboring life at one point. So much of what drives cosmic exploration involves the quest to learn whether or not we're alone in the Universe — as an intelligent species, or as life at all. Mars being so close compared with the rest of the cosmos — it's a slam dunk as a place you want to go visit.

What have Spirit and Opportunity, NASA's two Mars Exploration Rovers, been discovering up there?

Until now, all the evidence for water on Mars has been strongly circumstantial. It looks like water was there, because the surface features resemble features on Earth that we know are made by water. But you don't really know until you analyze the rocks. And geologists are good at that. Geologists can look at a rock and tell you a whole story: how old the rock is, what happened to it while it formed, how it got there. They do this with clever, subtle measurements of the rock's structure, form, and chemical composition. So geologists can read the history of the rocks of Mars and thereby read the history of the Martian surface. And the most recent evidence from the rovers confirms what we strongly thought before, that in fact water enjoyed a major presence on Mars.

Of course, what's really driving the missions to Mars is the search for life; there's no question about it. The current rovers aren't carrying biological experiments; they're carrying chemical experiments. One of the chemical experiments involves identifying certain kinds of rocks that can form only in the presence of water. Those rocks were found – so that's an important first step.

Suppose fossils or other signs of life are discovered on Mars. What are the implications?

There are two implications: one extraordinary, the other fun but not extraordinary.

There is evidence to suggest that Mars was wet before Earth was wet. If that's the case, maybe Mars had life before Earth had life. We know that meteors jump between the planets a fact established only in the past ten years, or so. A big meteor hits Mars, and thrusts rocks into space that then land on Earth; we know that this happens. We also know that certain classes of bacteria have very high thresholds for radiation and for temperature: they're extremophiles, they love extremes. Combine the fact that extremophiles exist, that rocks move from planet to planet, and that Mars was wet before Earth was wet, and you have to admit the possibility that life traveled from Mars to Earth as stowaways in the cracks of rocks. Which would make all humans — all life on Earth — the descendants of Martians.

Now, that's fun but not extraordinary, because we don't learn anything new about biology. We would all share common DNA. Biologists would continue to revel in the biodiversity of life as we know it, but at the end of the day there'd still be only a sample of one: DNA-based life. Whereas if the identify of life on Mars were encoded by other means, there'd be no end of inquiry, no end of revolutionary research — we will have discovered the first truly new form of life.

What do you think are the odds of finding evidence of life on Mars?

High — certainly higher than 50 percent. And by life, I refer to fossil life, not current life; and simple life, bacteria or microbes. Water is the key.

The invention of the telescope in the 17th century provided scientists with unprecedented, and ever-clearer, views of Mars. Much remained hazy, however, leaving observers plenty of room in which to exercise their imaginations. The result was a tantalizing vision of an oddly Earth-like Mars, a planet inhabited by "little green men" who built canals and plotted invasions-in short, a vision of a Mars too strange to be true.

Christiaan Huygens, a Dutch astronomer, sparked this vision in 1659 with a sketch of Mars based on his observations; the drawing showed shifting areas of light and dark, which to Huygens indicated the presence of vegetation that changed with the seasons. A century later, the American astronomer Frederic William Herschel observed dark areas of Mars, which he concluded were oceans, and shifting patterns of lighter areas, which he believed were "clouds and vapors." The "inhabitants" of Mars, he speculated in 1774, "probably enjoy a situation similar to our own."

In 1877, Italian astronomer Giovanni Schiaparelli reported seeing straight lines on the surface of Mars. He named them canali — "channels" in Italian. In other countries, however, the word was mistranslated as "canals," or waterways constructed by an advanced civilization. Schiaparelli drew maps of these canali. Other enthusiastic astronomers claimed the canali ended in large dark spots they called "oases." The notion of a wet Mars gained force in 1894, when telescope observations of Mars revealed polar ice caps that grew in winter and shrank in summer. Many astronomers interpreted these seasonal changes as evidence of a climate suitable for farming. Most, however, were not convinced. In his 1907 book, Is Mars Habitable? Alfred Russel Wallace argued (correctly, it turned out) that Mars was a frozen desert. Wallace predicted that the ice caps were more likely frozen carbon dioxide than water. (On that front, he was only half right: Scientists today suspect that Mars's ice caps may be a mixture of frozen carbon dioxide and frozen water.)

In 1908, the American astronomer Percival Lowell argued that Schiaparelli's canali were actual canals dug by intelligent Martians to transport water from the polar ice caps for irrigation. From his observatory in Arizona, Lowell mapped hundreds of supposed canals. Lowell's many articles and books, including Mars as the Abode of Life in 1908, did much to popularize the idea of a race of intelligent Martian farmers. (Lowell also saw "canals" on the surface of Venus, an observation that no other astronomer could duplicate. Recently, a team of ophthalmologists concluded that in fact Lowell had made the aperture of his telescope so small that he'd effectively turned it into a mirror: the Venusian "canals" were actually the reflected shadows of the blood vessels in the back of his eyeball.)

Debunking Lowell's theories, Alfred Russel Wallace countered that only "a race of madmen" would build open canals to carry water on Mars because the planet was so cold and airless that any liquid water would instantly evaporate. Nonetheless, by the 1920s, influenced by Lowell's active imagination, some popular illustrations of Mars included light and dark spots labeled "Vegetation," "Marshes," "Snow Clad Mountains," and "Hurricanes Dispersing the Mists." Some people argued that the canal-like lines many observers thought they saw on Mars were actually huge pictures drawn on the Martian surface to attract our attention "for the purpose of interplanetary communication."

Unfortunately for scientists, if not for science fiction fans, these theories attracted much interest among the general public. Mars hysteria peaked in 1938, when Orson Welles produced a radio adaptation of H. G. Wells's book "War of the Worlds," which describes a Martian invasion of Earth. Broadcasting the fictional story in the form of live news reports, Welles created widespread panic among gullible listeners. By the 1950s, men from Mars had become a staple of B movies. In the 1953 film "Invaders from Mars," for instance, a young boy sees a flying saucer land in his backyard. Big, green Martian zombies, controlled by a tentacled, disembodied head in a glass bubble, emerge from the spaceship to take over the town. Evidently these Martians had mastered interplanetary travel but had nothing better to do with their time than pester their neighbors.

Science eventually overtook science fiction. The first close-up pictures of the Martian surface, sent back by Mariner IV in 1965, revealed a cratered, moonlike wasteland. In 1972, however, NASA's Mariner IX photographed huge volcanoes and giant canyons that, if Earth was any model, appeared to have been cut by former waterways — a sign, perhaps, that water may indeed have been abundant on Mars at some time in the past. In 1976, two Viking orbiters made detailed maps of Mars, and two Viking landers set down on the planet. The landers tested the Martian soil for signs of life. They didn't find any, but they did send back dramatic photographs of the Martian surface.

The best large-scale images of Mars to date come from NASA's Mars Global Surveyor, which has been orbiting the planet since the year 2000, and the European Space Agency's Mars Express orbiter, which arrived in December 2003. Their photographs are at least 50 times more detailed than the earlier Viking photographs. Scientists are still sifting through this trove of data. So far, they have found evidence of surprisingly recent volcanic activity and, most exciting, of what appear to be water-carved canyons and gullies. And the two NASA rovers, Spirit and Opportunity, which are currently roaming the Martian surface on opposite sides of the planet, are providing scientists on Earth with a steady stream of data — leaving less room for the imagination, perhaps, but prompting a new vision of Mars no less exciting for its basis in fact.

In recent decades, as scientists have learned more about the geology of Mars, they have been increasingly impressed by the planet's physical likeness to Earth. Mars rotates at roughly the same rate as Earth does, so one Martian day (or "sol") is approximately 24 hours long. Mars has polar ice caps, and it is tilted slightly on its axis, so the planet has seasons. There is clear evidence that Mars had volcanoes, although their activity ceased long ago. And now, there are strong signs that Mars had vast quantities of liquid, briny water.

For all the similarities, however, Mars does possess unique geophysical traitstraits that early on set Mars on a developmental path distinct from Earth's. "I like to think that Mars is the mirror of Earth," says Denton Ebel, a curator of meteorites with the American Museum of Natural History's Department of Earth and Planetary Sciences. "It was once a lot like Earth was once. It is now, perhaps, the way Earth might become in the distant future."

Mars, like the other terrestrial planets (Earth, Venus, and Mercury), formed about four billion years ago from the accretion of asteroid-like boulders and other rocky, protoplanetary debris. Mars, however, was forming near the gas giant Jupiter. With its strong gravitational pull, Jupiter managed to divert a great deal of rocky debris that might otherwise have become part of Mars. As a result, Mars is only about one-third the size of Earth.

That smallness made all the difference. Small bodies cool more quickly than larger ones; over its long history, Mars lost much of the internal heat from its metallic core. The core supplied enough heat to drive volcanic eruptions and the circulation of water within the ground, but not continental drift on the scale seen on Earth. (What are now the highlands of Mars, in the southern hemisphere, may once have been a supercontinent that "froze" in place when the planet's upper mantle and crust became immobile.) Then again, says Ebel, studies of several meteorites from Mars — five are on display at the American Museum of Natural History — indicate that Martian volcanoes were active as little as 150 million years ago. "That's pretty recent, in geological time."

Meanwhile, back on early Earth, tectonic activity was going strong. The larger planet retained its internal heat, and with it a hot core of partially molten metal. As Earth rotates, the molten core churns and acts as a dynamo, generating a strong magnetic field – something Mars lacks. Unshielded, the Martian surface receives a heavy dose of deadly cosmic rays and is buffeted by the solar wind, which over time has stripped away Mars's atmosphere. Had Earth formed nearer to Jupiter, conditions might have turned out less favorably for its inhabitants. "The evolution and habitability of the planet are direct consequences of its initial conditions — how big it is, and where it is in the Solar System," Ebel says.

In March 2004, the NASA rovers Spirit and Opportunity found evidence that, like Earth, Mars once possessed large bodies of briny water, perhaps even a sea, on its surface. The discovery of acid-sulphate salts on the Martian surface indicate that the briny sea evaporated at some point. But when that occurred, and where all the water went, is unclear. "What happened to the water?" Ebel muses. "We don't know. The key missing evidence is the timing."

One possibility is that the water wound up in the polar ice caps, both of which are now known to contain frozen water in addition to frozen carbon dioxide, or "dry ice". "There's a lot of water in there, more than was previously thought," Ebel says. Scientists are trying to model the Martian atmosphere, to determine how carbon dioxide and water move around the surface of Mars and, by working backward in time, where it came from and went. But the orbit of Mars is far more eccentric than Earth's, thanks to the gravitational influence of Jupiter. As a result, Ebel explains, "Figuring out which part of the planet was cooler and which warmer, and when in history, is a challenging puzzle."

There's also a strong possibility that water, frozen or liquid, may still exist beneath the surface of Mars. (The Martian climate is too cold for liquid water to exist aboveground.) The European Space Agency's Mars Express orbiter is equipped with ground-penetrating radar, which can detect signs of frozen water beneath the rocky Martian surface — if it's there to be found.

The existence of both volcanic activity and water on Mars raise the tantalizing possibility that the planet might have harbored life at one time in its history. "Water and heat are two very important ingredients for life," Ebel says. If life did exist on Mars, he adds, it likely thrived deep underground, protected from the freezing temperatures and deadly rain of cosmic particles aboveground. (Microbes-and those are the only fossilized life-forms scientists would expect to find on Mars-are known to live deep below Earth's surface.) Alas, fossil-hunting is not something the Spirit and Opportunity rovers are designed to do; the first astrobiology experiments won't arrive on Mars until sometime in 2010.

Even if Mars did harbor microbial life, Ebel says, it almost certainly didn't have enough of it for a long enough period of time to alter the planet's climate. On Earth, the emergence of photosynthetic, oxygen-producing microbes – organisms that could use sunlight to convert carbon dioxide into carbohydrates and oxygen-dramatically altered the early atmosphere. The rise of oxygen, between 2.4 billion and 2.2 billion years ago, in turn made possible the eventual evolution of more complex forms of life, including humans. In contrast, the Martian atmosphere today – what little of it remains – is still mostly carbon dioxide, as it likely was from the outset.

Many physical factors kept Mars from developing into the planet that Earth is today. But might Earth one day come to resemble Mars? "A lot would have to happen between now and then," Ebel says. First, Earth would have to cool considerably; its convective core would then stop churning. Its magnetic field would also fade, making the surface vulnerable to both the solar wind, which would eventually strip away the atmosphere, and cosmic rays, which would effectively sterilize the planet. "But don't worry," Ebel adds, "that future is still billions of years away. By then, who knows? Maybe humans will have colonized some other corner of the cosmos – maybe a quiet little planet, maybe someplace like Mars."

In March 2004, two robotic rovers from NASA delivered clear evidence that liquid waterperhaps large bodies of itexisted on the surface of Mars at some point in the planet's past. But could liquid water still exist on Mars today? The likelihood would seem to be impossibly low: the temperature on the Martian surface averages between -70°C and -100°C (-95°F and -150°F), far below the freezing point of water. In such cold, water should freeze to a depth of several kilometers.

Yet a series of satellite photographs taken by NASA's Mars Global Surveyor, which has been mapping Mars since 1999, raise the exciting possibility that liquid water may have existed there very recently. The photos, examined by scientists Michael Malin and Ken Edgett of Malin Space Science Systems, reveal gullies on Mars seemingly identical to landforms that, on Earth, are created by flowing water. From appearances, water seems to have flowed out and down the sides of Martian craters and canyons. The scientists point to three distinguishing features: a clear starting point, or alcove, where the liquid seems to have come through the wall of a crater or valley; channels like those carved by running water on Earth; and a mound of rubble, or apron, at the bottom that looks like it was dumped by flowing water. (To examine the evidence yourself, click on the accompanying interactive, "Did Water Carve These Canyons?")

"The physical resemblance is stunning," says Nick Schneider, a geoscientist at the University of Colorado. "That's what water on craters looks like." In some photos, several gullies appear in a row. On Earth, these formations, called "weeping layers," form when water flows underground along a porous layer of rock. Wherever these layers end at exposed canyon or valley walls, water seeps out at several points along the layer. Photographs of what appear to be similar weeping layers suggest further evidence of liquid water. Malin and Edgett offered additional evidence in an article published in the journal Science. But Bruce Jakosky, also of the University of Colorado, perhaps summed up their case best: "It comes down to a very technical argument that these are water-carved, and that is: 'Dammit, they look water-carved!'"

One striking feature of the gullies is how young they appear to be. They slice through windswept dunes, which are some of the most rapidly changing features on Mars. They are also remarkably free of dust and impact craters, which cover almost everything else on Mars. According to Malin and Edgett, some of the gullies look to be so recent that they may still be actively forming today. Yet how could that be, if the Martian surface is so cold? Any liquid water should be frozen solid. Scientists are wracking their brains for a logical explanation. Several theories have been proposed, with no clear answer.

One possibility is that the water is very salty. There is plenty of salt on Mars, and salt can significantly lower the freezing point of groundwater. (That's why people sometimes use salt to melt the ice on their sidewalks.) To stay unfrozen on Mars, however, the water would need be so thick with salt that it probably would no longer flow like water. Another possibility is that the water is (or was) spilling from underground quickly enough to avoid freezing. But even then, the water would have to start out warm – heated, say, by a nearby volcano or a vein of subsurface magma. (A similar process produces geysers and hot springs at Yellowstone National Park.) So far, however, scientists have seen no sign of active Martian volcanoes or other potential heat sources.

It could also be that the gullies formed in the recent past, perhaps during a temporary warm spell, before Mars's climate cooled down to its present icy state. Steve Clifford, a scientist with the Lunar and Planetary Institute, points out that Mars wobbles on its axis, and that those wobbles could have prompted extreme changes in the planet's climate. If a planet has a tilted axis, then its northern and southern hemispheres will see more daylight during some parts of the year ("summer") and less during others ("winter"); hence the seasons. The greater the tilt, the more extreme the seasons.

Currently Mars sits at a tilt identical to Earth's. But in the past that tilt has varied drastically, from zero degrees (no tilt) to 60 degrees (tipped two-thirds of the way on its side). "That's an amazing variation," Clifford says. When the tilt was greater than 45 degrees, even the Martian poles – which today are frozen – could have received enough sunshine and heat to start melting. Likewise, underground ice elsewhere on the planet could have easily warmed up enough to melt, flow onto the surface, and carve the gullies seen today.

An entirely different theory for the Martian gullies suggests that a mixture of dirt and carbon dioxide gas, not water, was responsible. Michael Carr, a scientist with the U.S. Geological Survey and the author of the book "Water on Mars," notes that under certain conditions, dry particles can become "fluidized" and flow like water. This sometimes occurs on Earth – for example, when particles of ash and gas pour from a volcano, or on the rare occasion when a pocket of carbon dioxide bursts from a hillside and sends debris spilling down a slope. Something similar could have happened on Mars. "When we observe these kind of deposits with channels, which are pretty common on Earth, that's your first reaction, that water was involved," says Ken Tanaka, another USGS scientist. But since the environment on Mars is so different from Earth, he adds, "I think you want to keep your options open."