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A Tale of Two Rovers

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 fantasiesvisions 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.

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Mars Exploration Rovers

NASA/JPL


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."

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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 lifemost likely fossils of microbeson 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 ripplesstrong 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 rocksrocks 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."

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