03/24/04

Peering closely at a sofa-sized rock outcrop jutting from a dry, dusty Martian crater, the rover Opportunity has found strong evidence the area once lay beneath a shallow sea.

A rock that scientists dubbed "Last Chance" shows a telltale pattern of layering that looks like a series of smiles. The unique rippling signature is a record in stone of the waves that once lapped on a wetter ancient Mars, researchers reported Tuesday.

Three weeks ago, Opportunity gathered chemical and geological clues from the same rock outcrop that indicated salty water had once soaked it. But it was unclear at the time whether the water pooled on the surface or had bubbled up from below ground.

The rippling pattern, which scientists found by electronically pasting together a "mosaic" of 152 postage stamp-sized images taken by Opportunity's microscopic camera, confirmed that the water was on the surface. Analysis of the wavy layering, called "cross-bedding," determined the water was at least 2 inches deep, maybe much deeper, and moving about 1 mph.

The discovery of liquid surface water in Mars' past increases the chance that life may have taken hold in the briny pools, as it did on Earth.

"We believe Opportunity is parked on what was once the shoreline of a salty sea," said Cornell University scientist Steve Squyres, principal investigator for the science equipment aboard Opportunity and its twin robot Spirit. "This was a shallow sea, a salt flat. We don't know that life was there, but we have an environment that is very suitable."

NASA researchers don't know yet the water's source, how long it remained on the surface, whether it was present in areas beyond the flat equatorial plain called Meridiani Planum where Opportunity is roaming, or what happened to it.

Nevertheless, the confirmation that water once pooled on at least a portion of Mars something that scientists had suspected for decades has "profound implications for future exploration," NASA Administrator Sean O'Keefe said at a Washington, D.C., news briefing.

Already, the space agency is planning to expand and extend the missions of Spirit and Opportunity through September if their batteries hold out.

And the targets for an orbiting satellite that NASA will send to Mars in 2005 and a more advanced, nuclear-powered rover that will launch in 2009 will probably have to be adjusted to take into account the new water findings at Meridiani.

"If you have an interest in searching for fossils on Mars, this is the place you want to go," said NASA Associate Administrator Ed Weiler.

The types of rocks that captured the ripples of the ancient Martian water also are "exceptionally good at preserving evidence of microbial life" in the form of fossils, Squyres said. However, the size of fossils left by algae, for example, would be too small to be detected by the cameras on Spirit and Opportunity. "You'd have a hard time seeing such things," said Dave Rubin, a sedimentologist with the U.S. Geological Survey.

Rubin was one of six non- NASA scientists whom the space agency asked to review the rock photos Opportunity took, to get an outside opinion.

Rubin said he was "astonished" by how alike the rippling pattern was to something one could commonly find on Earth. "You can go out to your nearest beach or creek and take your shovel and dig in and see some of these same kind of structures," he said.

While it is possible that wind, rather than waves, could pile up sediment grains in a similar smile-shaped pattern, the process probably still would require water just below the surface, he said. Given the evidence, NASA's theory of gently flowing bodies of saltwater "is the best explanation for those rocks," Rubin said.

Case Western Reserve Univer sity planetary geologist Ralph Harvey was more cautious Tuesday. Repeated evaporation of small amounts of water could have a similar effect, he said.

"I'm not going to believe in huge bodies of water until I see a beached whale," said Harvey, who was not part of the review team but is an expert in Martian meteorites. "I'm not even sure I'd go for a salty puddle. I'd probably go for the salty, moist soil." A good way of determining how widespread the water was will be to examine other outcroppings in greater detail, he said.

That is exactly what NASA plans to do, with both Opportunity and Spirit. On Monday, Opportunity climbed out of the shallow crater where it had landed 57 days earlier. NASA controllers plan to send it to a nearby crater that may have larger exposures of the same kind of sedimentary rock that captured the water ripples. More rock "enables you to look farther and farther into the Martian past, at a greater slice of Martian history," Squyres said.

Spirit will spend the next several months traveling to an expanse of rock called the Columbia Hills a mile and a half from its current location and much farther than the rover was designed to travel. The sediment layers at Columbia may hold other water clues.

"I'm just humbled by what Mars has shown us already," said Jim Garvin, NASA's lead Mars scientist. "You can bet it's not done showing us its secrets."

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