Three weeks ago, NASA announced definitive evidence that Mars once featured an abundance of water supporting a habitable environment. But major questions remained. Today, scientists unveiled photographs from NASA's Opportunity rover showing cross-bedded sedimentary rocks indicating that at least at one point on the martian surface - Meridiani Planum - a shallow, salty sea once ebbed and flowed.

"Why is this finding important? I think it's important for two reasons," said Steve Squyres, principal investigator for the Mars Exploration Rovers project. "First of all, if we're correct in our interpretation, this was a habitable environment on Mars, this was a shallow sea. It's a salt flat. These are the kinds of environments that are very suitable for life. We don't know that life was there, but we have an environment that was suitable for life.

"Second reason it's important is it's potential for the preservation of evidence. If you have liquid water and you precipitate minerals from liquid water, that process of precipitation, which can be very rapid, can actually trap inside the crystals, inside the mineral grains that are grown, whatever is in the water, whatever is in there chemically, biochemically, biologically, evidence that can be trapped and can be preserved for very long periods of time. I think Meridiani Planum would be a great place to go for a sample return mission."

The Opportunity rover landed by chance in a shallow crater in Meridiani Planum in late January. Scientists were elated to discover exposed bedrock in the crater and for the past month and a half, Opportunity has been studying those rocks in exhaustive detail. The rover finally climbed out of the crater Monday, after beaming back a treasure trove of data.

"Three weeks ago, we gathered in this same room and we provided evidence at that time that the rocks at Meridiani Planum once had water seep slowly through them, changing their chemistry, changing their texture in distinctive ways that we can look at," Squyres said. "What's happened since then is we have found what I believe is strong evidence that the rocks themselves are sediments that were laid down in liquid water.

"It's a fundamental distinction," he said. "It's like the difference between water you can draw from a well and water you can swim in. So this is a fundamentally different sort of environment we're talking about here. The evidence for this, the evidence that Opportunity is now parked on what was once the shoreline of a salty sea on Mars comes in two kinds."

First, spectrometers aboard Opportunity discovered widely varying amounts of bromine in the rocks. Squyres said that was a "fairly fundamental finding because it's characteristic of what you see in rocks on Earth that were formed by evaporation of sea water. So this was a chemical clue that some kind of evaporative process had been responsible for forming these rocks."

But bromine alone wasn't enough to prove whether Opportunity had discovered sediments deposited in free-standing water. For that, the science team turned from chemistry to geology, using the rover's microscope camera to build up a remarkably detailed mosaic showing the sort of cross bedding that only occurs in layers that were deposited in the presence of slowly flowing water.

The evidence was sent to a team of six independent scientists with no direct connection to the Mars rover program. And they agreed that the only likely explanation was the presence of a long-vanished sea.

"I was astonished ... to see sedimentary structures like we see on Earth and you can go out to your nearest beach or creek and take a shovel and dig in and see some of these same kinds of structures," said Dave Rubin, a sedimentologist with the U.S. Geological Survey.

Said Squyres, "There's still a lot we don't know. Yes, we believe Opportunity is parked on what was once the shoreline of a salty sea. But we don't know how laterally extensive this water was, we don't know how long it was there, we don't know how common this is at other places on Mars. The main thing is we have the capability in the future to find out."

He was referring to the Mars Reconnaissance Orbiter, a spy satellite-class science craft scheduled for launch in August 2005, and a nuclear-powered roving laboratory set for arrival by the end of the decade. In the meantime, two NASA orbiters and one built by the European Space Agency are continuing to map the red planet while Opportunity and its twin, Spirit, creep about the surface searching for clues about the persistence of water in the distant past.

Spirit landed on the other side of Mars, inside a crater that scientists believe once harbored a vast lake. Spirit has yet to find the sort of definitive evidence of water that Opportunity has uncovered, instead sending back vistas of igneous rock-strewn plains. But a cluster of hills some 1.2 miles away, named in honor of the shuttle Columbia's fallen crew, may feature lakebed sediments and Squyres is hopeful Spirit will last long enough to reach them.

Opportunity, now outside its landing crater, is heading for another crater 2,300 feet away that may feature exposed rocks from deeper underground.

"We expect to be able to find exposures of maybe many meters of this same kind of rock," Squyres said. "And what that enables you to do is look further and further and further into the martian past, look at a greater slice of martian history."

The big question, of course, is whether the rovers or any future craft might find evidence of past or present life. Squyres said the rovers' cameras are not able to resolve the sort of microscopic life that might flourish in a shallow, salty sea. For that, one would need to return rock samples to Earth for laboratory analysis.

"It appears the rocks at Meridiani were not just altered or modified by water, they were actually formed in water, perhaps a shallow, salty sea," said NASA science chief Ed Weiler. "This is a profound discovery, it has profound implications for astrobiology. If you have an interest in searching for fossils on Mars, this is the first place you want to go."

Stay tuned.