PASADENA, Calif. -- Mars had a shallow pool of briny water on its surface long ago, NASA said Tuesday in announcing what could be the strongest evidence yet that the now-dry Red Planet was once hospitable to life.
The space agency's scientists announced earlier this month that the Opportunity rover found evidence of water in Mars' distant past. But it was unclear whether the water was in the soil or on the surface. The new findings suggest there was a pool of saltwater at least two inches deep.
A rocky outcropping examined by the rover had ripple patterns and concentrations of salt -- considered telltale signs that the rock formed in standing water.
"We think Opportunity is now parked on what was once the shoreline of a salty sea on Mars," said Cornell University astronomer Steve Squyres, the mission's main scientist.
The findings add to the growing body of evidence that the Red Planet was once a wetter and possibly warmer place that may have been conducive to life.
"This is a profound discovery, it has profound implications for astrobiology, and I'd like to say if you have an interest in searching for fossils on Mars, this is the first place you'd want to go," said Ed Weiler, NASA's associate administrator for space science.
Although Squyres referred to the water as a sea, scientists said it is not clear how big the body of water might have been or whether it was a fixed feature and not just a desert basin that flooded periodically.
The evidence also does not indicate when water covered the broad and flat region where Opportunity landed, called Meridiani Planum, or for how long. Nor does it indicate if any organisms actually lived on Mars.
If life did flourish at the site when it was awash in water, the type of rock found there is capable of preserving evidence of any biological material, he said.
"If we are correct in our interpretation, this was a habitable environment," Squyres said. "These are the kinds of environments that are very suitable for life."
The findings were presented at a televised news conference at the headquarters of the National Aeronautics and Space Administration in Washington.
Weiler noted that a group of outside scientists was brought in to review the rover mission scientists' findings before they were announced.
The findings give NASA impetus to expand its Mars exploration program to learn whether microbes ever existed there and, ultimately, whether people can live there, Weiler said.
NASA plans to send a more sophisticated rover to Mars in 2009 to probe for signs of life. In 2013, the space agency plans to send a robotic mission that would collect rock and soil samples and bring them back to Earth for more detailed analysis.
NASA intends to send one or more unmanned missions to Mars every 26 months. President Bush recently proposed a manned mission to visit the planet but did not set a timeline for such an undertaking, which probably remains decades away.
Today, Mars is largely dry and cold. It contains trace amounts of water vapor in its atmosphere and large caps of frozen water at its poles. Spacecraft also have detected significant amounts of ice mixed in the martian soil at high latitudes.
For decades, spacecraft in orbit around Mars have also detected evidence that large amounts of liquid water once flowed across the surface, carving vast and sinuous networks of channels.
But the new findings provide the first definitive evidence from rocks on the surface of Mars that liquid water once pooled on the surface.
Opportunity carried out detailed analyses of the finely layered rocks at its landing site, snapping 152 microscopic images of one feature alone.
The close-ups revealed that the sediments that bonded together to form the rocks were shaped into ripples by water that stood at least two inches deep, said mission team member John Grotzinger of the Massachusetts Institute of Technology. The water flowed over the site at about 1 mph, Grotzinger said.
The analyses also bolstered previously disclosed evidence that suggested the rocks contained a salt called bromine, which would have precipitated out of the water as it evaporated.
NASA's Jet Propulsion Laboratory launched Opportunity and its twin, Spirit, on a $820 million mission to probe Mars for evidence of water in its past.
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