WASHINGTON — Water percolating through the soil once created a friendly environment that would have been ideal for life to flourish on Mars, NASA scientists say.
It isn't known how long this environment lasted or if any organism actually developed. But scientists directing robot rovers prowling the Martian surface said Tuesday the evidence now is clear that some rocks "were once soaked with liquid water."
"The ground would have been suitable for life," said Steve Squyres of Cornell University, the lead investigator for science instruments on the rover Opportunity. "That doesn't mean life was there. We don't know that."
Mars now is cold and dry, and there is no apparent evidence of life on its barren surface.
But Squyres said chemical and geological clues gathered by Opportunity give dramatic proof that at some time in its past, liquid water coursed over the rocks and soils.
Such conditions on Earth, Squyres said at a news conference, "would be capable of supporting life."
"We believe that that place on Mars for some period of time was a habitable environment," he said.
Squyres said it is not known how long the life-environment lasted, if the water collected in surface pools or underground, and when in the long history of Mars the liquid water existed. Answers to those questions, he said, probably will require missions that scoop up Martian samples and bring them to laboratories on Earth.
NASA researchers from the Jet Propulsion Laboratory in Pasadena, Calif., who are guiding the exploration of Mars by Opportunity and its twin, Spirit, said the primary goal of the rover mission was find evidence of Martian water.
That goal was accomplished, they said Tuesday, when Opportunity used all of its instruments to study a fine, layered rock called El Capitan. The rock is embedded in the wall of the crater where the six-wheeled robot began its journey on Mars.
"We've been attacking that outcrop with everything we have," said Squyres. The payoff is chemical and geological evidence of a water history at that one spot.
Benton C. Clark III, a Lockheed Martin Space Systems scientist and a member of the rover team, said that when Opportunity used an abrasion tool to bore into the rock it found "an astounding amount of salt" crystallized inside.
"The only way you can form such large concentrations of salt is dissolve it in water and allow the water to evaporate," Clark said.
Clark said the salts may have been dissolved in water and then crystallized as the water evaporated. Salts of bromine and chlorine, he said, are deposited in patterned layers that match the evaporation sequence found on Earth when briny water pools dry up.
The scientists also found what they call "blueberries," small, globular-shaped inclusions in the rock that can be formed by water. The inclusions are rather like blueberries in a muffin, hence the name. Some of the spherical objects have rolled into a small basin, called the "blueberry bowl" by scientists, and will be analyzed further by Opportunity to confirm their water origin.
Images also show voids the size of pennies randomly distributed in the martian rocks. Such voids, called vugs, are often formed in rocks on Earth when chemicals crystallize and then erode away, leaving behind an empty space.
Discovery of an iron sulfate mineral called jarosite adds to the evidence, said Squyres.
"This is a mineral you've got to have water to make," he said.
Other processes, such as volcanoes or meteorite impacts, could account for the "blueberries" or the vugs, but Squyres said the random distribution of the rock inclusions make water the most likely source.
Asked how long ago water might have existed on Mars, Squyres said: "It's very, very difficult to infer age simply by looking at pictures, by measuring this kind of composition. What you really need is samples brought back." He did say that whatever process in the past produced the water, "There's nothing like this going on Mars today."
The twin rovers now on Mars will not return to earth but will remain on the red planet after they stop functioning. President Bush has proposed sending manned flights to the moon and Mars.
Opportunity landed five weeks ago near an exposed bedrock embedded in the wall of a small crater. Spirit landed half a planet way, on a flat plain. It is now heading toward a crater.
The scientists said they plan further studies of El Capitan and other rocks in the crater outcropping. Such work, said Squyres, is expected to help confirm the water history on Mars and perhaps produce more surprises.
"Stay tuned," he said.