"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."
Ed Weiler, NASA's associate administrator for space science, said the agency's "ultimate quest at Mars is to answer the age-old question: Was there life, is there life on Mars? Today's results are a giant leap toward achieving that long-term goal."
Mars now is cold and dry and there is no apparent evidence of life on its barren surface. Nor have robot craft that have visited the planet found any direct evidence of life.
"This is a window into the past of Mars that's totally different from anything we see happening at the surface today," said Squyres.
But Squyres said Tuesday that chemical and geological clues gathered by Opportunity give dramatic proof that liquid water coursed over the rocks and soils at that spot on Mars at some time in the planet's history.
On Earth, such conditions "would be capable of supporting life," said Squyres. "We believe that that place on Mars for some period of time was a habitable environment."
Opportunity and its twin, Spirit, were launched to Mars last year to search out signs of water in the geology of two very different landing sites, reports CBS News Space Consultant Bill Harwood. Spirit landed inside Gusev Crater, which scientists believe once held a vast lake. So far, Spirit has encountered primarily volcanic rocks and soil, but scientists are hopeful lakebed deposits may be found in a nearby crater or elsewhere.
Opportunity landed on the other side of the red planet, in an Oklahoma-size region known as Meridiani Planum where hematite, a mineral that forms in the presence of water, had been detected from orbit. But today's announcement did not involve hematite. By pure chance, the rover came to rest in a small crater that featured exposed bedrock.
"Ever since Opportunity touched down on Meridiani Planum the night of Jan. 24 and we first opened our eyes and took a look around and saw this marvelous outcrop of layered bedrock literally right in front of us, we've been trying to puzzle out what this outcrop has been trying to tell us," Squyres said.
National Aeronautics and Space Administration scientists said they still don't known how long a life-friendly environment lasted, whether the water collected in surface pools or underground or when in the long history of Mars the liquid water existed. Answers to those questions, Squyres 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 to find evidence of Martian water.
That goal was accomplished, they said Tuesday, when Opportunity used all of its instruments to study and sample 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 in an area called Meridiani Planum, believed to be the smoothest, flattest region on Mars.
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" crystalized 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 crystalized as the water evaporated. Salts of bromine and chlorine, he said, were deposited in patterned layers that match the evaporation sequence found on Earth when briny water pools dry up.
The scientists also found what they called "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 crystalize 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 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.
CBS News Space Consultant William Harwood has covered America's space program full time for nearly 20 years, focusing on space shuttle operations, planetary exploration and astronomy. Based at the Kennedy Space Center in Florida, Harwood provides up-to-the-minute space reports for CBS News and regularly contributes to Spaceflight Now and The Washington Post.