"Opportunity has landed in an area of Mars where liquid water once drenched the surface," said Edward Weiler, associate NASA administrator for space science, at a news conference. "This area would have been a good, habitable environment."
A study of a fine, layered rock by the rover detected evidence of sulfates and other minerals that form in the presence of water. The finding does suggest that if there had been life present when the rocks were formed, then the living conditions could have permitted an organism to flourish. The study, however, has found no direct evidence of living organisms.
"The ground would have been suitable for life," said Steve Squyres, lead investigator for science instruments on Opportunity. "That doesn't mean life was there. We don't know that."
Opportunity found "an astounding amount of salt," said Benton C. Clark III, a member of the rover team. This was found not only on the surface but confirmed when the rover bored into a rock outcropping with its abrasion tool.
"The only way you can form such large concentrations of salt is dissolve it in water and allow the water to evaporate," Clark said.
NASA launched the Mars Exploration Rover mission to see whether "at least one part of Mars had a persistently wet environment that could possibly have been hospitable to life," James Garvin, a lead NASA scientist, said in a statement. "Today we have strong evidence for an exciting answer: Yes."
Opportunity and its twin rover, Spirit, are controlled by a team of scientists working at the Jet Propulsion Laboratory in Pasadena, Calif. Scientists there earlier in the week said they had found exciting results from the work of Opportunity. Details were not immediately available.
Squyres, a Cornell University scientist and principal investigator for the science instruments on Opportunity, said the rover's study of formations near its landing site show that liquid water once flowed there, changing the chemistry and composition of the rocks.
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."
The twin Mars 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.
The rover conducted a chemical analysis of the outcrop, including a rock named El Capitan by scientists, and found a concentration of sulphur rich in magnesium, iron and other sulfate salts. Opportunity's instrument also detected jarosite, an iron sulfate mineral.
On Earth, such minerals would have formed in water and the presence of jarosite suggests an acid-rich lake or hot springs environment, scientists said.
John Grotzinger, a geologist from the Massachusetts Institute of Technology in Cambridge, said the evidence of water also includes three direct visual observations: the presence in El Capitan of small voids, called vugs; the presence of spherules, and the layering of the rock.
Images shows that El Capitan is pocked with .4-inch long indentations or voids that may have once contained salt minerals. Such voids, or vugs, form when crystals of salt minerals aggregate within a rock sitting in salty water. Later processes cause the crystals to disappear, leaving behind the voids within the rock.
BB-sized particles, called spherules, also formed in the rock. These can be formed from molten droplets originating from meteor impacts or from volcanic action, or they can precipitate from solution inside of porous rock.
NASA scientists said that since the spherules are randomly distributed they probably formed in water. If they were of volcanic or impact origin, the spherules would probably concentrate in rock layers that were exposed at the time of those events, the researchers said in a statement.
The rock also has layers in a pattern called cross-bedding that can be formed by water or wind action, the statement said.
More study of the target rocks is planned. Officials said they will maneuver the six-wheeled rover closer to the outcrop to get closer, more detailed views.