The robot explorer Opportunity has seen clear evidence of the main goal of Mars exploration - that water once flowed or pooled on the Red Planet's surface.

"Opportunity has landed in an area of Mars where liquid water once drenched the surface," NASA associate administrator Ed Weiler told a news conference. "Moreover, this area would have been good habitable environment." 

A study of a fine, layered rock by the rover detected evidence of sulphates and other minerals that form in the presence of water. The finding suggests 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 life.

"NASA launched the Mars Exploration Rover mission specifically to check 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, California.

Scientists there earlier in the week said they had found exciting results from the work of Opportunity. Details were not immediately available.

Rock composition

Steve 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.

More study of the targeted rocks
on the red planet, is planned

"We've been able to read the telltale clues the water left behind, giving us confidence in that conclusion," Squyres said in a statement.

Additional studies will determine if the rocks were laid down by minerals formed at the bottom of a salty lake or sea.

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 sulphate salts. Opportunity's instrument also detected jarosite, an iron sulphate mineral.

Lake or hot springs

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.

"We've been able to read the telltale clues the water left behind, giving us confidence in that conclusion"

Steve Squyres,
Scientist and principal investigator based at Cornell University

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 one centimetre 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.

Meteor, volcanic action

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 crossbedding that can be formed by water or wind action, the statement said.

More study of the target rocks is planned. Officials said they will manoeuvre the six-wheeled rover closer to the outcrop to get closer, more detailed views.