Weiler hailed the mission by Opportunity and its twin probe Spirit as a "giant leap" toward answering the question of whether life ever existed on the Red Planet.

Two US robot probes blasted off from Earth in mid-2003 and have been exploring the Martian surface at opposite poles of Mars since January.

Steve Squyres, the scientific head of the Mars mission, stressed that there was no sign yet that life has existed on the planet. But he told a press conference that an outcrop of rocks in the area where Opportunity has been conducting research had provided signs of water.

"We've concluded that the rocks here were once soaked in liquid water," Squyres told a press conference at the NASA headquarters.

He said there was strong evidence that water had "acted" on the rocks, including the high sulfur content and hollows in rocks where crystals have developed.

"All of those clues together have led us to the conclusion these are probably what geologists call concretions, which form when there is liquid water in a rock, stuff is dissolved in it and it begins to precipitate ... begins to solidify from solution," said Squires of Cornell University.

NASA based its findings on exposed bedrock in a small crater close to Opportunity's landing site in the Meridiani Planum. The robot has spent much of the last three weeks on a study of a single rock NASA scientists dubbed El Capitan.

Data taken by an X-ray spectrometer aboard the robot and relayed to Earth showed a high concentration of sulfur, which is likely to have precipitated out of solution at the bottom of a salty lake or sea.

"The chemical form of this sulfur appears to be in magnesium, iron or other sulfate salts," according to Benton Clark of Lockheed Martin Space Systems in Denver.

"The only way you can form such large concentrations of salt is dissolve it in water and allow the water to evaporate," Clark said.

"Elements that can form chloride or even bromide salts have also been detected."

Spirit and Opportunity are expected keep looking for signs of life at least until April.

"We have tantalizing clues and we're planning to evaluate this possibility in the near future," said John Grotzinger, a sedimentary geologist at the Massachusetts Institute of Technology, in Cambridge, Massachusetts.

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