Spirit landed on January 3, 2004, while its twin probe Opportunity touched down three weeks later. Both launched one of the most successful space missions of all time.

"I never, ever would have imagined the opportunity to be literally standing here a year later and say, yet again, we're back and we're still on Mars," outgoing National Aeronautics and Space Administration administrator Sean O'Keefe told scientists running the Mars mission at the Jet Propulsion Laboratory (JPL) this week.

O'Keefe, who is expected to continue at NASA until a successor is named in February, said the twin geological rovers were "extraordinary pieces of machinery" that have yielded very important discoveries -- the biggest of 2004, according to Science magazine.

Above all, O'Keefe noted the evidence Opportunity found that a vast deposit of salt-water once sloshed around the Planum Meridiani in Mars' valley-shaped equatorial region.

Spirit, which landed in the newly named Gusev crater, fully opposite Opportunity's landing site, in December found goethite in a rock formation, another sign there once was water on Mars since the mineral is formed only in the presence of water in ice, liquid or vapor form, NASA said.

"What it tells us is that the climate, the atmosphere of our closest neighbor was once dramatically different and perhaps conducive to life," said O'Keefe, who addressed the JPL scientists in Pasadena, California, on the anniversary of Spirit's arrival on Mars.

"Understanding why that changed may well provide a whole new perspective of our own place in the solar system, in this galaxy and, indeed, in the broader universe," he added.

For David Grinspoon, planetary scientist at Southwest Research Institute in Boulder, Colorado, the probes' findings "also tells us that we must go back to Mars with equipment to look for fossils or chemical signs of past Martian life."

"The rocks don't lie. Their chemistry and detailed texture reveal the story," he wrote in an op-ed piece published Tuesday by The Los Angeles Times. Grinspoon also referred to other signs of a Mars' watery past seen in many photos of valleys and ancient lakes beamed back by the two probes and previous Mars missions.

"Before Opportunity, such a search would have seemed far-fetched; now it is not only reasonable but obligatory," said the author of "Planetary Planets: The Natural Philosophy of Alien Life."

"If Mars was wet but never 'alive', then perhaps the conditions we believe to be necessary for life are not sufficient," wrote Grinspoon.

"In that case, dumb luck might have played a larger role in our existence than we like to think, and the universe might be a lonely place indeed for inhabited worlds like ours," he added.

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