Two U.S. robot rovers are still gathering scientific data about the history of water on Mars, one year after the first one landed. It is about nine months longer than the robots were expected to last. The mission team gathered at the Jet Propulsion Laboratory in Pasadena, California Monday to celebrate the first anniversary of the landing and to reveal new discoveries being made on Mars.

To the astonishment of mission managers and scientists, the pair of six-wheeled rolling geology carts called Spirit and Opportunity, which touched down three weeks apart last January, continue to send back high quality images and other data about their surroundings on opposite sides of Mars.

"To me, the most remarkable story of today is that the rovers are still going," said Cornell University geologist Stephen Squyres, who leads the team controlling the Opportunity rover. It was Opportunity that found chemical evidence in exposed bedrock last March that a shallow saltwater sea once flowed on Mars. A few months later, Spirit also found rocks altered by water on the other martian hemisphere, evidence suggesting that water percolated through soil underground as the rocks were forming.

"The legacy, the lasting legacy of this mission, is going to turn out to be the recognition by people here on this planet that our sister planet Mars once had habitable conditions on its surface," Mr. Squyres said. "What that meant for the origin of life, what it meant for the evolution of life, that's for the future Mars program to determine."

Scientists had not expected the two rovers to last this long. But Mars helped by having a less frigid winter than anticipated and by kicking up less than the predicted amount of dust to deposit on the solar energy panels. This has allowed the rovers' batteries to retain a strong charge. Mechanical problems have been minor.

NASA Administrator Sean O'Keefe, who recently announced he will soon be resigning, says the successful arrival of the two rovers a year ago did much to boost the morale of a nation hurt by the 2001 terrorist attacks and the morale of a space agency that had lost seven astronauts aboard the ill-fated mission of the shuttle Columbia in 2003.

"It was at a time when we really needed it most," he said. "It was an accomplishment that positively reassured us that we are and continue to do great things."

But mission scientists are not resting on their accomplishments. Stephen Squyres says Spirit has encountered a rock nicknamed "Wishstone" with a composition unlike anything they have seen on Mars, but which could be yet another indicator of Mars' wet past.

While rocks tend to have grains of a uniform size, Wishstone's grains vary in size and have sharp angles, suggesting it was formed suddenly in an explosive event like a meteor impact or volcanic eruption. The Cornell geologist says it also has much more phosphorous than previously analyzed martian rocks. This could be because the rock initially had a high phosphorous content or because the rock was washed by water rich in phosphorous salts called phosphates.

"If it is what happened, then it speaks of a water chemistry dramatically different from what we saw just 500 meters away on the west spur," he said. "So it's telling us something dramatically different about the water chemistry, somehow the water chemistry changing with position or changing with time."

As geologists analyze rover findings, NASA is preparing to launch another spacecraft in seven months to join two other U.S. satellites already orbiting Mars. The newest one will do advance imaging for landing sites for another planned rover.

The head of NASA's Mars exploration program, Iranian-born Firouz Naderi, says it is part of a long-term plan to eventually return to the Earth rock and soil samples that could contain fossils of martian life.

"Ours is to look for areas that show the highest potential for having been a habitat, areas that exhibit the sign of having had of having water and complex carbon chemistry, which most astrobiologists see as essential for the emergence of life," he said.

In the very long term, the United States hopes to send humans to explore Mars, but has set no date for such a mission.