ater is the elixir of life, and scientists reported almost certain evidence today that the tiny crater that the Mars rover Opportunity has been scooting around for the last month was once soaked in it.

The finding greatly increases the prospects that Mars was a much more hospitable planet early in its history, possibly even amenable to the rise of life.

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The scientists do not know what kind of wet environment existed at the rover's landing site: perhaps groundwater percolating up through volcanic ash, perhaps a lake bed that dried up, perhaps something else.

Nevertheless, "we believe at this place on Mars for some period in time, it was a habitable environment," said Dr. Steven W. Squyres, a professor of astronomy at Cornell and the principal investigator for the mission. "This is the kind of place that would have been suitable for life."

He quickly added: "Now that doesn't mean life was there. We don't know that."

Dr. Squyres said he could not say how long the area remained wet or when it was wet, except that it was not recently.

Highlighting the significance of the findings, the National Aeronautics and Space Administration held the news conference not at the Jet Propulsion Laboratory in Pasadena, Calif. where the scientists have been working, but instead flew the scientists to NASA headquarters in Washington to make the announcement.

"Our ultimate quest at Mars is to answer the age-old question, `Was there life, is there life on Mars?' " Dr. Edward J. Weiler, NASA's associate administrator for space science, said at the news conference this afternoon. "Today's results are a giant leap toward achieving that long-term goal."

NASA officials also used the opportunity to promote coming missions and the initiative President Bush announced in January to send astronauts back to the moon and eventually to Mars.

"These results from Mars are already laying the foundation for the new vision of robotic and human exploration of the solar system and beyond that was announced by our president from this very stage just six weeks ago," Dr. Weiler said.

Nowadays, the surface of Mars is cold and arid, but persistent speculation, based on huge canyons and channels carved in parts of the surface, is that the atmosphere was once thick and warm enough to allow liquid water to exist on the surface. Another possibility is that Mars has always been cold, except for brief episodes of torrential rains following volcanic eruptions or meteor impacts.

The mission of the two rovers that NASA landed on Mars in January is to search for signs of past water. At least in a small crater on the flat plains of Meridiani Planum, the landing site of the Opportunity rover, scientists have succeeded.

Since its arrival Jan. 25, the Opportunity has spotted suggestive hints of past water — fine layers in bedrock that might be sedimentary rock deposited at the bottom of a lake or sea and an iron mineral that usually forms in the presence of water. However, in both cases, viable alternate explanations also exist. The layers could be volcanic ash or sediments carried by wind. The iron mineral can also form directly from lava.

The scientists still do not yet know whether the rocks formed in water, but as to whether water subsequently altered the minerals in the rocks, "the answer to that we believe, definitively, is yes," Dr. Squyres said.

Close-up examination of the bedrock, exposed along the rim of the crater that the Opportunity is in, provided four lines of evidence.

The most compelling evidence is large quantities of jarosite, a mineral that contains iron, sulfur — and trapped water. "This is a mineral that you've got to have water around to make it," Dr. Squyres said.

Instruments also measured high levels of sulfur in the rocks, probably in the form of sulfur salts. "The only way you can form such large concentrations of salt on Earth normally is to dissolve it in water and have the water evaporate," said Dr. Benton C. Clark III, chief scientist of space exploration at Lockheed Martin Space Systems and a member of the science team.

Photographs also show holes in the rocks roughly the shape and size of pennies. The scientists believe these are places where minerals carried by water formed crystals that subsequently dissolved or fell out.

The final evidence is the curious round pebbles, nicknamed "blueberries," that are scattered around the surface and are also embedded in the bedrock. The blueberries, the scientists said, are objections known as concretions that form within sedimentary rocks.

Dr. John P. Grotzinger, professor of earth sciences at Massachusetts Institute of Technology, said other explanations were ruled out because the pebbles did not displace the layers around them, indicating that they formed within the rock, and they were evenly distributed throughout the rock. Had the pebbles been, for instance, glass beads formed from molten rock from a volcanic eruption of a meteor impact, the pebbles would have pressed down the layers where they struck and be found only in certain layers, he said.

The discoveries make Meridiani Planum a promising candidate for a future robotic mission, at least a decade away, that would bring pieces of Mars back to Earth for closer examination.

Next, the Opportunity rover will cozy up to a section of the bedrock nicknamed Big Bend where scientists may find evidence that the rocks not only sat in water but were also formed in water. Cursory photographs already show ripples and angled layers that might indicate sediment that was pushed around by flowing water.

"We don't have an answer to that one yet," Dr. Squyres said. "We may have something for you in another week to two weeks."

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