The space rover Opportunity has discovered that the ancient surface of Mars was soaked in water -- and may therefore have supported life -- NASA scientists announced today.

Using an array of photographs, spectrometer data and scientific deductions, scientists said that a rock near where the rover landed offered the most detailed, convincing and precise evidence that water once existed on the red planet.

"Opportunity has landed in an area of Mars where water once drenched the surface," said Ed Weiler, associate administrator of NASA's Office of Space Science. The evidence, he added, was "a giant leap" toward determining whether life could have existed on Mars.

Long the subject of space fantasies and human terrors, the proof of water -- and the possibility of life -- that scientists unveiled today was at once more mundane and more exciting than anything dreamed up by Orson Welles and Hollywood.

No little green men were in sight, but the evidence sent back by the rover's cameras and robotic instruments offered what scientists called unmistakable evidence that the planet once held water.

A rock near the rover's landing site that scientists have dubbed El Capitan offered multiple intersecting clues of the existence of water, said Steve Squyres, a Cornell scientist and principal investigator of the Mars Exploration Rover.

Little spherical objects, which Squyres said looked "like blueberries in a muffin," stood out on the rock surface. The objects suggested that water within the rocks had concreted around nuclei, forming the spheres, Squyres said.

"Concretions form when there is liquid water inside rock," said Squyres.

Scientists also discovered tell-tale signs of crystals, and spectrometer analyses -- taken after the rover drilled several millimeters into the surface of the rock -- showed the presence of sulfur.

Benton C. Clark III, chief scientist of space exploration at Lockheed Martin Space Systems Astronautics Operations, said the rocks probably contained magnesium sulfate -- a drier version of what can be purchased here on earth at any pharmacy: Epsom salts. The compound again points to the presence of water.

"This is a milestone day for planetary exploration because for a long, long time we believed that Mars was warm and wet," said David Eicher, editor of Astronomy Magazine. "We know now this is the case."

"Everything you need to have for life was there in abundance long ago," said Eicher. "It is a stunning discovery."

The El Capitan outcrop is about 66 feet wide and nearly a foot high, said John Grotzinger, a Massachusetts Institute of Technology scientist and a geologist with the project. It is deeply layered.

Another nearby rock, which scientists have dubbed Last Chance, has details suggestive of "crossbedding" -- patterns in the rock consistent with sediment particles that were moved around in a flowing current.

That current, Grotzinger said, could have been water -- but it could also have been air or volcanic gases.

"What an amazing time to be alive doing science on Mars," exclaimed Jim Garvin, NASA's lead scientist for Mars and the Moon. The discovery, he said, would alter future missions.

The Mars Reconnaissance Orbiter, he said, would next look for evidence of ancient seas. And NASA would begin work on one of the most challenging tasks ever devised the space agency: "These rocks could be a wonderful preservation environment for ancient habitats on Mars," said Garvin. NASA scientists, he added, "want to bring rocks home."

Opportunity arrived on Mars on Jan. 24, landing on a broad plain called Meridiani. Shrouded in protective airbags, the rover bounced to a stop inside the crater from which the robot rover has conducted its investigations so far. Opportunity's six-wheeled twin sister, Spirit, landed on the other side the planet three weeks earlier and is currently navigating toward a crater nicknamed Bonneville.