The landing site of the Mars rover Opportunity was once drenched with water, providing an environment that could have supported life, NASA scientists announced at a press conference today.

Opportunity has been studying an outcropping of apparently ancient bedrock that's about the height of a curb.

The geologic signs of Martian water include Opportunity's discovery that the outcrop was full of sulfate salt deposits arranged in layers, with embedded spherical particles that can be formed by liquid water.

Initially, scientists thought the puzzling BB-sized spherules, as they are called, were lapilli, bits of volcanic material that may have formed from droplets of molten volcanic glass or meteor impact ejecta. But further study, including up-close looks at cross sections of these "blueberries," as scientists referred to them, led researchers to believe they are "concretions" of material, once dissolved in water, that gradually settled in porous rock.

"It grows around in a spherical shape until it fills in the pores of a rock," Squyres said.

One key to how the sphere's formed is the fact that they do not deform the layers in which they are embedded, as might have happened if the spheres were created in volcanic eruptions and then fell to the surface.

"Had they dropped in, they would have pushed the layers down," said John Grotzinger, a rover science team geologist from MIT. And had the spheres dropped in during catastrophic events, they would not be spread so randomly through all the rock layers, he said.

Opportunity also found odd indentations about the size and shape of a penny in the rock outcrop. Squyres said geologists see similar formations on Earth where crystals that once sat in a rock face had eroded away over time or dissolved due to a change in water chemistry.

More convincing were closer observations with two instruments on the rover arm, the Moessbauer and alpha particle X-ray spectrometers, as well as the mast-mounted miniature thermal emission spectrometer. Those devices found a pattern of salt deposition found in the slow evaporation of water.

"This is a window into the past on Mars unlike anything that is happening today," Squyres said.

Water is essential to life as we know it. Clark said the discovery, especially the sulfate deposits, could make the past existence of life on Mars very possible.

"There are organisms that use sulfate as an energy source," he said.

Bob Craddock, science advisor to the under secretary for science at the Smithsonian Institution, said the new findings gave him goose bumps. "This is why we put landers on the surface," he said.

Craddock is an expert on Mars geology and is not involved in the rover mission. He said the NASA scientists were being "very scientific" in their cautious approach to interpreting the findings.

Craddock told SPACE.com he's leaning toward standing water as the explanation for the many lines of evidence presented today. The presence of one mineral in particular, called kieserite, is typically left behind only when a sea evaporates, he said.

Don't expect any evidence of life to be detected by the current rovers, however. Scientists agree that if life ever existed on Mars, it did not get beyond simple, microbial stages. Remains of such life on Earth are typically detected not as conventional fossils but via mineral or chemical signatures only in very well preserved rock.

"Unless we find a dinosaur bed, we're not going to be able to [find signs of past life] in situ very easily," Craddock said. Instead, samples would need to be returned to an Earth lab for more study, he said.

Opportunity and its identical robot twin, Spirit, are part of an $820 million mission designed to search for water on Mars. Both rovers operate near the equator on opposite sides of the planet.

Opportunity has gleaned other images and measurements hinting at the existence of Martian water. Its landing site is rife with the mineral hematite, which typically forms in the presence of water. The hematite is more abundant outside the shallow depression where the rover is now.

Over the next week or two, the robot will try to characterize the hematite to add to the understanding of the planet's water history.

Scientists then want to look into how extensive the water was. There are no clear shoreline features suggesting any boundary to any possible ancient lake or ocean in Meridiani Planum.

Opportunity will trek nearly a half-mile (740 meters) eastward to a crater called Endurance, which has a bright rim that might be a larger version of the outcrop just investigated. Endurance is about 525 feet (160 meters) wide and 100 feet (30 meters) deep.

Then, if Opportunity lasts, the robot would head south about 1.2 miles (2 kilometers) toward mottled light-and-dark terrain, where bright areas seen from Martian orbit might be more outcroppings of bedrock.

Next Steps for NASA

Jim Garvin, NASA's lead scientist for Mars and the Moon, said the Meridiani Planum is now an early frontrunner for a Mars sample return mission. That's primarily because it still has the field pretty much to itself.

Garvin said that one of the problems confronting NASA's earlier plans to launch a Mars sample return mission is that scientists could only conjecture as to where a promising touch down site would be. Now with Opportunity's findings, the agency has at least one solid idea as to where to look.

"Now we have a candidate," Garvin said.

Whether Meridiani Planum remains the frontrunner, however, depends on what NASA spacecraft find between now and the end of the decade when planning for a Mars sample return mission is expected to be in full swing. Other missions -- including that of the Spirit rover -- will likely be adjusted to make the best use of what Opportunity has learned.

Garvin also said that the findings will be used to fine tune the planned Mars Reconnaissance Orbiter to locate and identify outcroppings similar to those at Meridiani Planum.