Oceans of data point to ancient Martian sea.

A robotic rover on Mars has radioed to Earth strong evidence that some rocks near the Red Planet's equator formed from sediments in a shallow, ancient ocean. In announcing the finding, scientists have identified a promising site at which to look for remains of life.

"If you have any interest in searching for fossils on Mars, then this is the place you want to go,' declares Ed Weiler, NASA associate administrator for space science in Washington, D.C. Lead Mars-rover scientist Steve Squyres of Cornell University and his colleagues unveiled the images, taken with the rover Opportunity, at a March 23 NASA press briefing.

The surface of the region explored by Opportunity is bone-dry. But detailed images that the rover has taken of a shallow crater, dubbed Eagle, indicate that an outcrop of rocks there was laid down by a briny body of flowing water. Opportunity landed on Jan. 25 in the crater, which is part of an equatorial plain called Meridiani Planum.

"We think Opportunity is parked on what was once the shoreline of a salty sea," says Squyres.

The rover's magnifying lens shows that rocks in the outcrop contain fine, rippled layers that are at various angles rather than in orderly, parallel rows. The layers can best be explained as having been caused by ripples of water flowing above a sandy surface, says rover scientist John Grotzinger of the Massachusetts Institute of Technology.

He estimates that the water was at least 5 centimeters deep and gently flowed at 10 cm to 50 cm per second. There's no indication yet of how long the body of water persisted.

The new findings enlarge the water story that made a splash 3 weeks ago (SN: 3/6/04,p. 147). Scientists then announced that data gathered by Opportunity showed that rocks in Eagle had at one time been soaked by water, leaving behind a residue of sulfate minerals and BB-size particles.

The evidence announced this week suggests that the rocks were not only chemically altered by water seeping through them but had been deposited by a persistent, standing body of water.

"These Mars sedimentary structures look just like what we see on Earth;' says Dave Rubin of the U.S. Geological Survey in Santa Cruz, Calif. Rubin is a member of an independent team of scientists that reviewed the findings.

If these Mars rocks are, in fact, just like those on Earth, they would be well suited to preserve remains of organisms, says James B. Garvin, lead scientist for Mars exploration at NASA headquarters in Washington, D.C. Opportunity isn't equipped to search for fossils, but a mission planned for launch early next decade is intended to collect samples and return them to Earth.

Opportunity is now headed for another crater, named Endurance, which lies 700 meters away from Eagle. Rover images taken at a distance suggest that Endurance may also contain sedimentary rocks. If close-up images corroborate this, it would provide an indication of how extensive the ancient Martian water was.