A U.S. rover on Mars has taken close-up images of an unusual soil and pebble mixture that scientists are hoping will help tell them whether water once flowed there.

If you were walking next to the Opportunity rover, you might think you were on a strange beach. You would be standing on a thin layer of gray sand over a very fine red dust, with light colored pebbles about three centimeters wide scattered across the surface.

The configuration has intrigued mission scientists in California, who have gotten their first close look at the soil at the bottom of a very shallow crater.

"We've discovered some interesting things," said lead mission geologist Steven Squyres of Cornell University. "There are some features in this soil unlike anything that's been seen on Mars before. We've got a number of different components in this soil. They are mixed in different ratios in different places. Some places are rich in the sand, some places are rich in the redder stuff, some places have a lot more of these pebbles, some have fewer of the pebbles. We're looking for correlations there. We haven't got a complete story yet. This stuff is falling into place as we go."

Because the irregularly shaped pebbles have rounded edges, scientists believe they could have been worn by water. They have already found traces of a mineral called hematite in the soil, an iron compound that often forms in water.

But, according to Mr. Squyres, it is too early to conclude that water flowed at this site. "With respect to extrapolating from a few grains of sand to a story about water on Mars, [it's] a little hard to do at this point," he explained.

The Cornell geologist said Opportunity is being prepared to drive six meters to a nearby rock formation where its instruments have sensed a higher concentration of hematite. If the rover's instruments can prove the hematite formed in water and not by a dry volcanic process, as is sometimes the case, the evidence would suggest the Red Planet had environmental conditions that could have supported life.

Opportunity's twin on the other side of Mars, the Spirit rover, has been undergoing a memory cleansing. Excess data left over from its seven-month voyage to Mars clogged one part of its memory two weeks ago, forcing its computer to continually restart and interrupting its efforts to collect science data

Engineers had hoped to have this section of Spirit's memory reformatted earlier in the week. But deputy mission manager Mark Adler says that they cautiously practiced for four days on a test model rover to make sure erasing it would not have unwanted side effects, such as destroying computer programs needed to run the vehicle.

"It's not an operation we do lightly," he said. "We did go through a lot of testing to make sure that the operations do exactly what we expect. You've got to make sure that it works right and we've done that, and so we're doing it on the spacecraft right now."

Mission engineers say they are confident the rover will operate normally again.