Mars rovers: new evidence of past water.

Twin rovers on opposite sides of the Red Planet have found additional evidence that liquid water once flowed there, scientists announced last week during a telephone briefing. Designed to last only 3 months, the rovers have been reporting data back to Earth since January (seep. 253).

On the southwestern slope of the shallow. stadium-size crater in which it landed, the rover Opportunity has found that several flat rocks bear a geometric network of fractures. The fractures resemble mud cracks on Earth, which form when water-soaked soil dries and contracts, notes rover researcher John Grotzinger of the Massachusetts Institute of Technology.

Opportunity had found evidence that a body of standing water long ago deposited other rocks at the rover's landing site (SN: 3/27/04, p. 195). That watery era would have occurred before the region eroded into a vast plain, called Meridiani Planum.

The new data suggest that rocks at Meridiani were exposed to water twice--once when they formed from lake or ocean sediments and then briefly again after the impact that excavated the crater. During this later era, the water-deposited rocks got a thorough soaking and cracked as they dried out, suggest Grotzinger and rover principle investigator Steve Squyres of Cornell University.

"When we saw these polygonal crack patterns, right away we thought of a secondary water event significantly later than the episode that created the rocks," says Grotzinger. One source of the water could have been frost deposits that melted during a warm spell. It's even possible that there was a pond inside the crater, Squyres speculates.

However, a second episode of water is not the only explanation for the fractures, Squyres notes. The cracks, found in a rock called Esther and several others, may represent the final drying out from a single wet period in which the rocks formed. Or the same three that gouged the crater could have generated the cracks.

Scientists are now directing the rover to examine the pattern of cracks in a boulder called Wopmay, which lies near Escher. That study may narrow the number of explanations, Squyres says.

Halfway around Mars, the rover Spirit has scraped the patina from a dime-size patch of the lock called Ebenezer. From the color ,and texture of the surface, scientists had guessed that Ebenezer, which lies in an area known as Columbia Hills, might be a volcanic rock that had remained unaltered for eons. But spectra of the scraped spot reveal that Ebenezer, like many other rocks examined by Spirit, has been enriched in phosphorus, sulfur, sodium, chlorine, and bromine.

Each of these elements is easily transported by liquid water. Squyres suggests that the elements dissolved out of nearby rocks during a wet era and that later. when the water evaporated, they were deposited in high concentration in Ebenezer and other rocks.

"Every single rock in [Columbia] Hills shows signs of alteration by water," says Squyres.