The most recent data from the Mars rover Opportunity indicate that the section of the Red Planet it is studying was once soaking wet, NASA scientists announced yesterday. "We've been able to read the telltale clues the water left behind, giving us confidence in that conclusion," says principal investigator Steve Squyres of Cornell University.

Since its January 24th landing in a small crater in the Meridiani Planum region of Mars, Opportunity has been exploring its surroundings, particularly a rock outcrop on the inner slope of the crater. As the robot collected more information on the bedrock, the planets watery history became clear, the NASA group says. "Liquid water once flowed through these rocks. It changed their texture, and it changed their chemistry," Squyres asserts. According to the team, four lines of evidence point to the existence of liquid water. First, small spherical objects embedded in the rock are similar to so-called concretions, which form when minerals precipitate out of water. Second, close-up shots reveal irregularly shaped holes at various angles in the rock, the imprints of crystals formed by water. Third, spectrometry analyses indicate that the rock contains enormous quantities of sulfur, which the researchers attribute to it being awash in sulfate salts, a telltale sign of liquid water. Finally, a second spectrometer detected the mineral jarosite, an iron-sulfate hydrate, in the outcrop. This mineral is found on Earth, though rarely, and is made only in the presence of water. "We believe at this place on Mars, for some point in time, this was a habitable environment," Squyres remarks. "This is the kind of place that would have been suitable for life. That doesnt mean life was there; we dont know that."

Indeed, plenty of questions about Martian water remain. Although the researchers are confident that liquid water flowed over or through the rocks, they do not yet know whether the rocks were laid down in liquid water. More detailed pictures taken in the days to come could help resolve this unknown, however. In addition, the age of the rocks Opportunity is studying cannot be determined, because the most effective methods for dating require samples to be returned to a laboratory on Earth.

In the coming weeks, Opportunitys handlers plan to explore other regions of the outcrop called Big Bend and hope to send it more than 740 meters out to a crater dubbed Endurance. Says project scientist Joy Crisp, "We need to take a close look at the outcropping, and broaden our view to get a better understanding of the geology of the region, which is about the size of Oklahoma."