The Mars Exploration Rover, Spirit, covered 637 meters in its first 90 Martian days, or sols (a sol is 40 minutes longer than an Earth day). Its trek through the Gusev Crater has not revealed any evidence for the ancient lake bed that geologists thought might be there, but water may have played some part in the formation of certain observed rock features.

One of the engineering goals for the two rovers, Spirit and Opportunity, was to have each explore the Martian terrain for at least 90 sols. Spirit, which landed on January 4, 2004--21 days before Opportunity--has been operational for more than 200 sols and counting. In 11 articles in the August 5 issue of Science, Spirit's science team reviews the data gathered from the nominal 90-sol mission.

Spirit landed in the eastern portion of the Gusev Crater, a 160-kilometer-wide basin at the foot of a large channel called Ma'adim Vallis. Mission planners speculated that water once flowed down this channel and pooled in Gusev. If so, it would have left behind sedimentary rocks.

Unfortunately, volcanic rock now covers the crater's surface, thus obscuring any signs of water that might lie beneath. Surprised and disappointed by this finding, scientists sent the rover to the nearby Bonneville Crater in hopes that the relatively recent impact may have exposed or dislodged an underlying sedimentary layer. But a search of the site failed to turn up geologic proof of an ancient lake. "Apparently, Bonneville didn't dig deep enough," observes Jim Bell of Cornell University, the lead author of one of the reports.

Still, along the way, Spirit has encountered some rocks with veins and surface coatings that may have been deposited by water. And the team has high expectations for Spirit's current enterprise to excavate the crater's low-lying Columbia Hills, which may have escaped the lava flows that carpeted the surrounding plains. "We may have a chance of finding a smoking gun in those hills," Bell remarks.

As winter approaches in Gusev, the decreased sunlight will mean less solar power to run the rover's electronics. The question is, how much longer can Spirit continue its quest for water before it runs out of steam? --Michael Schirber