Less than a week after NASA scientists announced that data from the Mars rover Opportunity point to a wet history for the Red Planet, the Spirit rover has found additional evidence of Martian water. The twin rovers are canvassing opposite sides of the faraway planet and are encountering vastly different clues. The latest finding comes from a dark volcanic rock, nicknamed Humphrey, from Marss Gusev Crater.

"If we found this rock on Earth, we would say it is a volcanic rock that had a little fluid moving through it," explains Ray Arvidson of Washington University, who works on the rovers scientific instruments. The two-foot-tall rock has bright material in its crevices and cracks that appear to be minerals that crystallized out of water. (The false-color image above depicts the area of interest.) To ensure that these characteristics were not the result of surface dust intruding into the crevices over the years, investigators instructed Spirit to scrape away the rocks exterior. Although the team is confident that the rock had contact with liquid water, the amount of water that the crystals suggest is significantly less than the amount believed to be present at Opportunitys landing site at Meridiani Planum. "Mars is a diverse planet," Arvidson notes.

Spirit is on its way to the 150-meter-wide Bonneville crater, which is less than 90 meters away. The scientists plan to drill into another, as yet unselected, dark volcanic rock along the way. Meanwhile, Opportunitys attempt to penetrate a rock dubbed "Flat Rock" over the weekend was not successful. So far the rock abrasion tool appears to be functioning normally, but there has been no discernible impression made on the stone. "Our activities are getting increasingly complex," remarks Opportunity mission manager Matt Wallace. "Between the two rovers, weve had a terrific 100 days on Mars. This last week has been particularly exciting."