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"We'll both be at the rims of craters," he said of the two rovers' science teams, "one thinking about going in and the other thinking about going out onto the plain." Not counting occasional backup moves, Spirit has driven 171 meters (561 feet) from its lander. It has about half that distance still to go before reaching the crater rim. The terrain ahead looks different than what's behind, however. "It's rockier, but we're after rocks," Arvidson said. Spirit Mission Manager Jennifer Harris said the rover can traverse the rockier type of ground in front of it. As it approached the edge of a small depression in the ground earlier this week, the rover identified the slope as a potential hazard, and "did the right thing" by stopping and seeking an alternate route, she said. However, engineers are also planning to transmit new software to both rovers in a few weeks to improve onboard navigation capabilities. "We want to be more robust for the terrain we're seeing," Trosper said. The software revisions will also allow engineers to turn off a heater in Opportunity's arm, which has been wasting some power by going on during cold hours even when not needed. As it heads toward "Bonneville" to look for older rocks from beneath the region's current surface layer, Spirit is stopping frequently to examine soil and rocks along the way. Observations with its microscope at one wavy patch of windblown soil allowed scientists to study how martian winds affect the landscape. Coarser grains are concentrated on the crests, with finer grains more dominant in the troughs, a characteristic of "ripples" rather than of dunes, which are shaped by stronger winds. "This gives us a better understanding of the current erosion process due to winds on Mars," Shane Thompson, a science team collaborator from Arizona State University, said. A sequence of newly processed photos from Opportunity showed dust gradually obscuring the Sun during a blue-sky martian sunset. Dr. Jim Bell, lead scientist for the Mars rovers' panoramic cameras, said of the collection of images, "It's inspirational and beautiful, but there's good science in there, too." The amount of dust indicated by Opportunity's observations of the Sun is about twice as much as NASA's Mars Pathfinder lander saw in 1997 from another site on Mars. The rovers' main task is to explore their landing sites for evidence in the rocks and soil about whether the sites' past environments were ever watery and possibly suitable for sustaining life.
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