The weather forecast for Mars: Cold. Breezy. Dust devils a hundred yards wide. This is late summer in Gusev Crater, where the Spirit rover is beginning to move around on the surface. While the ground might be a balmy 44 degrees Fahrenheit, the air above it climbs to only 12 or 15 degrees. Although there are no weather instruments on the twin rovers — Opportunity lands in about a week on the other side of the planet — the robotic geologists can help the science team at NASA’s Jet Propulsion Laboratory learn more about Mars’ mysterious atmosphere.