Soil stuck to the wheels of a U.S. rover on Mars is causing mission scientists to wonder if it is mud. Two rovers are on the red planet seeking evidence of liquid water that might once have supported life. The Spirit rover has arrived at a small depression in the ground nicknamed Laguna Hollow and has wiggled its wheels to disturb the very fine soil. This kind of maneuver allows its instruments to sense minerals below the surface. When the vehicle backed up, scientists noticed that some of the fine soil stuck to its wheel.