This image provided on Thursday, Sept. 1, 2011, by NASA shows the agency's Mars Exploration Rover Opportunity using its camera to take this picture showing the rover's arm extended toward a light-toned rock, "Tisdale 2," during the 2,695th Martian day on Aug. 23, 2011. The solar-powered rover beamed back images of the horizon, soil and nearby rocks that are unlike any it has seen during its seven years roaming the Martian plains. (AP Photo/NASA)

This image provided on Thursday, Sept. 1, 2011, by NASA shows the agency's Mars Exploration Rover Opportunity using its camera to take this picture showing the rover's arm extended toward a light-toned rock, "Tisdale 2," during the 2,695th Martian day on Aug. 23, 2011. The solar-powered rover beamed back images of the horizon, soil and nearby rocks that are unlike any it has seen during its seven years roaming the Martian plains. (AP Photo/NASA)