Martian rock investigations. Mars Exploration Rover Opportunity examining bedrock on the Martian surface. This rock, nick-named 'El Capitan', is situated along the rim of the crater in which Opportunity landed on 24 January 2004. The rover's robotic arm (instrument deployment device, upper left) is seen poised to use its rock abrasion tool (RAT) to grind the rock surface. Two of the rover's wheels are at lower right and left. Opportunity will use its microscopic imager and spectrometers to examine the freshly-exposed rock for clues to the rock's formation and composition. This image was taken on 23 February 2004 by Opportunity's front hazard-avoidance camera.