Martian rock investigations. Holes (centre and upper centre) made in Martian rock by the Mars Exploration Rover Opportunity. This rock, nick- named 'El Capitan', is situated along the rim of the crater in which Opportunity landed on 24 January 2004. The rover used its rock abrasion tool (RAT) to drill the holes on Martian days 30 and 34 of the mission. The microscopic imager and spectrometers revealed that sulphate minerals are present in the rocks. These could only have formed in the presence of water, confirming that water was once found on Mars. Tyre tracks from the rover's wheels are seen at lower right and left. Image taken on 1 March 2004.