Martian rocks. True-colour image of Martian rocks on the slope of the 'Low Ridge' feature of the Gusev Crater. The dark, pockmarked rocks (one seen in the foreground) are thought to be volcanic in origin. The foreground boulder is 40 centimetres wide. This image was obtained by the Mars Exploration Rover Spirit. The colours of this image were created by combining camera filters to approximate what a human would see. The false- colour equivalent image is R360/250. Spirit landed in the Gusev Crater on Mars in January 2004, and obtained this image on 13 April 2006, the 810th Martian day of the rover's mission.