Martian rock. Image of the Martian rock 'Humphrey', showing a hole (centre) made by the rock abrasion tool on the Mars Exploration Rover Spirit. Spirit landed on Mars on 4 January 2004, and carried out this grinding process on the 60th Martian day, or sol, of its mission. The purpose of the grinding was to expose new layers of rock for chemical analysis by Spirit's spectrometers. The mineral composition of the rock may reveal how it was formed and whether this area of the Martian surface was once covered in water. The circular depression measures 46 millimetres in diameter. This image was taken with Spirit's navigation camera.