Martian rock. Surface of the Martian rock 'Adirondack', showing a depression made by the the rock abrasion tool on the Mars Exploration Rover Spirit. Spirit landed on Mars on 4 January 2004, and carried out the grinding process on 6 February 2004. It was the first time a rock had ever been ground on Mars. 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 (mm) in diameter and is nearly 3 mm deep.