Endurance Crater, Mars. Coloured image taken by NASA's Mars Exploration Rover Opportunity in Endurance Crater. The rover has been studying the layers of rock in this crater in order to learn about the geology of this part of Mars. Three holes have been drilled by the rover's Rock Abrasion Tool (RAT), from top left to centre bottom. The holes are approximately 4.5 centimetres across and are used to enable analysis of the rock beneath the surface, and hence the geological history; the earliest layers lie on the top, whereas the older layers are deeper into the crater. The top layers have been shown to be sulphate-rich. Photographed in June 2004.