Gusev crater, Mars. Artwork of the Gusev crater (midcentre), the Apollinaris Patera volcano (upper left) and river Maadim Vallis (lower centre). Gusev has a diameter of 90 kilometres and is thought to have been formed three to four billion years ago by an asteroid impact. Scientists believe that water formed a river that drained into the crater. On January 3 2004, NASAs Mars rover Spirit landed in the crater. The rover has been able to grind away the surface cover on rocks found in the crater and analyse the minerals inside. In March 2004, Spirit found evidence of minerals crystallised out of water within a rock found in Gusev.
