Water-filled Gusev crater, Mars. Artwork of the Gusev crater (large indentation, mid-centre), as seen millions of years ago when Gusev was filled with water and mud. At the top right, Ma'adim Vallis, the river to Gusev, is visible. Gusev has a diameter of 90 kilometres and is thought to have been formed three to four billion years ago by an asteroid. On January 3 2004, NASA's Mars rover 'Spirit' landed in the now arid crater. The rover has been able to grind away the surface cover on rocks found in the crater and in March 2004, found evidence of minerals crystallised out of water.