This is an oblique view of part of the Argyre Basin on Mars. The huge and ancient impact crater (870 kilometers across) has been heavily eroded by water, ice and wind. It is though that the basin once held a partially frozen lake, and sinuous sediment features on the lake bed were probably formed by rivers flowing beneath the ice. Several more recent impact craters dot the basin floor. The thin Martian atmosphere, composed mainly of carbon dioxide with small amounts of nitrogen and other gases, is visible above the limb.