Impact crater size comparison. Artwork comparing the diameters of impact craters found in the solar system relative to the size of the United States. From outer circle inwards, the craters are: Valhalla on the Jovian moon Callisto (3,900 kilometres in diameter), Aitken Basin on the lunar south pole (2,500 km), Hellas Basin on Mars (2,100 km), Argyre Basin on Mars (1,800 km), Caloris on Mercury (1,550 km), Mare Imbrium on the lunar surface (1,130 km), Vredefort in South Africa (300 km) and the Chicxulub crater on the Yucatan Peninsula (180 km). Impact craters are created when an asteroid or comet hits the surface. Craters larger than 300 km are refered to as impact basins. USA topographic map originally from the USGS.