Impact basin
An impact basin is a very large impact crater, usually more than about 300 kilometres across on the Moon.
An impact basin is an enormous crater, so large that its structure is different from smaller craters. On the Moon, craters bigger than about 300 kilometres are usually called basins. They often have several concentric rings of mountains, which is why they are also called multi-ring basins.
The Moon's maria fill many of its basins, such as the Imbrium basin, about 1,100 kilometres across, now flooded with dark basalt lava. Mars has the Hellas basin, around 2,300 kilometres wide. Mercury has the Caloris basin, and the asteroid Vesta has Rheasilvia, almost as wide as Vesta itself.
Basins formed mostly early in Solar System history, when large impacts were more frequent. Dating their rocks, such as impact melts from the Moon, helps reconstruct that history. On Earth, plate tectonics and erosion have erased most ancient basins, though some very old impact structures, such as Vredefort, may originally have been basin-sized.
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