Multi-ring basin
A huge impact structure surrounded by several concentric rings of mountains or cliffs.
The very largest impacts make multi-ring basins, with two or more circular rings of mountains or cliffs spaced around a central depression. Their diameters run to hundreds or even thousands of kilometres. The outer rings are thought to form as the crust collapses inwards along great circular faults, possibly helped by weaker, warmer rock flowing beneath the crust, but the exact process is still debated.
These basins record the most violent period of the Solar System's history, when large bodies struck young planets and moons. On the Moon many were later flooded by basaltic lava, which formed the dark maria visible with the naked eye. Samples of their impact melt brought back by the Apollo missions have been dated to establish when the basins formed.
The classic example is Mare Orientale on the western edge of the Moon. Its outer ring, the Cordillera Mountains, is about 930 kilometres across and encloses inner rings like a giant bullseye. Because it is young for a basin and only partly flooded, its structure is unusually clear in spacecraft images, though from Earth it is seen almost edge-on. Valhalla on Jupiter's moon Callisto has many rings, which are mostly ridges and troughs in the icy crust.
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