CALORIS

large basin measuring about 1,300 km (810 miles) in diameter, centred in the northern hemisphere of the planet Mercury. It was discovered in 1974 by the U.S. Mariner 10 planetary probe. The Caloris basin was created by the impact of a large meteorite early in Mercury's geologic history. The interior of the circular structure is occupied by fractured plains that may be composed of either impact melt or lava extruded shortly after the impact. The central plains are surrounded by a 1,300-kilometre-diameter ring of mountains 12 km high. The creation of the Caloris basin was a catastrophic event in the history of Mercury. In addition to the tectonic and volcanic disturbances produced by the rapid collapse of the original impact crater, the ejecta blanket buried a substantial area of the planet's surface. Furthermore, it is speculated that the shock waves from the impact were focused on the opposite side of the planet, producing a distinctive area of hilly terrain crisscrossed by straight valleys.