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What is the Caloris impact basin on Mercury?

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Messenger: Mercury — Mercury Mercury as seen by the Messenger probe, January 14, 2008. This image shows half of the hemisphere missed by Mariner 10 in 1974–75 and was snapped by Messenger's Wide Angle Camera when it was about 27,000 km (17,000 miles) from the planet.

The Caloris basin is the most prominent impact structure on Mercury. Formed approximately 3.9 billion years ago, it is one of the planet's youngest large multiringed impact basins.12

Physical Characteristics

The basin spans roughly 1,550 km (960 miles) in diameter. Its interior features smooth plains characterized by a complex pattern of ridges and fractures, including over 200 radial fractures. The basin is surrounded by a rim of irregular mountains reaching up to 3 km (2 miles) in height—the tallest on the planet—along with a secondary outer escarpment.12

Geological Context

The surrounding region includes ejecta terrains and plains that show significant evidence of volcanism, such as volcanic vents identified by the Messenger mission. Notably, the area directly opposite Caloris on the other side of the planet contains "weirdly contorted terrain," which likely resulted from the focusing of seismic waves generated by the massive impact.21