Caloris basin
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Mosaic of the Caloris basin based on photographs by the MESSENGER orbiter.
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| Planet | Mercury |
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| Coordinates | 30°30′N 189°48′W / 30.5°N 189.8°WCoordinates: 30°30′N 189°48′W / 30.5°N 189.8°W |
| Diameter | 1,550 km (963 mi) |
| Eponym | Latin for "heat" |
Caloris Planitia is a plain within a large impact basin on Mercury, informally named Caloris, about 1,550 km (960 mi) in diameter. It is one of the largest impact basins in the Solar System. "Calor" is Latin for "heat" and the basin is so-named because the Sun is almost directly overhead every second time Mercury passes perihelion. The crater, discovered in 1974, is surrounded by a ring of mountains approximately 2 km (1.2 mi) tall.
Caloris was discovered on images taken by the Mariner 10 probe in 1974. It was situated on the terminator—the line dividing the daytime and nighttime hemispheres—at the time the probe passed by, and so half of the crater could not be imaged. Later, on January 15, 2008, one of the first photos of the planet taken by the MESSENGER probe revealed the crater in its entirety.
The basin was initially estimated to be about 810 mi (1,300 km) in diameter, though this was increased to 960 mi (1,540 km) based on subsequent images taken by MESSENGER. It is ringed by mountains up to 2 km (1.2 mi) high. Inside the crater walls, the floor of the crater is filled by lava plains, similar to the maria of the Moon. These plains are superposed by explosive vents associated with pyroclastic material. Outside the walls, material ejected in the impact which created the basin extends for 1,000 km (620 mi), and concentric rings surround the crater.
In the center of the basin is a region containing numerous radial troughs that appear to be extensional faults, with a 40 km (25 mi) crater located near the center of the pattern. The exact cause of this pattern of troughs is not currently known. The feature is named Pantheon Fossae.
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