Appearance
The Caloris Basin 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 crater was initially estimated to be about 1,300 km (810 mi) in diameter, though this was increased to 1,550 km (960 mi) 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. 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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