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The largest crater in the Solar System is located on the Moon. The impact crater is named the South Pole-Aitken basin. It is in the neighborhood of 2,500 km in diameter and is 13 km deep. It is the largest, oldest, and deepest recognizable basin on the Moon. The area is mainly located on the far side of the Moon, so it can not be visualized by Earth bound telescopes. Because of its location, depth, and the height of the crater walls, the majority of the basin is in constant shadow.
The only way to directly observe the basin was to send spacecraft to orbit the Moon. The Lunar Orbiter Program launched by NASA in the 1960s returned the first images of the crater. The altimeters aboard the Apollo 15 and 16 missions were used to measure the depth of the northern edge of the crater. The first complete geologic map of the basin was published by the United States Geological Survey in 1978, but very little was known until the Galileo and Clementine spacecraft visited in the 1990s. Multi-spectral images have shown that the basin contains more iron oxide and titanium dioxide than normal. The topography of the basin was mapped using altimeter data and the analysis of stereo image pairs taken during the Clementine mission.
Although it is called an impact crater, scientists are not 100% sure what created it. Scientists know that if a large asteroid had struck the Moon directly on the pole, it would have thrown up vast amounts of mantle material from as deep as 200 km below the surface. That material is missing from the surface in and around the basin, so scientists think that an object may have hit the surface at a low angle, not digging into the surface deeply.
NASA statements show that the North and South Poles of the Moon may contain a combined six billion metric tons of water ice. The Lunar Prospector also found that the water is in concentrations of .3% to 1%, mixed in with the lunar soil, called regolith. Clementine estimated that there could be from 110 million to 1.1 billion tons of water ice in a 5,500 square mile area around the lunar South Pole. This is enough water to support 2000 average people for over 500 years and could be a source of water for any lunar surface space stations.
The South Pole-Aitken basin is the largest crater in the Solar System. Whether it was caused by a direct impact or a glancing blow, it may contain the water that is needed to supply an colonization of the Moon and could be a source of minerals in the future.
We have recorded a whole series of podcasts about the Solar System at Astronomy Cast. Check them out here.