South Pole-Aitken Basin
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The South Pole-Aitken basin is an impact crater on Earth's Moon. Roughly 2500 kilometers in diameter and 13 kilometers deep, it is the largest known impact crater in the entire solar system. The only impact basin close to it in size is the 2100 kilometer Hellas Planitia on Mars. This basin was named for the two points at the extreme ends, with Aitken crater overlaying the northern end and the southern lunar pole at the other end.
The existence of the basin was suspected as early as 1962, but global photography by the Lunar Orbiter program in the mid-1960s confirmed its existence. The first complete map was published in 1978 by D. Stuart-Alexander of the United States Geological Survey. Little was known about the basin until the 1990s, when the spacecraft Galileo and Clementine visited the Moon. The basin contains more FeO and TiO2 than lunar highlands typically have, and has a darker appearance. Possibilities for the distinctive chemical composition are the presence of an unusual rock type or types, widespread distribution of ponds of iron-rich basalts like those in the lunar maria, exposure of lower crustal rocks with a different bulk composition from the surface, or the presence of rocks dug up from the lunar mantle. The origin of the material is not known with certainty at this time, however.
Geophysicist experts in impact dynamics are convinced that a normal impact could not have produced the basin without digging up vast amounts of mantle materials, but observations thus far have been highly inconclusive about whether there is any mantle material present at all. This suggests that the basin was not formed by a typical high-velocity impact, but may instead have been formed by a low-velocity projectile that hit at a low angle (about 30 degrees or less), and hence did not dig very deeply into the Moon. Such a glancing impact would have sent much of the resulting debris back into space surrounding the Moon and Earth, which may have provided a source of projectiles to make other lunar basins, many of which may have been made in a narrow time interval between 3.85 and 3.95 billion years ago. This is still largely speculation, however.
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