South Pole–Aitken basin
The South Pole–Aitken basin is an immense impact crater on the far side of the Moon. At roughly in diameter and between deep, it is one of the largest known impact craters in the Solar System. It is the largest, oldest, and deepest basin recognized on the Moon. It is estimated that it was formed approximately 4.2 to 4.3 billion years ago, during the Pre-Nectarian epoch. It was named for two features on opposite sides of the basin: the lunar South Pole at one end and the crater Aitken on the northern end. The outer rim of this basin can be seen from Earth as a huge mountain chain located on the Moon's southern limb, sometimes informally called "Leibnitz mountains".
On 3 January 2019, the Chang'e 4, a Chinese spacecraft, landed in the basin, specifically within a crater called Von Kármán. In May 2019, scientists announced that a large mass of material had been identified deep within the crater. Chang'e 6 aims to collect sample from this crater, specifically within the Apollo basin.
Discovery
The existence of a giant far side basin was suspected as early as 1962 based on early Soviet probe images, but it was not until wide-field photographs taken by the US Lunar Orbiter program became available in 1966-7 that geologists recognized its true size. Laser altimeter data obtained during the Apollo 15 and 16 missions showed that the northern portion of this basin was very deep, but since these data were only available along the near-equatorial ground tracks of the orbiting command and service modules, the topography of the rest of the basin remained unknown. The geologic map showing the northern half of this basin and with its edge depicted was published in 1978 by the United States Geological Survey. Little was known about the basin until the 1990s, when the spacecraft Galileo and Clementine visited the Moon. Multispectral images obtained from these missions showed that this basin contains more FeO and TiO2 than typical lunar highlands, and hence has a darker appearance. The topography of the basin was mapped in its entirety for the first time using altimeter data and the analysis of stereo image pairs taken during the Clementine mission. Most recently, the composition of this basin has been further constrained by the analysis of data obtained from a gamma-ray spectrometer that was on board the Lunar Prospector mission.Physical characteristics
The South Pole–Aitken basin is the largest, deepest and oldest basin recognized on the Moon. The lowest elevations of the Moon are located within the South Pole–Aitken basin. The Moon's tallest mountains are found around the basin's rim – they have summit elevations of up to 8,500 m and base-to-peak heights of up to 7,000 m. Because of this basin's great size, the crust at this locale is expected to be thinner than typical as a result of the large amount of material that was excavated due to an impact. Crustal thickness maps constructed using the Moon's topography and gravity field imply a thickness of about 30 km beneath the floor of this basin, in comparison to 60–80 km around it and the global average of about 50 km.The composition of the basin, as estimated from the Galileo, Clementine, and Lunar Prospector missions, appears to be different from typical highland regions. Most importantly, none of the samples obtained from the American Apollo and Russian Luna missions, nor the handful of identified lunar meteorites, have comparable compositions. The orbital data indicate that the floor of the basin has slightly elevated abundances of iron, titanium, and thorium. In terms of mineralogy, the basin floor is much richer in clinopyroxene and orthopyroxene than the surrounding highlands, which are largely anorthositic. Several possibilities exist for this distinctive chemical signature: one is that it might simply represent lower crustal materials that are somewhat richer in iron, titanium and thorium than the upper crust; another is that the composition reflects the widespread distribution of ponds of iron-rich basalts, similar to those that make up the lunar maria; alternatively, the rocks in the basin could contain a component from the lunar mantle if the basin excavated all the way through the crust; and, finally, it is possible that a large portion of the lunar surface surrounding the basin was melted during the impact event, and differentiation of this impact melt sheet could have given rise to additional geochemical anomalies. Complicating the matter is the possibility that several processes have contributed to the basin's anomalous geochemical signature. Ultimately, the origin of the anomalous composition of the basin is not known with certainty and will likely require a sample return mission to determine.
In 1994, the Clementine Mission detected several regions of water ice within the basin. These areas were mapped in greater detail by the Lunar Prospector mission in 1998 and several missions since then.