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 two features on opposing sides; the crater Aitken on the northern end and the southern lunar pole at the other end. The outer rim of this basin can be seen from Earth as a huge chain of mountains located on the lunar southern limb, sometimes called "Leibnitz mountains", although this name has not been considered official by the International Astronomical Union.
The lowest elevations of the Moon (about -6 km) are located within the South Pole-Aitken basin, and the highest elevations (about +8 km) are found on this basin's north-eastern rim. 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 would have been excavated during this impact event. Crustal thickness maps constructed using the Moon's topography and gravity field imply a thickness of about 15 km beneath the floor of this basin, in comparison to the global average of about 50 km.
The composition of this basin, as estimated from the Galileo, Clementine and Lunar Prospector missions, show that it is different than 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 a composition that is comparable. The orbital data indicate that the floor of this 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 that are largely anorthositic. Several possibilities exist for this distinctive chemical signature. One is that this composition might simply represent lower crustal materials that are somewhat more rich in iron, titanium and thorium than the upper crust. Another possibility is that this composition reflects the widespread distribution of ponds of iron-rich basalts, similar to those that make up the lunar maria. Alternatively, the rocks here could contain a component from the lunar mantle if the basin excavated all the way through the crust. The origin of the anomalous composition of this basin is not known with certainty at this time, however, and a sample return mission will most likely be required to settle this debate. Complicating matters is the fact that all three of the above hypotheses could contribute to the anomalous geochemical signature of this giant crater. Furthermore, it is possible that a large portion of the lunar surface in the vicinity of this basin was melted during the impact event, and differentiation of this impact melt sheet could have given rise to additional geochemical anomalies.