Moon's Largest Impact Basin

The South Pole-Aitken basin is the largest and oldest recognized impact basin on the Moon. It's diameter is roughly 2,500 km or 1,550 miles. The Moon's circumference is just under 11,000 km, meaning the basin stretches across nearly a quarter of the Moon. In the LROC WAC mosaic below, which is centered on the middle of the basin, you can see SPA as an area of relatively low reflectance extending from the crater Aitken in the north and all the way down to the South Pole. Topographic data from LOLA can also help to give a sense of the enormous effect the SPA impact had on the Moon - the basin is more than 8 km deep.

A tratigraphic relationships show that SPA is the oldest impact basin on the Moon, but scientists are intensely interested in just how old it is. Lunar samples suggest that most of the major basins on the Moon formed around 3.9 billion years ago in a period called the late heavy bombardment. By this time most of the large debris within the solar system should have already accreted to form the planets, so such a large number of big impacts occurring at nearly the same time may have been due to unusual gravitational dynamics in the early Solar System.

The Constellation region of interest, highlighted in the NAC detail above and outlined in the WAC mosaic was selected , because it is in a deep portion of the basin, where a large volume of melt would be expected. Some of this melt would remain as a significant component of the soil, and an analysis of a carefully selected suite of samples from this region would reveal the age of the oldest lunar impact basin.