Lunar Gravity Anomalies and South Pole-Aitken

The Bouguer lunar gravity field, determined up to an unprecedented precision from a combination of high-resolution gravity data from the Gravity Recovery and Interior Laboratory (GRAIL) mission and high-resolution topography from the Lunar Orbiter Laser Altimeter (LOLA) instrument onboard the Lunar Reconnaissance Orbiter (LRO), is dominated by the South Pole-Aitken (SP-A) basin that covers approximately 15% of the lunar surface. The major component of the basin's Bouguer gravity (after a 2550 kg m^-3 uniform density topographic correction) is a long-wavelength positive gravity anomaly of 800 mGal that is the result of mantle uplift beneath a thin crust. In the region of the south pole below latitude 80S there are several large impact basins inside and outside the topographic boundary of SP-A. Together these large basins dominate the region below 80S. We present an in-depth analysis of the Bouguer anomalies in this area.

In the length scale of 10-50 km, the full thickness of the crust, the rims of most of these large craters, such as Scott, show up in the Bouguer gravity as positive anomalies while the interiors appear as mostly negative. Some craters, such as Schrodinger-Zeemann, inside SP-A, the interior anomalies are large and positive. For Bouguer anomalies with the shortest length scales, 10-20 km, the spatial distribution appears much lower inside SP-A than outside but with no particular pattern, although there is some suggestion of a greater concentration inside Scott crater. At longer wavelengths, length scales of 40 to 50 km, the pattern of anomalies follow the boundary of the SP-A both inside and outside the basin suggesting that a sequence of anomalies exist at a depth of 45 km at the base of the crust, probably the result of density variations or changes in crustal thickness.