Implications of Bouguer Gravity Structure Under Major Lunar Basins

Analysis of the lunar Bouguer gravity field under major basins reveals how gravity varies with spherical harmonic degree L and, potentially, with depth (to relate the two we use a relationship based on point masses).  We have studied 19 lunar basins based upon a GRAIL 1200 degree and order gravity model (GRGM1200B).  The vertical component of Bouguer gravity shows how the gravity is distributed in spherical harmonic degree between the lowest degree, 2, and the highest degree, 1200. Under each basin, this gravity spectrum of accelerations per individual spherical harmonic degree shows a benign region for L from 800 to 100, a range of approximately 20 km immediately below the surface, consistent with the observation that the upper crust is largely homogenous (Zuber et al., 2013). A region of more varied gravity signal occurs down to L~20, approximately 60 km deeper. The basin gravity signal merges with the deep interior at L~10, approximately 150 km below the surface. A set of profiles over latitude or longitude through an individual basin anomaly shows how the magnitude of the gravity signal changes with depth as it passes from the annular moat to the central high of the anomaly; all of which takes place between L~100-20, a depth range estimated to be ~20-80 km.  However, all basins are different to some extent. Outside of the basin anomaly the gravity spectra are relatively benign from just below the surface to L~40, a depth of approximately 45 km and consistent with the approximate average thickness of the lunar crust.  An exception to the general characteristics of the spectra of basins is South Pole-Aitken (SPA) which indicates a structure with few variations that is very similar to the regions that have near zero Bouguer gravity at the surface with no large anomalies in the top 100 km. We interpret this result for SP-A as a result of its largely compensated state.