3-D Density Structure of the Lunar Mascon Basins Revealed by a High-Efficient Gravity Inversion of the GRAIL Data

The lunar mascon basins are characterized by high gravity anomalies and thin crust. Study of the density structure beneath the mascon basins can help to understand the origin of these gravity anomalies and infer their formation and evolution. We propose an efficient forward gravity method based on the 3-D Gauss-Legendre quadrature (GLQ) and Fast Fourier Transforms combined with the adaptive discretization strategy to ensure high accuracy. The numerical example demonstrates that computational efficiency is increased by about three orders of magnitude compared to the traditional 3D GLQ method. We employ this forward method in a 3-D inversion of the gravity data derived from the lunar gravity model GL1500E. The inverted results show prominent high-density structures (namely, mascons) representing significant mantle uplift and thinned crust beneath most impact basins. Marked low-density rings surround the mascons. Most of the corresponding low-density anomalies extend from the near-surface to the Moho, indicating a thick, low-density crust. Our density model is consistent with the formation processes of mascons that an impact causes collapse of the transient crater and then the shocked mantle drives mantle flow. The low-density rings surrounding the mascons may stem from the crust thickening following an impact and extensive fracturing of the crustal column.