Shallow Crustal Structure of the Lunar Farside From High‐Resolution GRAIL Gravity

Abstract The structure of the lunar crust preserves key records of the Moon's origin and long‐term evolution. Using the latest high‐resolution gravity field from the Gravity Recovery and Interior Laboratory (GRAIL) mission together with Lunar Orbiter Laser Altimeter (LOLA) topography, we investigate the shallow crustal structure of the lunar farside. Applying the effective density spectrum technique with a two‐layer formulation, we constrain the densities of the upper and lower layers and the depth of their interface. Our results reveal a thin (average 4.7 km), low‐density (average 2,111 kg ) surficial layer, likely representing a highly porous megaregolith formed by basin ejecta and impact fragmentation, overlying a higher density unit (average 2,739 kg ) consistent with a less porous, anorthositic crust. From the inferred upper‐layer density, we estimate crustal porosity and find that the porosity decreases systematically with increasing interface depth, while exhibiting substantial lateral variability at a given depth. This variability is closely linked to differences in cumulative impact modification. Residual porosity below ∼2 km shows a statistically significant negative correlation with N(20), indicating that repeated small impacts have progressively compacted the shallow crust and reduced its pore space.