Lateral and vertical heterogeneity of the lunar South Pole–Aitken basin derived from gravity and topography

Abstract

Gravity field in the South Pole–Aitken (SPA) basin at the lunar farside reflects multiple superposed cratering and accumulated damage from billions of years of impactor history. This study applied localized gravity–topography analysis from Gravity Recovery and Interior Laboratory (GRAIL) gravity field and Lunar Orbiter Laser Altimeter (LOLA) topography to estimate effective bulk density and porosity within the basin, and determine how the crust varies laterally and with depth. Results show that SPA exhibits pronounced lateral heterogeneity in its upper crust, with a mean bulk density of ∼2620 kg m^–3 and a mean porosity of ~11.7%. Layered analysis further indicates a depth-dependent structure: lower-degree harmonic bands, representing deeper structures, are denser and less porous than higher-degree bands that are proxies for the shallower crust. The derived bulk density decreases from 2653 kg m^–3 at degrees 150–300 to 2350 kg m^–3 at degrees 750–850, while porosity increases from 10.1% to 20.5%. Results imply that the SPA basin contains a mechanically damaged, porous uppermost crust overlying a denser and less porous subsurface. Furthermore, it underscores the structural complexity of the SPA basin, in which crustal heterogeneity arises not only from the original formation event but from years of subsequent, long-term modification of the crust.