Variations of porosity in intermediate-sized lunar impact basins

Abstract

The Moon is cratered with hundreds of well-preserved impact basins, and shared bombardment history with Earth makes lunar basins a good proxy for the processes affecting early Earth. Basin-forming impact events alter the bulk density and porosity of the targeted crust, and measurement of these quantities offers a means by which we can investigate the mechanisms of basin formation. In this work, we calculate the bulk density of the crust in the vicinity of 50 basins with diameters of 170–360 km—a size range that has been neglected by previous studies—with the goal of understanding the deformational mechanisms present during impact events. Our bulk density estimates are derived from gravity data from NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission and topography data from the Lunar Orbiter Laser Altimeter (LOLA) using a new technique similar to the Nettleton method in terrestrial gravimetry. By making assumptions about crustal compositions across the Moon, we can furthermore estimate porosity in and around the basins. Low porosity is observed in the interior of 76% of the basins studied, and this trend becomes more apparent when basins in the Feldspathic Highlands Terrane are observed alone. Overall, impact events appear to decrease porosity inside basins and increase porosity outside basins, suggesting the prevalence of impact melting within the basins and fracturing and ejecta deposition in the exteriors. Our results are seemingly inconsistent with previous studies that measured high porosity values inside basins from residual Bouguer anomalies. However, these disparate results may be reconciled if impact melting processes are confined to the uppermost kilometers of the lunar crust and are underlain by heavy impact-induced fracturing.