Density Structure of the Moon's Crust Beneath the Polar Regions as Revealed by Gravity Data

Abstract

Impact cratering is a geologic process that can fracture crustal materials and change their physical properties. On the Moon, the cumulative effects of impact cratering have given rise to a surficial fine‐grained impact gardened regolith that is 10s of meters thick, along with deeper layers of thick impact ejecta and in situ fractured crust. Collectively, the thick ejecta deposits and underlying fractured crust are referred to as the megaregolith, but its physical properties and thickness remain poorly understood. We use high resolution gravity data from the Gravity Recovery and Interior Laboratory (GRAIL) mission to investigate how density and porosity vary with depth below the surface. We employ localized multitaper spectral analyses with a Bayesian inversion approach that uses Markov Chain Monte Carlo sampling of density profiles comprised of constant density layers. Analyses are restricted to the polar regions where the quality of the gravity models is the best. Our results reveal a distinct two‐layer density structure that is characterized by an upper megaregolith layer that is about 1–2 km thick with porosities of ∼15%–20% and a less porous underlying layer with porosities of 7%–13%. These two layers are interpreted as being composed of allochthonous impact ejecta and autochthonous in situ fractured crust, respectively. The depth of the lower layer extends to at least 11 km beyond which our inversion approach loses sensitivity to density. We show that these levels of porosity reduce the bulk seismic velocities and thermal conductivity of the crust.