Can the Moon's Center of Mass–Center of Figure Offset Be Explained With a Uniform Primordial Crust?

Abstract

A fundamental constraint on the Moon's interior mass distribution is the 1.935-km lunar center of mass (COM)–center of figure (COF) offset. Extant constraints on the mass asymmetry that generates the COM-COF offset—commonly attributed to a crustal thickness asymmetry wherein the nearside crust is thinner than that of the farside—do not permit a unique solution for the lunar interior structure. Using simple analytical models of isostasy and porosity evolution, we quantify potential contributions to the lunar mass asymmetry from nearside-farside asymmetries (specifically, spherical harmonic degree-1 variations) in porosity, crustal basalts, and dense late-stage magma ocean cumulates. We demonstrate that these asymmetries could simultaneously explain the COM-COF offset and allow for a lunar crust that formed with globally uniform thickness and porosity. Scenarios with an excess of ∼10–44 km of late-stage cumulates in the nearside relative to the farside allow for full ranges of 5%–12% nearside anorthosite porosity, 1–2 km of excess nearside basalts, and nearside crustal thickness of either 30 km or 38 km. Furthermore, under specific conditions (30-km nearside crust with low porosity and high late-stage cumulate density of ∼3,600 kg/m³), the COM-COF offset permits an initially uniform crust as well as a present-day crust with uniform thickness. While observational constraints do not favor perfectly symmetric present-day crustal thickness, our analyses highlight the importance of higher fidelity characterization of the lunar interior structure and the use of caution in investigations that fundamentally rely on lunar crustal thickness constraints.