The Contribution of Impact Basins and Mascons to the Lunar Figure: Evidence for Lunar True Polar Wander, and a Past Low-Eccentricity, Synchronous Lunar Orbit

As first noted 200 years ago by P.S. Laplace, the Moon's rotational and tidal bulges are significantly larger than expected, given the Moon's present orbital and rotational state. This excess deformation has been ascribed to a fossil figure, frozen in when the Moon was closer to the Earth. However, the observed figure (quantified by the Moon's moments of inertia, or degree-2 gravity field) is only consistent with an eccentric and/or non-synchronous orbit, contrary to our understanding of the Moon's formation and evolution. Here, we show that lunar mascons and impact basins have a significant contribution to the observed lunar figure. Removing their contribution reveals a misaligned fossil figure consistent with an early epoch of true polar wander driven by the formation of the South Pole-Aitken Basin. This mass anomaly corrected figure is also consistent with an elastic lithosphere that formed while the Moon was on an early low-eccentricity, synchronous lunar orbit. This new self-consistent model solves a long-standing problem in planetary science, and will inform future studies of the Moon's dynamical evolution and early dynamo, and motivates future exploration of South Pole-Aitken.