Evidence for Lunar True Polar Wander, and a Past Low-Eccentricity, Synchronous Lunar Orbit

TL;DR

This study reveals that the Moon's ancient figure was shaped by true polar wander and a low-eccentricity, synchronous orbit, challenging previous eccentric orbit assumptions and clarifying lunar deformation origins.

Contribution

The paper demonstrates that lunar mascons and impact basins significantly influence the lunar figure, supporting a model of early true polar wander and a synchronous orbit.

Findings

Lunar mascons and impact basins affect the observed lunar figure.

A fossil figure consistent with true polar wander is identified.

The model supports a low-eccentricity, synchronous early lunar orbit.

Abstract

As first noted 200 years ago by 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 is only consistent with an eccentric and 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) and 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.