Early Dynamics of the Lunar Core

Abstract

The Moon is known to have a small liquid core, and it is thought that in the distant past the core may have produced strong magnetic fields recorded in lunar samples. Here we implement a numerical model of lunar orbital and rotational dynamics that includes the effects of a liquid core. In agreement with previous work, we find that the lunar core is dynamically decoupled from the lunar mantle, and that this decoupling happened very early in lunar history. Our model predicts that the lunar core rotates sub-synchronously, and the difference between the core and the mantle rotational rates was significant when the Moon had a high forced obliquity during and after the Cassini State transition. We find that the presence of the lunar liquid core further destabilizes synchronous rotation of the mantle for a wide range of semimajor axes centered around the Cassini State transition. CMB torques make it even more likely that the Moon experienced large-scale inclination damping during the Cassini State transition. We present estimates for the mutual core-mantle obliquity as a function of Earth-Moon distance, and we discuss plausible absolute time-lines for this evolution. We conclude that our results are consistent with the hypothesis of a precession-driven early lunar dynamo and may explain the variability of the inferred orientation of the past lunar dynamo.

arXiv commentAccepted for JGR:Planets

References in corpus (8)

• The origin of the Moon within a terrestrial synestia
• Satellite dynamics on the Laplace surface
• Tidal evolution of the Moon from a high-obliquity, high-angular-momentum Earth
• Lunar Laser Ranging Science: Gravitational Physics and Lunar Interior and Geodesy
• Determining parameters of Moon's orbital and rotational motion from LLR observations using GRAIL and IERS-recommended models
• Collisionless Encounters and the Origin of the Lunar Inclination
• Dynamical Instabilities in High-Obliquity Systems
• The Cassini State of the Moon's inner core