A past lunar dynamo thermally driven by the precession of its inner core

Abstract

The Cassini state equilibrium associated with the precession of the Moon predicts that the mantle, fluid core and solid inner core precess at different angles. We present estimates of the dissipation from viscous friction associated with the differential precession at the core-mantle boundary (CMB), $Q_{cmb}$, and at the inner core boundary (ICB), $Q_{icb}$, as a function of the evolving lunar orbit. We focus on the latter and show that, provided the inner core was larger than 100 km, $Q_{icb}$ may have been as high as $10^{10}-10^{11}$ W for most of the lunar history for a broad range of core density models. This is larger than the power required to maintain the fluid core in an adiabatic state, therefore the heat released by the differential precession at the ICB can drive a past lunar dynamo by thermal convection. This dynamo can outlive the dynamo from precession at the CMB and may have shutoff only relatively recently. Estimates of the magnetic field strength at the lunar surface are of the order of a few $\mu$T, compatible with the lunar paleomagnetic intensities recorded after 3 Ga. We further show that it is possible that a transition of the Cassini state associated with the inner core may have occurred as a result of the evolution of the lunar orbit. The heat flux associated with $Q_{icb}$ can be of the order of a few mW m${}^{-2}$, which should slow down inner core growth and be included in thermal evolution models of the lunar core.

arXiv comment33 pages, 7 figures

References in corpus (9)

• Tidal evolution of the Moon from a high-obliquity, high-angular-momentum Earth
• Precession-driven dynamos in a full sphere and the role of large scale cyclonic vortices
• Precessing spherical shells: flows, dissipation, dynamo and the lunar core
• Tidal instability in a rotating and differentially heated ellipsoidal shell
• Observational constraint on the radius and oblateness of the lunar core-mantle boundary
• Collisionless Encounters and the Origin of the Lunar Inclination
• Magnetic fields driven by tidal mixing in radiative stars
• Fossil field decay due to nonlinear tides in massive binaries
• The Cassini State of the Moon's inner core

Cited by in corpus (3)

• Persistent but weak magnetic field at Moon's midlife revealed by Chang'e-5 basalt
• Seven decades of exploring planetary interiors with rotating convection experiments
• Viscous dissipation in the fluid core of the Moon