Resonant lunar tides of Earth's core and basal magma ocean
Abstract
Earth's magnetic field is generated by fluid motion in the liquid-metal core and has been active for billions of years. However, prior to the onset of inner-core growth, the sources of mechanical power that drove the geodynamo remain uncertain. During this period, the core may have been overlain by a basal magma ocean (BMO), creating two immiscible fluid layers separated by a density interface, beneath the solid mantle. We develop a theory for lunar tides in this core--BMO system, in which the tidal potential acts on the density contrast between the two layers. The resulting dynamics differ fundamentally from previous models driven by imposed deformation of the core's bounding envelope. In the inviscid limit, the response transitions from an equilibrium tide to an interfacial gravity wave as forcing frequency increases. The two regimes are separated by a resonance that occurs when the forcing frequency matches the natural frequency of the degree-2, order-2 interfacial mode. The inviscid core flow is formally identical to the canonical elliptical vortex, linking the problem to the theory of elliptical instability. Finite viscosity regularises the resonance, introduces phase lag and generates oscillatory boundary layers. Combined with parameterised models of lunar recession and BMO crystallisation, the theory predicts enhanced core-boundary ellipticity, core-flow speed, magnetic Reynolds number and instability metrics, particularly near resonance. These results identify a previously unexplored mechanism for tidally driven flow in differentiated planetary bodies and show that a BMO can enhance tidal coupling to the core, potentially contributing to dynamo action.
Related
- Incompatibility of iron in post-perovskite and the stability of basal magma oceans in super-Earths
- Magma Buoyancy Couples Lunar Mantle Convection and Magmatism: Long-Lasting Mare Volcanism and Radius Change in a 3-D Regional Model
- Rattling Resonant Chains with Dynamical Tides and Parametric Resonance Passage
- Coupled evolution of Earth's H2-rich proto-atmosphere and oxidized magma ocean: Efficient H2O production through dissolved H2 transport and atmospheric buffering of the magma ocean redox state
- Coupled Evolution of Earth’s H2-rich Proto-atmosphere and Oxidized Magma Ocean: Efficient H2O Production through Dissolved H2 Transport and Atmospheric Buffering of the Magma Ocean Redox State
BibTeX
@misc{kiernan2026resonant,
title = {{Resonant lunar tides of Earth's core and basal magma ocean}},
author = {Murray B. C. Kiernan and Hamish C. F. C. Hay and David W. Rees Jones and James F. J. Bryson and Richard F. Katz},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2606.16340},
eprint = {2606.16340},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2606.16340},
note = {CC BY-SA 4.0},
}
RIS
TY - UNPB
TI - Resonant lunar tides of Earth's core and basal magma ocean
AU - Murray B. C. Kiernan
AU - Hamish C. F. C. Hay
AU - David W. Rees Jones
AU - James F. J. Bryson
AU - Richard F. Katz
PY - 2026
DA - 2026/10/20
PB - arXiv
DO - 10.48550/arxiv.2606.16340
UR - https://arxiv.org/abs/2606.16340
AB - Earth's magnetic field is generated by fluid motion in the liquid-metal core and has been active for billions of years. However, prior to the onset of inner-core growth, the sources of mechanical power that drove the geodynamo remain uncertain. During this period, the core may have been overlain by a basal magma ocean (BMO), creating two immiscible fluid layers separated by a density interface, beneath the solid mantle. We develop a theory for lunar tides in this core--BMO system, in which the tidal potential acts on the density contrast between the two layers. The resulting dynamics differ fundamentally from previous models driven by imposed deformation of the core's bounding envelope. In the inviscid limit, the response transitions from an equilibrium tide to an interfacial gravity wave as forcing frequency increases. The two regimes are separated by a resonance that occurs when the forcing frequency matches the natural frequency of the degree-2, order-2 interfacial mode. The inviscid core flow is formally identical to the canonical elliptical vortex, linking the problem to the theory of elliptical instability. Finite viscosity regularises the resonance, introduces phase lag and generates oscillatory boundary layers. Combined with parameterised models of lunar recession and BMO crystallisation, the theory predicts enhanced core-boundary ellipticity, core-flow speed, magnetic Reynolds number and instability metrics, particularly near resonance. These results identify a previously unexplored mechanism for tidally driven flow in differentiated planetary bodies and show that a BMO can enhance tidal coupling to the core, potentially contributing to dynamo action.
ER -