Did lunar tides sustain the early Earth's dynamo?

TL;DR

This study evaluates whether lunar tidal forces could have powered the early Earth's magnetic field, concluding that tidal forcing alone was likely insufficient, thus implying other mechanisms were responsible for the ancient geodynamo.

Contribution

The paper introduces new scaling laws for the Earth's magnetic field amplitude based on tidal forcing and turbulence regimes, providing insights into the viability of tidal-driven dynamo models.

Findings

Tidal forcing could have triggered core turbulence before -3.25 Gy.

Scaling laws suggest tidal forcing alone was too weak to sustain the early Earth's dynamo.

Other mechanisms, such as compositional convection, are more likely responsible for the ancient magnetic field.

Abstract

Geological data show that, early in its history, the Earth had a large-scale magnetic field with an amplitude comparable to the one of the present geomagnetic field. However, its origin remains enigmatic and various mechanisms have been proposed to explain the Earth's field over geological time scales. Here, we critically evaluate whether tidal forcing could explain the ancient geodynamo, by combining constraints from geophysical models of the Earth-Moon system and predictions from turbulence studies. Our analysis shows that lunar tidal forcing could have been sufficiently strong before $-3.25$ Gy to trigger turbulence within the Earth's core, and potentially to sustain dynamo action during that interval. Then, we propose new scaling laws for the magnetic field amplitude $B$. We expect the latter to scale as $B \propto \beta^{4/3}$, where $\beta$ is the equatorial ellipticity of the liquid core, if the turbulence involves weak interactions of three-dimensional inertial waves. Alternatively, in the regime of strong tidal forcing, the expected scaling becomes $B \propto \beta$. When extrapolated to the Earth's core, it suggests that tidal forcing alone was too weak to possibly explain the ancient geomagnetic field. Therefore, our study indirectly favours another origin for the early Earth's dynamo on long time scales (e.g. exsolution of light elements atop the core, or thermal convection due to secular cooling).