Did lunar tides sustain the early Earth's geodynamo?

Abstract: Geological data show that, early in its history, the Earth had a large-scale magnetic field with an amplitude comparable to that 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 early Earth's geodynamo, by combining constraints from geophysical models of the Earth-Moon system and predictions from rotating turbulence studies. We show that the Moon's tides could be strong enough before $-3.25$ Gy to trigger turbulence in the Earth's core, and possibly dynamo action in the mean time. Then, we propose new scaling laws for the magnetic field amplitude $B$. We show that $B \propto \beta^{4/3}$, where $\beta$ is the typical equatorial ellipticity of the liquid core, if the turbulence involves weak interactions of three-dimensional inertial waves. Otherwise, we would expect $B \propto \beta$ if the amplitude of tidal forcing were strong enough. When extrapolated to the Earth's core, it suggests that tidal forcing alone was too weak to possibly explain the ancient paleomagnetic field. Therefore, our study indirectly favours another origin for the early Earth's geodynamo on long time scales (e.g. the exsolution of light elements atop the core, or thermal convection due to secular cooling).