Ancient geodynamo driven by lunar tides beneath a basal magma ocean

Dynamo action in Earth\'s liquid-iron core has generated a magnetic field for at least 3.4 billion years. Prior the onset of solidification that formed the inner core at about 1 Ga, the energy source driving the geodynamo is unknown. Contemporaneously, the bottom of the mantle may have been fully molten, forming a basal magma ocean. We propose that the boundary between this silicate magma and the immiscible, liquid core was susceptible to tides driven by the Moon’s gravity. We present theoretical predictions for the laminar component of this tidal flow. Our results indicate that a tidal resonance provided enough energy to sustain dynamo action for ~3.5 Gyr by turbulent magnetic induction. Lunar tides may thus have played a key role in generating Earth\'s ancient magnetic field, which shielded early life from solar radiation.