Ancient geodynamo driven by lunar tides beneath a basal magma ocean

Earth’s magnetic field is generated by dynamo action in the liquid-iron core
and has existed for at least 3.4 billion years. The energy source driving the geo-
dynamo before nucleation of the inner core around 1 billion years ago remains
unknown. During this interval, 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 compo-
nent of this tidal flow. Our results show that a tidal resonance provided enough
energy to sustain dynamo action by turbulent magnetic induction. Lunar tides
thus played a key role in generating Earth’s ancient magnetic field, which shielded
early life from solar radiation.