An impact driven dynamo for the early Moon

Recent palaeomagnetic and seismological studies have strengthened the suggestion that the Moon once possessed a core dynamo. Despite its importance as a constraint on lunar evolution, there is currently no consensus on how such a dynamo was driven. Two groups working independently have arrived at the idea that the lunar dynamo was powered by mechanical stirring of the liquid core. Dwyer et al. investigate the mechanism of a dynamo driven by continuous mechanical stirring arising from the differential motion between the solid silicate mantle and the liquid core beneath. They show that the fluid motions and the power required to drive a dynamo operating continuously for more than 1 billion years are readily obtained by such mechanical stirring. Le Bars et al. propose a model whereby the dynamo action comes from impact-induced changes in the Moon's rotation rate. They show that basin-forming impact events are energetic enough to have unlocked the Moon from synchronous rotation, and that the subsequent large-scale fluid flows in the core, excited by the tidal distortion of the core–mantle boundary, could have powered a lunar dynamo. The origin of lunar magnetic anomalies1,2,3,4,5 remains unresolved after their discovery more than four decades ago. A commonly invoked hypothesis is that the Moon might once have possessed a thermally driven core dynamo3, but this theory is problematical given the small size of the core and the required surface magnetic field strengths6. An alternative hypothesis is that impact events might have amplified ambient fields near the antipodes of the largest basins7, but many magnetic anomalies exist that are not associated with basin antipodes. Here we propose a new model for magnetic field generation, in which dynamo action comes from impact-induced changes in the Moon’s rotation rate. Basin-forming impact events are energetic enough to have unlocked the Moon from synchronous rotation8, and we demonstrate that the subsequent large-scale fluid flows in the core, excited by the tidal distortion of the core–mantle boundary9, could have powered a lunar dynamo. Predicted surface magnetic field strengths are on the order of several microteslas, consistent with palaeomagnetic measurements5, and the duration of these fields is sufficient to explain the central magnetic anomalies associated with several large impact basins.