Ancient lunar dynamo

Abstract

The differential rotation between the Moon's core and mantle may have powered the ancient lunar dynamo, either continuously over several hundred million years or intermittently after large impacts. See Letters p.212 & p.215 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.