Tidal pull of the Earth strips the proto-Moon of its volatiles
Abstract
Prevailing models for the formation of the Moon invoke a giant impact between a planetary embryo and the proto-Earth Canup2004, CukStewart2012. Despite similarities in the isotopic and chemical abundances of refractory elements compared to Earth's mantle, the Moon is depleted in volatiles WolfAnders1980. Current models favour devolatilisation via incomplete condensation of the proto-Moon in an Earth-Moon debris-disk CharnozMichaut2015,Canup2015,Lock2018. However the physics of this protolunar disk is poorly understood and thermal escape of gas is inhibited by the Earth's strong gravitational field NakajimaStevenson2014. Here we investigate a simple process, wherein the Earth's tidal pull promotes intense hydrodynamic escape from the liquid surface of a molten proto-Moon assembling at 3-6 Earth radii. Such tidally-driven atmospheric escape persisting for less than 1 Kyr at temperatures 1600-1700 K reproduces the measured lunar depletion in K and Na.
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