Tidal pull of the Earth strips the proto-Moon of its volatiles.

Prevailing models for the formation of the Moon invoke a giant impact between a planetary embryo and the proto-Earth \citep{Canup_2004, Cuk_Stewart_2012}. Despite similarities in the isotopic and chemical abundances of refractory elements compared to Earth's mantle, the Moon is depleted in volatiles \citep{Wolf_Anders_1980}. Current models favour devolatilisation via incomplete condensation of the proto-Moon in an Earth-Moon debris-disk \citep{Charnoz_Michaut_2015,Canup_2015,Lock_2018}. However the physics of this protolunar disk is poorly understood and thermal escape of gas is inhibited by the Earth's strong gravitational field \citep{Nakajima_Stevenson_2014}. 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 $\sim 1600-1700$ K reproduces the measured lunar depletion in K and Na.