  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.

Earth and Planetary Astrophysics;Solar and Stellar Astrophysics