  Returning humans to the Moon presents an unprecedented opportunity to
determine the origin of volatiles stored in the permanently shaded regions
(PSRs), which trace the history of lunar volcanic activity, solar wind surface
chemistry, and volatile delivery to the Earth and Moon through impacts of
comets, asteroids, and micrometeoroids. So far, the source of the volatiles
sampled by the Lunar Crater Observation and Sensing Satellite (LCROSS) plume
has remained undetermined. We show here that the source could not be volcanic
outgassing and the composition is best explained by cometary impacts. Ruling
out a volcanic source means that volatiles in the top 1-3 meters of the Cabeus
PSR regolith may be younger than the latest volcanic outgassing event (~1
billion years ago; Gya).
