  The formation of the solar system's terrestrial planets concluded with a
period of giant impacts. Previous works examining the volatile loss caused by
the impact shock in the moon-forming impact find atmospheric losses of at most
20-30 per cent and essentially no loss of oceans. However, giant impacts also
result in thermal heating, which can lead to significant atmospheric escape via
a Parker-type wind. Here we show that H2O and other high-mean molecular weight
outgassed species can be efficiently lost through this thermal wind if present
in a hydrogen-dominated atmosphere, substantially altering the final volatile
inventory of terrestrial planets. Specifically, we demonstrate that a giant
impact of a Mars-sized embryo with a proto-Earth can remove several Earth
oceans' worth of H2O, and other heavier volatile species, together with a
primordial hydrogen-dominated atmosphere. These results may offer an
explanation for the observed depletion in Earth's light noble gas budget and
for its depleted xenon inventory, which suggest that Earth underwent
significant atmospheric loss by the end of its accretion. Because planetary
embryos are massive enough to accrete primordial hydrogen envelopes and because
giant impacts are stochastic and occur concurrently with other early
atmospheric evolutionary processes, our results suggest a wide diversity in
terrestrial planet volatile budgets.
