  The fate of surface water on Venus is one of the most important outstanding
problems in comparative planetology. Here a new concept is proposed to explain
water removal on a steam-covered proto Venus, referred to as impact-driven
planetary desiccation. Since a steam atmosphere is photochemically unstable,
water vapor dissociates into hydrogen and oxygen. Then, hydrogen escapes easily
into space through hydrodynamic escape driven by strong extreme ultraviolet
radiation from the young Sun. The focus is on the intense impact bombardment
during the terminal stage of planetary accretion as generators of a significant
amount of reducing agent. The fine-grained ejecta remove the residual oxygen,
the counter part of escaped hydrogen, via the oxidation of iron-bearing rocks
in a hot atmosphere. Thus, hypervelocity impacts cause net desiccation of the
planetary surface. I constructed a stochastic cratering model using a Monte
Carlo approach to investigate the cumulative mass of nonoxidized, ejected rocks
due to the intense impact bombardment. It is shown that a thick steam
atmosphere with a mass equivalent to that of the terrestrial oceans would be
removed. The cumulative mass of rocky ejecta released into the atmosphere
reaches 1 wt% of the host planet, which is 10000 times of the current mass of
the Earths atmosphere. These results strongly suggest that chemical reactions
between such large amounts of ejecta and planetary atmospheres are among the
key factors required to understand atmospheric mass and its composition, not
only in the Solar System but also in extrasolar systems.
