  The influence of the hydrogen hydrodynamic upper atmosphere escape, driven by
the solar soft X-ray and extreme ultraviolet radiation (XUV) flux, on an
expected magma ocean outgassed steam atmosphere of early Venus is studied. By
assuming that the young Sun was either a weak or moderate active young G star,
we estimated the water loss from a hydrogen dominated thermosphere due to the
absorption of the solar XUV flux and the precipitation of solar wind produced
energetic hydrogen atoms (ENAs). The production of ENAs and their interaction
with the hydrodynamic extended upper atmosphere, including collision-related
feedback processes, have been calculated by means of Monte Carlo models. ENAs
that collide in the upper atmosphere deposit their energy and heat the
surrounding gas mainly above the main XUV energy deposition layer. It is shown
that precipitating ENAs modify the thermal structure of the upper atmosphere,
but the enhancement of the thermal escape rates caused by these energetic
hydrogen atoms is negligible. Our results also indicate that the majority of
oxygen arising from dissociated H$_2$O molecules is left behind during the
first 100 Myr. It is thus suggested that the main part of the remaining oxygen
has been absorbed by crustal oxidation.
