  Here we investigate how small amounts of hydrogen (much smaller than the mass
of the exoplanet) above a magma ocean on a rocky exoplanet may modify the
atmospheric chemistry and atmospheric escape.We use a chemical model of a magma
ocean coupled to a gas equilibrium code. An energy-limited model is used to
compute atmospheric escape. The composition of the vapor above a magma ocean is
drastically modified by hydrogen, even for very modest amounts of H ($\ll
10^{-6}$ planetary mass). Hydrogen consumes much of the O$_2$(g), which, in
turn, promotes the evaporation of metals and metal oxides (SiO, Mg, Na, K, Fe)
from the magma ocean. Vast amounts of H$_2$O are produced by the same process.
At high hydrogen pressures, new hydrogenated species such as SiH$_4$ form in
the atmosphere. In all cases, H, H$_2$, and H$_2$O are the dominant
nonmetal-bearing volatile species. Sodium is the dominant atmospheric
metal-bearing species at T$<$ 2000K and low H content, whereas Fe is dominant
at high H content and low temperature, while SiO predominates at T>3000 K. We
find that the atmospheric Mg/Fe, Mg/Si, and Na/Si ratios deviate from those in
the underlying planet and from the stellar composition. As such, their
determination may constrain the planet's mantle composition and H content. As
the presence of hydrogen promotes the evaporation of silicate mantles, it is
conceivable that some high-density, irradiated exoplanets may have started life
as hydrogen-bearing planets and that part of their silicate mantle evaporated
(up to a few $10 \%$ of Si, O, and Fe) and was subsequently lost owing to the
reducing role of H. Even very small amounts of H can alter the atmospheric
composition and promote the evaporation to space of heavy species derived from
the molten silicate mantle of rocky planets.
