of water can also be inhibited by the presence of a cold-trap if non-condensing species such as N$_2$ or CO$_2$ make up a large fraction of the atmosphere (@WordsworthPierrehumbert13). An initially pure H$_2$O atmosphere cannot have a cold-trap, but a cold-trap can form as O$_2$ builds up, inhibiting H$_2$O escape (@WordsworthPierrehumbert14). If the star’s activity is strong enough to drive rapid atmospheric escape however, we might expect rapid oxygen escape to take place in such a case, possibly removing any accumulated atmospheric O$_2$. Studying the case of GJ 1132b, which is an Earth-mass planet orbiting a low mass M dwarf, @Schaefer16 found that most of the O$_2$ produced by the dissociation and loss of water vapor is lost to space, whereas a much smaller fraction is absorbed into the planet.

Most atmospheric loss processes take place in response to the magnetic activity of the host star, which is responsible for winds and high-energy X-ray and ultraviolet radiation. A star’s X-ray and ultraviolet spectrum (referred to here as ‘XUV’ and defined as 1 to 400 nm) is emitted by the photosphere and the magnetically heated chromosphere and corona (@Fontenla16). For a solar mass star, the chromosphere and corona is responsible for 