in response to the star’s bolometric radiation for planets on short period orbits (@OwenWu16), or most notably in response to the star’s X-ray and ultraviolet emission for planets orbiting active stars (@Tian05; @Erkaev16; @Kubyshkina18). Heating can also take place due to other mechanisms, mostly in response to the star’s wind (@Cohen14; @Lichtenegger16). For example, @Chassefiere96 and @Chassefiere97 suggested for the case of early Venus that energetic neutral atoms (ENAs) created in charge exchange reactions between neutral atmospheric particles and the early solar wind could heat the atmosphere significantly and drive escape, though @Lichtenegger16 found that this mechanism might not lead to significant additional escape.

Water vapor in a planet’s upper atmosphere is photodissociated by the high-energy radiative spectrum of its host star creates a large number of other chemical species such as OH, O$_2$, and O$_3$, and causes most of the thermosphere to be filled neutral and ionized hydrogen and oxygen atoms which can flow away from the planet hydrodynamically (@Kasting83; @Guo19). Due to its lower mass, hydrogen is lost more rapidly than oxygen, though how different these loss rates are depends on several factors, including the total loss rate and which loss process dominates. This preferential loss of H 