Nonthermal Hydrogen Loss at Mars: Contributions of Photochemical Mechanisms to Escape and Identification of Key Processes

Hydrogen loss to space is a key control on the evolution of the Martian atmosphere and the desiccation of the red planet. Thermal escape is thought to be the dominant loss process, but both forward modeling studies and remote sensing observations have indicated the presence of a second, higher‐temperature “nonthermal” or “hot” hydrogen component, some fraction of which also escapes. Exothermic reactions and charge/momentum exchange processes produce hydrogen atoms with energy above the escape energy, but H loss via many of these mechanisms has never been studied, and the relative importance of thermal and nonthermal escape at Mars remains uncertain. Here we estimate hydrogen escape fluxes via 47 mechanisms, using newly developed escape probability profiles. We find that HCO+ dissociative recombination is the most important of the mechanisms, accounting for 30%–50% of the nonthermal escape. The reaction CO2+ ${\text{CO}}_{2}^{+}$ + H2 is also important, producing roughly as much escaping H as momentum exchange between hot O and H. Total nonthermal escape from the mechanisms considered amounts to 39% (27%) of thermal escape, for low (high) solar activity. Our escape probability profiles are applicable to any thermospheric hot H production mechanism and can be used to explore seasonal and longer‐term variations, allowing for a deeper understanding of desiccation drivers over various timescales. We highlight the most important mechanisms and suggest that some may be important at Venus, where nonthermal escape dominates and much of the literature centers on charge exchange reactions, which do not result in significant escape in this study. Plain Language Summary The climate of Mars has become drier over time, partially due to hydrogen and oxygen atoms escaping from the planet's upper atmosphere. This can happen when the atoms have sufficient energy to overcome Mars' gravitational pull. Some fraction of escaping hydrogen—known as “hot” hydrogen—gains this energy through chemical processes, but the extent to which this contributes to the total hydrogen escape rate remains uncertain. Here we estimate the rate of hydrogen escape from 47 different chemical mechanisms, by considering the altitude above the surface at which they are produced and the energy that they might be given when they are formed. We find the mechanisms producing the most escaping hydrogen and estimate how quickly hot hydrogen is being lost to space. The most important mechanism, of those we cons