HCO<sup>+</sup> Dissociative Recombination: A Significant Driver of Nonthermal Hydrogen Loss at Mars
Bethan Gregory, Rodney D. Elliott, Justin Deighan, H. Gröller, Michael Chaffin
Abstract
Abstract Hydrogen escape to space has shaped Mars' atmospheric evolution, driving significant water loss. An unknown fraction of atmospheric H lost acquires its escape energy from photochemical processes, with multiple observational studies suggesting much higher densities of such “hot” H than models predict. Here, we show that a previously unconsidered mechanism, HCO + dissociative recombination, produces more escaping hot H than any previously studied process, potentially accounting for more than 50% of escape during solar minimum aphelion conditions and ∼5% of the expected long‐term average loss. This hot H is predicted to impact observed brightness profiles negligibly, posing a significant challenge to the interpretation of spacecraft remote sensing observations. This mechanism's efficiency is largely due to the high (63%–83%) albedo of the planet to H at 1–10 eV energies, indicating the likely importance of dozens of similar photochemical mechanisms for the desiccation of Mars, Venus and planets throughout the universe.