Nonthermal hydrogen loss at Mars: Contributions of photochemical mechanisms to escape and identification of key processes

TL;DR

This study quantifies hydrogen escape mechanisms at Mars, revealing that nonthermal processes, especially HCO$^+$ dissociative recombination, significantly contribute to atmospheric loss, with implications for planetary evolution.

Contribution

The paper introduces new escape probability profiles for 47 mechanisms, identifying key processes like HCO$^+$ dissociative recombination as dominant in nonthermal hydrogen escape at Mars.

Findings

HCO$^+$ dissociative recombination accounts for 30-50% of nonthermal escape.

Nonthermal escape constitutes up to 39% of thermal escape during low solar activity.

Escape probability profiles enable analysis of seasonal and long-term variations.

Abstract

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 CO$_2^+$ + H$_2$ 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.