HCO+ Dissociative Recombination: A Significant Driver of Nonthermal Hydrogen Loss at Mars. Issue 1 (24th January 2023)

• Record Type: Journal Article
• Authors: Gregory, Bethan S.; Elliott, Rodney D.; Deighan, Justin; Gröller, Hannes; Chaffin, Michael S.
• Is Part Of: Journal of geophysical research. Volume 128:Issue 1(2023)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2023-01-24
• Subjects: planetary atmospheric evolution -- hydrogen escape -- nonthermal hydrogen -- Mars -- terrestrial planets -- remote sensing; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2022JE007576
• Languages: English
• ISSNs: 2169-9097

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. Plain Language Summary: The escape of hydrogen atoms from the upper atmosphere of Mars has led to significant loss of water from the planet. Hydrogen needs energy to escape Mars' gravitational pull, which can be sourced from the energy given to H atoms when they are produced by chemical reactions—this results in "hot" hydrogen. However, the importance of hot hydrogen escape is unknown, and there is a disparity between predictions from observations and models. Here, we show that one reaction, HCO + dissociative recombination (HCO + + e − → CO + H), which has never been considered before, produces more escaping hot H than any process previously studied. In some seasonal conditions, it accounts for more than half the H loss, while accounting for 5% on longer timescales. Despite its importance, we predict that the hot H produced by this process is very difficult for spacecraft to observe. This mechanism is effective for escape because most H atoms produced by chemical reactions at high altitudes escape, so dozens of similar mechanisms are likely to be important at Mars. HCO + dissociative recombination is probably significant at Venus, where hot H escape is dominant, and perhaps at other rocky planets outside the solar system. Key Points: We investigate nonthermal hydrogen escape via HCO + dissociative recombination for the first time This is likely the dominant nonthermal mechanism, potentially responsible for >50% of the total escape under certain conditions Though important for H loss, we predict that this mechanism impacts brightness negligibly, posing a challenge for spacecraft observations