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Mars is a natural laboratory to study how atmospheric escape shapes planetary habitability. It is now well established that a significant amount of the Mars atmosphere has been lost to space (e.g., Jakosky et al., 2018). This escape is fractionating-the relative escape efficiency is different for members of an isotope pair, such as deuterium (D) and hydrogen (H). Because on Mars, D and H are found primarily in water, D/H fractionation indicates a history of water loss (Owen et al., 1988). Understanding escape fractionation therefore contributes to understanding the long-term loss of the atmosphere and desiccation of the planet.Geological studies indicate that Mars has likely lost 500+ meters global equivalent layer (GEL) of water (Lasue et al., 2013, and references therein), but atmospheric modeling studies typically do not find the same result, instead arriving at a smaller number of 100-250 m GEL (

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.