The surface of Mars is marked with ample evidence of its wetter past. Today, water on Mars exists only in the polar caps, subsurface ice, and atmosphere, but geomorphological and geochemical evidence points to significant alteration of the surface by liquid water. The presence of compounds like jarosite and hematite indicate past pooling and evaporation (Klingelhöfer et al., 2004; Squyres et al., 2004), while substantial evidence of hydrated silicates supports the theory that ancient river deltas, lake beds, catastrophic flood channels, and dendritic valley networks were formed by water (Ehlmann & Edwards, 2014; M. H. Carr & Head, 2010, and references therein). Because the contemporary Martian climate cannot support liquid water on the surface, Mars must have once had a thicker and warmer atmosphere. The Mars science community generally agrees that the atmosphere has escaped over time, with a significant amount in the form of thermal escape of H, in which a fraction of H atoms are hot enough that their velocity exceeds the escape velocity. Because H is primarily found in water on Mars, this has effectively desiccated the planet (Jakosky et al., 2018).

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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 (

Although deuterium (D) on Mars has received substantial attention, the deuterated ionosphere remains relatively unstudied. This means that we also know very little about non-thermal D escape from Mars, since it is primarily driven by excess energy imparted to atoms produced in ion-neutral reactions. Most D escape from Mars is expected to be non-thermal, highlighting a gap in our understanding of water loss from Mars. In this work, we set out to fill this knowledge gap. To accomplish our goals, we use an upgraded 1D photochemical model that fully couples ions and neutrals and does not assume photochemical equilibrium. To our knowledge, such a model has not been applied to Mars previously. We model the atmosphere during solar minimum, mean, and maximum, and find that the deuterated ionosphere behaves similarly to the H-bearing ionosphere, but that non-thermal escape on the order of 8000-9000 cm-2s-1 dominates atomic D loss under all solar conditions. The total fractionation factor, f, is 0.04–0.07, and integrated water loss is 147–158 m GEL. This is still less than geomorphological estimates. Deuterated ions at Mars are likely difficult to measure with current techniques due to low densities and mass degeneracies with more abundant H ions. Future missions wishing to measure the deuterated ionosphere in situ will need to develop innovative techniques to do so.