Water, a key requirement for life, once shaped the martian surface in huge amounts, but today is only a small fraction of the planet’s volatile inventory. Mars’ high D/H (deuterium/hydrogen) ratio, 4-6× the Earth value, suggests that much of the missing water has escaped to space; H, the less massive isotope, more easily escapes the planet’s gravity. However, estimates of water loss from atmospheric models are typically a factor of ∼4 lower than geomorphological estimates. Much literature has focused on H escape, with less emphasis on D escape and the fractionation factor (the relative escape efficiency of D compared to H), despite the importance of the D/H ratio to understanding of the water cycle of Mars.

To approach these problems, I have developed the most comprehensive photochemical model, named bluejay, of the martian atmosphere to date, which I use in this thesis to explore D and H escape and water loss from Mars. The first chapter covers how the thermal Jeans escape fractionation factor varies with temperature and atmospheric water, revealing it to be smaller than in previous studies and hinting at the importance of non-thermal D escape. In the second chapter, I present the first predictions of the deuterated ionosphere and photochemical non-thermal D escape, and identify the driving chemical reactions. Finally, I investigate the response of the D/H ratio and escape to seasonal variations, finding that both thermospheric temperatures and mesospheric water induce large changes in the atomic D/H ratio within a Mars season due to dynamical differences of H and D. 

Even with the inclusion of non-thermal D escape, modeled integrated water loss is still lower than geomorphological estimates by a factor of ∼3, indicating a future need to better understand past epochs and additional avenues for water loss. Nevertheless, these results present the most detailed look at the deuterated atmosphere of Mars thus far. The work here also provides a baseline for future photochemical modeling investigations, as bluejay may continue to be used to study Mars or other rocky bodies with atmospheres.