The Evolution of Deuterium and Hydrogen in the Martian Upper Atmosphere

Understanding the ratio of deuterium to hydrogen (D/H) in the atmosphere of Mars is key to understanding the history of water and its escape into space. Observations of HDO and H2O near the surface have shown factor of two-three differences depending on the hemisphere and season. By contrast, observations by multiple spacecraft have now shown order of magnitude changes in the densities and escape fluxes of both H and D with an annual cycle in the upper atmosphere of Mars. The maxima occur shortly after perihelion, during southern summer, and are associated with increased levels of water in the middle atmosphere as well as increased solar UV flux at Mars. Work remains to sort out the relative influence of solar EUV flux in the upper atmosphere and transport of H and D from the lower atmosphere, which can be affected by global circulation and dust storms.