Photochemical Modeling of Upper Atmospheric D/H on Mars

Abstract:

Elevation of the Martian deuterium/hydrogen (D/H) ratio when compared to the Earth is typically interpreted as evidence for significant loss of water to space over time. Studies of the Martian atmospheric D/H ratio as measured in water from ground- and Earth-orbit-based observatories have found values between 1-10× the Earth Standard Mean Ocean Water (SMOW) value depending on geographic location and the presence of dust storms (Villanueva et al., 2015, Villanueva et al., 2020), with typical values falling within 4-6× SMOW (Owen et al., 1988; Bjoraker et al., 1989; V. Krasnopolsky et al., 1997; Encrenaz et al., 2018; Vandaele et al., 2019).

Recent results from the Imaging UltraViolet Spectrograph (IUVS) instrument onboard the MAVEN Mars orbiter have measured an upper atmospheric D/H ratio in the atomic species higher than the ratio measured in water (Clarke et al., 2019). Unlike atomic D and H, which reach altitudes up to the exobase (~200 km), where they can then escape, water vapor is typically restricted to the middle and lower atmosphere on Mars (below ~100 km), but new analysis of the MAVEN Neutral Gas and Ion Mass Spectrometer (NGIMS) data has indicated the significant presence of water ions in the thermosphere due to elevated vertical transport during the dust storm season (Stone et al., 2019). These water ions play a significant part in altering atomic H abundances, and therefore escape, but their effects on the D/H ratio have not been studied.

Here, we update our 1D photochemical model (Cangi et al., 2020) to include a self-consistent ionosphere, and present new results on the coupling between thermospheric water ions, upper atmospheric atomic D and H, and D/H fractionation (which quantifies efficiency of escape of D vs H) as a function of atmospheric temperature and total water vapor. We also compare with the results from the MAVEN mission and give an outlook for future synergy between D/H modeling and mission data, including how it can be applied to atmospheres on other Solar System planets or extrasolar planets.