Estimate of the D/H Ratio in the Martian Upper Atmosphere from the Low Spectral Resolution Mode of MAVEN/IUVS

Abstract Recent Mars Atmosphere and Volatile EvolutioN (MAVEN) observations have shown unexpectedly large deuterium Lyman‐α emissions near the Martian southern summer solstice. The deuterium Lyman‐α brightness can reach ∼10% of the hydrogen Lyman‐α brightness below 200 km. In this study, we propose a method to estimate the D/H ratio in the Martian upper atmosphere through analysis of the Lyman‐α vertical profiles without the ability to spectrally separate the two atomic emission lines. Lyman‐α vertical profiles measured by MAVEN/IUVS at four periods are analyzed. During southern summer, the derived D/H ratios at 200 km are larger than the HDO/H2O ratio measured in the lower atmosphere, yet the extrapolated D/H ratio from 200 to 80 km agrees with the HDO/H2O ratio. This larger D/H ratio at higher altitude is attributed to the more efficient escape of hydrogen atoms compared to deuterium atoms. The method we describe accounts for uncertainties related to constraining the temperature and density profiles of H and D in the lower thermosphere. Because spectrally resolved D and H Lyman‐α measurements are not always available, this method provides a way to estimate D/H variations in their absence. This method will be useful for analysis of relevant data sets from past and future Mars missions.