Water Loss from Mars: Energetic Deuterium and Its Effects on the D/H Ratio

How much water did Mars lose over time? Can it be estimated by looking at the present value of the D/H ratio at Mars? The current picture of martian water escape estimates that Mars has lost ~140 m of global equivalent layer (GEL) of water over its evolution history based on the enhanced D/H ratio of ~4-8 times the value at Earth. Recent analysis of D and H Lyman alpha observations with the Mars Atmosphere Volatile Evolution (MAVEN) mission show that the exospheric densities of atomic D and H at Mars undergo large changes in a short timescale of weeks. The Jeans escape process, which is the major contributor of H and D escape from planetary exospheres, is insufficient to account for such fast depletion of the column of D and H, raising the likelihood of the presence of an energetic population of the two species in the exosphere of Mars, created by non-thermal processes. The signature of non-thermal H has already been discovered at Mars with the Hubble Space Telescope (HST). This work will present new observations with HST tailored towards detecting non-thermal D, conducted on 19th July 2022 when D emission at Mars was at its brightest, close to Mars' southern summer solstice. The initial analysis of the data shows a D line integrated intensity of ~750 Rayleighs along with an altitude profile extending up to ~6500 km on the night side and ~10,000 km on the dayside. The shape of the altitude profile will be used to detect the present of energetic D atoms in the martian exosphere. We will present the modeling results from the HST data analysis and discuss the implications of the findings through this work.