Constraining Venus’ Water Loss History through D/H Ratio Observations from Ion Cyclotron Wave Analysis in the extended Exosphere

Venus, has been studied by numerous space missions over decades, yet its past water inventory remains poorly constrained, obscuring insights into planetary evolution and the emergence of habitability. A critical factor in understanding its history is the atmospheric deuterium-to-hydrogen (D/H) ratio, which offers insights into Venus' water loss and atmospheric escape mechanisms. Previous measurements of this ratio have been confined to altitudes near or below the exobase of about 250 km, based on remote sensing and in situ water vapor observations. In this study, we reanalyze magnetic field data from Venus Express to examine pick-up generated ion cyclotron waves (PCWs) caused by hydrogen ions. For the first time, we also apply the same method to study PCWs generated by the pick-up of deuterium ions, presenting the first altitude-dependent density profile of deuterium in Venus' extended exosphere. Our results also align with recent studies and indicate that the escape rates of deuterium and hydrogen fractionate insufficient to support the existence of a late-stage ocean on Venus, reinforcing the notion that Venus may have never been habitable due to its inability to retain significant water over time.