What does the unexpectedly high D/H ratio in Venus’ upper atmosphere imply for the existence of a late ocean?

Today, Venus is a dry planet with an atmosphere that contains very little water. The bulk D/H ratio in its atmosphere is enriched by a factor of ~120 compared to the Earth. This suggests that more of the lighter hydrogen escaped into space over time compared to the heavier deuterium, leading to the conclusion that the planet once hosted a much larger water reservoir than today. Recent climate studies even suggest that Venus could have hosted a temperate period with a liquid water ocean and habitable conditions up to ~0.7 Gyr ago [1]. If so, (i) the ocean must have evaporated afterwards with H and D being lost into space and O being either lost into space or sequestered into the surface, and (ii) the D/H ratio likely needed to fractionate from its initially low, Earth-like value toward its present bulk value since the time of ocean evaporation. Recent analysis of Venus Express data, however, suggest that the D/H ratio in Venus’ atmosphere increases with altitude, reaching values of D/H~0.2 in the mesosphere [2] and even ~0.4 in the exosphere [3]. Photochemical escape rates for D and H based on the analysis of exospheric ion cyclotron waves (see EGU abstract by Weichbold et al.) further suggest lower loss rates for H but higher ones for D, as expected before Venus’ unexpectedly high upper atmosphere D/H ratio was revealed. Based on these novel results, we re-evaluate the evolution of Venus’ water inventory and D/H ratio over time. Our study indicates that only a comparatively small amount of H and D could have been lost since the last resurfacing event (contributing to less than 1 m global equivalent layer of water) and that the D/H ratio likely has been fractionated toward high values already relatively early in Venus’ history, potentially during an early phase when the atmospheric escape of H transitioned from hydrodynamic toward Jeans escape indicating an early loss of most of Venus’ water reservoir. A habitable ocean, as late as 0.7 Gyr ago, can therefore hardly be compatible with the new findings on Venus’ upper atmosphere D/H ratio and the therewith connected escape rates of H and D. This supports recent findings that Venus has never been liquid-water habitable [4].References:[1] Way, M. J. and Del Genio, Anthony D., Venusian Habitable Climate Scenarios: Modeling Venus Through Time and Applications to Slowly Rotating Venus-Like Exoplanets, Journal of Geophysical Research (Planets), 125, 5, e06276, 2020, doi:10.1029/2019JE00627610.1002/essoar.10501118.3.[2] Mahieux, A., Viscardy, S., Yelle, R.V. et al., Unexpected increase of the deuterium to hydrogen ratio in the Venus mesosphere, Proceedings of the National Academy of Science, 121, 34, e2401638121, 2024, doi:10.1073/pnas.2401638121.[3] Weichbold, F., Lammer, H., Scherf, M. et al., First Detection of Deuterium in Venus's Extended Exosphere, 2025, preprint (Version 1) available at Research Square [https://doi.org/10.21203/rs.3.rs-7720153/v1][4] Constantinou, T., Shorttle, O., and Rimmer, P. B., A dry Venusian interior constrained by atmospheric chemistry, Nature Astronomy, 9, 189, 2025, doi:10.1038/s41550-024-02414-5.