Despite decades of exploration, the history of water on Venus remains uncertain. It is thought that the atmospheric D/H ratio, enriched by a factor of ~120 compared to Earth, indicates significant past water loss. Recent observations by the SPICAV instrument from Venus Express suggest that this isotope ratio increases with altitude, reaching unexpectedly high values in the upper atmosphere, implying enhanced deuterium, and hence water escape and a limited remaining water inventory.

Analysis of proton and deuterium pick-up ion cyclotron waves from Venus Express provides the first altitude-resolved constraints on deuterium in the extended exosphere. The results indicate lower hydrogen but higher deuterium escape rates than previously expected, indicating that most of Venus’ water was lost early in its history. These discovery challenges scenarios of long-lived habitable conditions or a recent liquid water ocean.

For reproducing the solar activity-dependent D and H density profiles in Venus’ exosphere, a Monte Carlo model for suprathermal atom production in Venus’ thermosphere and transport is applied to hydrogen-related reactions and so-called knock-on processes with suprathermal oxygen atoms. By simulating the generation and reproduction of the observation-related exospheric number density profiles, the model will help constrain escape rates and assess their role in shaping the evolution of Venus’ atmosphere.