Understanding the Importance of HDO and D/H on Venus

The study of HDO and the deuterium/hydrogen (D/H) ratio plays a critical role in reconstructing the past and present climate of Venus. These isotopic tracers offer valuable insights into the planet’s hydrological history, atmospheric escape processes, and potential ancient reservoirs of water. In particular, variations in D/H can shed light on the mechanisms that led Venus to become the hot, arid world we observe today.

In this work, we present the first fully three-dimensional simulation of the HDO cycle on Venus, by implementing HDO in both gas and liquid phases within the Venus Planetary Climate Model (VPCM). Our model allows us to explore the spatial and temporal behavior of HDO, and how it interacts with the atmospheric dynamics and cloud chemistry of Venus.

One of our key findings is that the vertical distribution of D/H is strongly influenced by processes previously neglected in earlier models. Notably, we show that the presence of deuterated sulfuric acid (HDSO₄) in Venusian clouds can significantly alter the D/H profile and must be taken into account in future studies.

We also assess the individual effects of isotope fractionation during three key processes: condensation, molecular diffusion, and photolysis. Our analysis reveals that photolysis-induced fractionation dominates the D/H ratio in the upper atmosphere, driving its increase to approximately 480 × VSMOW (Vienna Standard Mean Ocean Water) at altitudes near 130 km.

In addition, we observe notable diurnal variations in the abundances of H₂O, HDO, and D/H in the upper atmosphere, consistent with expectations based on solar-driven atmospheric dynamics.

Overall, our results highlight the complex interplay of physical and chemical processes controlling the distribution of water isotopologues on Venus. These findings not only refine our understanding of the present Venusian atmosphere but also provide essential constraints for reconstructing its climatic evolution and water loss history.