First Detection of Deuterium in Venus's Extended Exosphere

Venus, Earth’s twin planet, has been explored by numerous spacecraft, yet its water inventory and implications for planetary evolution remain uncertain. The deuterium-to-hydrogen (D/H) ratio is a key tracer for understanding the history of water on Venus. Here, we reanalyze Venus Express magnetic field data to study ion cyclotron waves (ICWs) generated by picked up H$^+$ ions and, for the first time, D$^+$ ions in the extended exosphere. From these wave signatures, we derive altitude-dependent neutral density profiles of both H atoms and D isotopes over several Venus radii. The profiles indicate that the observed ICWs are primarily driven by energetic suprathermal particle populations, which dominate the extended exosphere, and originate in the thermosphere via electron-dissociative recombination of hydrogen-bearing molecular ions (e.g., HCO$^+$, DCO$^+$, HCO$_2^+$, DCO$_2^+$, OH$^+$, OD$^+$, H$_2^+$, HD$^+$) controlling atmospheric hydrogen escape. Using these densities, we estimate the escape rates of H atoms and D isotopes to be approximately $\SI{2.82e25}{s^{-1}}$ and $\SI{1.5e24}{s^{-1}}$, respectively. The average D/H ratio near the exobase level is $\sim$0.44. It decreases to $\sim$0.33 at around 10 Venus radii. These unexpected high D/H ratios are in agreement with observations of the increase of the D/H ratio in the Venus mesosphere, showing elevated D/H ratios above the main cloud layer. These results reveal that fractionation processes in the upper atmosphere are altitude- and species-dependent and highlight a strong link between suprathermal particles, upper atmospheric chemistry, and water loss. Finally, our findings provide new constraints on Venus’s exosphere and the evolution of its water inventory.