Tracing Nonthermal Hydrogen and Deuterium From the Upper Atmosphere to the Venusian Exosphere

Recent studies of the Venusian upper atmosphere revealed a sudden increase in the deuterium-to-hydrogen ratio (D/H) above the main cloud layer using observations from the Solar Occultation in the InfraRed (SOIR) instrument onboard Venus Express. In addition, analyses of magnetic field and plasma measurements from Venus Express identified two distinct populations of hydrogen (H) and deuterium (D) in the upper atmosphere and exosphere of Venus: a thermal and a nonthermal component. Unlike the thermal population, the nonthermal H and D atoms possess sufficient energy to populate the extended exosphere and represent a major source of atmospheric escape.

The nonthermal H and D populations are believed to originate from photochemical reactions occurring in the upper atmosphere of Venus between altitudes of approximately 100 km and 250 km. In this study, we apply a Monte Carlo model to trace the trajectories and evolution of nonthermal H and D atoms from their source regions through the upper atmosphere and into the exosphere. The model follows the particles until they either become thermalized through collisions or escape into the extended exosphere.

By comparing the modeled exospheric populations with observations of H and D in the extended Venusian exosphere, we investigate which photochemical reactions are most important for populating the exosphere and driving the atmospheric escape of hydrogen and deuterium. These results provide new insights into the long-term evolution of water and atmospheric loss processes at Venus.