Hydrogen and helium escape on Venus via energy transfer from hot oxygen atoms

Abstract

ABSTRACT Due to the relatively strong gravity on Venus, heavy atmospheric neutrals are difficult to accelerate to the escape velocity. However, a variety of processes, such as the dissociative recombination of ionospheric O$_2^+$, are able to produce hot atoms which could deliver a significant amount of energy to light neutrals and drive their escape. In this study, we construct a Monte Carlo model to simulate atmospheric escape of three light species, H, H2, and He, on Venus via such a knock-on process. Two Venusian background atmosphere models are adopted, appropriate for solar minimum and maximum conditions. Various energy-dependent and species-dependent cross-sections, along with a common strongly forward scattering angle distribution, are used in our calculations. Our model results suggest that knock-on by hot O likely plays the dominant role in driving total atmospheric hydrogen and helium escape on Venus at the present epoch, with a significant portion contributed from regions below the exobase. Substantial variations are also revealed by our calculations. Of special interest is the modelled reduction in escape flux at high solar activities for all species, mainly associated with the enhancement in thermal O concentration near the exobase at high solar activities which hinders escape. Finally, model uncertainties due to several controlling factors, including the distribution of relevant light species in the background atmosphere, the plane-parallel approximation, and the finite O energy distribution, are evaluated.