On the origins of backscattered solar wind energetic neutral hydrogen from the Moon and Mercury

Abstract

The surfaces of planetary airless bodies are directly bombarded by solar particles. The most abundant of those particles are solar wind protons. In the case of the Moon, many in situ observations have shown that a significant portion of these incident solar-wind protons are backscattered as energetic neutral hydrogen atoms (ENA). Measurements of the energy flux distribution of these neutral particles provide a clue regarding the processes occurring in the surface regolith when impacting protons collide with the surface grains. In this work, we developed a model to describe the fate of protons through a regolith model and compared our results with Chandrayaan-1 ENA measurements. We combined a Monte Carlo approach to reconstruct the motion of these particles into a layer of grains with molecular dynamics to parametrize the interaction of an incident proton with a grain. Using simultaneous measurements of the incident plasma by Chandrayaan-1 SWIM, this detailed modelling allowed us to analyse the measurements performed by the Chandrayaan-1 CENA instrument when the Moon is in the solar wind and in the Earth's magnetosheath, and to highlight what could control the flux and energy properties of these backscattered neutral hydrogen particles. Predictions of the intensity and shape of the backscattered neutral hydrogen distribution at Mercury are derived from this calculation.