Solar wind scattering from the surface of Mercury: Lessons from the Moon

We discuss the surface-scattering of solar wind protons at Mercury based on observed scattering characteristics from lunar regolith. The properties of the impinging plasma are expected to be different between different regions on Mercury, and between Mercury and the Moon. Here, we review the expected Hermean plasma conditions and lunar empirical scattering models. We present observed and modeled energy spectra for scattered protons and hydrogen energetic neutral atoms (ENAs) for three cases of very different plasma conditions at the Moon. Then, we simulate scattering from the Hermean surface by applying the empirical models to four different scenarios of plasma precipitation on Mercury. The results suggest that surface-scattering is a strong source of ENAs at Mercury (up to ∼108 cm−2 s−1), which can be very useful for remote-sensing of the plasma conditions at the surface. Protons scattered from the surface back into space are also expected with high fluxes up to ∼107 cm−2 s−1, and may be important for wave generation and the filling in of the loss cone of mirroring and quasi-trapped populations. Scattered protons at the cusp region (of ∼106 cm−2 s−1) can potentially be detected by orbiters as outflowing protons within the loss cone.