Loss rates and time scales for sodium at Mercury

The time scales and loss rates for sodium in the exosphere of Mercury are studied here. Sodium comes from release processes occurring at the planetary surface; the amount of surface sodium that is available for release (mostly through thermal- or photon-stimulated desorption) is limited. Loss processes deplete the surface concentration of sodium, which is continuously refilled by diffusion from the interior of regolith grains or by chemical sputtering. Ejected sodium particles may either escape the gravity, also aided by the radiation pressure acceleration, or be photoionized, or fall back onto the surface. Falling particles may either stick to the surface or bounce. A Monte Carlo model, simulating all these processes, is used to obtain the exosphere densities, the global loss rates at different true anomaly angles, and typical time scales for small-term variations, taking into account planetʹs orbit and rotation speed. Assuming an impulsive event, which causes the enhancement of sodium in the exosphere, the model gives the time scales for the exosphere to recover to a steady-state condition. It is found that time scales go from one or two hour (close to perihelion) to half day (close to aphelion). The escape probability ranges from 20% at perihelion and aphelion up to 40% at true anomaly angles of about 60° and 300°.