Seasonal Effects of the Changing Photon Scattering Rates on Mercury’s Exospheric Structure

Abstract Emission in Mercury’s exosphere observed from the ground and by spacecraft is produced by resonant scattering of sunlight. The process of resonant scattering changes the structure of the exosphere owing to the transfer of momentum between the incident photons and the scattering atoms, resulting in a net antisunward-directed force known as radiation pressure. The photon scattering rate (the so-called g -value) and the magnitude of radiation pressure depend strongly on both Mercury’s distance from the Sun and the radial velocities of the scattering atoms relative to the Sun. We discuss four effects of the changing g -value over a Mercury year that require a model capable of tracking the positions and speeds of exospheric constituents in order to properly interpret emission data: (1) variations in the escape flux of atoms, (2) variations in the ratio of atoms that escape Mercury in neutral versus ionized form, (3) difficulties in determining where material was ejected from the surface based on the locations of the emitting atoms in the exosphere, and (4) systematic uncertainties in interpreting the column density of emitting gas using a constant g -value versus a variable g -value that takes into account the radial motion of the atoms relative to the Sun.