Magnetosphere, Exosphere, and Surface of Mercury

The discovery of an atomic sodium exosphere at Mercury raises the question of whether Mercury, like I0 at Jupiter, can maintain a heavy ion magnetosphere. We suggest that it does, and that heavy ions (mainly Na ÷) from the exosphere are typically accelerated to keV energies and make important or dominant contributions to the mass (~300 g sec -1) and energy ( 3 x 109 W) budget s of the magnetosphere. The sodium supply to the exosphere is largely from within Mercury itself, with external sources like meteoroid infall and the solar wind being relatively unimportant. Therefore Mercury is in the process of losing its semivolatiles. Photosputtering dominates charged particle sputtering and can maintain an adequate rate of Na ejection from the surface. © 1987 Academic