Mercuryʹs exosphere origins and relations to its magnetosphere and surface

Mariner 10, the only spacecraft that ever passed close to Mercury, revealed several unexpected characteristics: an intrinsic magnetosphere, the highest mean density of any Solar System terrestrial planet and a very thin non-collisional atmosphere. Mercuryʹs atmosphere is very poorly explored since only three atomic elements, H, He and O, were observed during the three flybys of Mariner 10. The measurements done by radio and solar occultations provided upper limits on the neutral and ion densities. These measurements pointed out the close connection between species in Mercuryʹs exosphere and its surface, which is also the case for the Moon. Mariner 10 observations also characterized the vertical distributions and the day to night contrasts of Mercuryʹs exosphere for its lightest components H and He (Broadfoot, A.L., et al., 1976. Mariner 10: Mercury atmosphere. Geophys. Res. Lett. 3, 577–580). han a decade later, the first observation from a ground-based observatory of Mercuryʹs sodium (Na) exospheric component was reported (Potter, A.E., Morgan, T.H., 1985. Discovery of sodium in the atmosphere of Mercury. Science 229, 651–653). Since then, potassium and more recently calcium have been identified in Mercuryʹs exosphere. The bright Na resonant scattering emission has been often observed since 1985. This large set of observations is now the best source of information on Mercuryʹs exospheric mechanisms of ejection, dynamics, sources and sinks. In particular, several of these observations provided evidence of prompt and delayed effects, both localized and global, for the very inhomogeneous Mercuryʹs Na exosphere. These inhomogenities have been interpreted as the trace of Mercuryʹs magnetosphere–solar wind interaction and have highlighted some of the main sources of exospheric material. Some of these features have been also interpreted as the trace of a global dayside to night side circulation of Mercuryʹs exosphere and therefore have highlighted also the relation between exospheric production and upper surface composition. lly, new sets of in situ measurements will be obtained within the next decade thanks to Messenger and Bepi-Colombo missions. Until then, ground-based observations and modelling will remain the only approaches to resolve questions on Mercuryʹs exosphere. Mercuryʹs exospheric composition and structure as they are presently known are described in this paper. The principal models for the main short and long times terms variations and local and global variations of Mercuryʹs exosphere are described. The mechanisms of production and their characteristics are also given. Mercuryʹs exosphere can also be seen as part of the coupled magnetosphere–upper surface–exosphere system and several of the links between these elements are essential to the interpretation of most of the ground-based observations. The relation between Mercuryʹs planet composition and its exospheric composition is also considered, as is the global recycling, sources and sinks of Mercuryʹs exosphere.