The sodium exosphere of Mercury and its dynamics

The exosphere is the upper layer of a planetary atmosphere, and the last planetary territory before the interplanetary medium. In case of planets without a proper atmosphere, as Mercury, the exosphere is also the only kind of ‘atmosphere’ the planet possesses, and it is directly in contact with the surface. Due to this peculiarity, its origin is mainly from the surface outgassing, through a complex series of processes that acts as sources. The exospheric composition is then strictly related to the planet surface, but also to the other many interactions that the exosphere experiences with: the solar wind radiation and particles, the intrinsic magnetic field and ions circulation, the interplanetary magnetic field, the (micro)meteoritic bombardment. In addition, the interaction with the previously cited elements may also cause depletion. As a consequence, the exosphere of Mercury experiences an intense spatial and temporal variability.

The resulting dynamics of the exosphere of Mercury is evidenced in the studies of the sodium component that is a perfect tracer of the hidden interactions with the surrounding environment.

MESSENGER mission in the last decade highlighted the strong interchange with the intrinsic magnetic field, and the Earth-based observations with the interplanetary magnetic field. In the next future, the BepiColombo mission to Mercury, launched in October 2018, with its instrumentation devoted to the study of the magnetic field, ions and neutral particles will contribute to a comprehensive explanation of the processes and interactions that generate, sustain and change it.