What Do We Know on Mercury’s and Moon Exospheres and Could We Learn from Their Comparison?

Abstract:

Mercury’s and the Moon exospheres have many common characteristics. They are surface bounded exospheres essentially formed from the interaction of our Sun with their regolith. However, their interaction with their plasma environment and their respective distance to the Sun induce many remarkable discrepancies between these two objects. Mercury’s exosphere is clearly much denser than the Moon’s but remains much more difficult to observe remotely or in situ. Mercury’s intrinsic magnetic field induces a complex and variable interaction between the solar wind and the surface whereas the Moon regularly interacts with various plasma types (solar wind, magnetospheric…). In terms of observables, they are very complementary. Only few exospheric species have been observed so far on both objects despite many attempts, but observations from the Moon surface or very close to it allowed detecting species not yet observed at Mercury. Many features induced by the solar wind interaction with the Moon surface have been identified: as an example the surface charge, the backscattered population, planetary ions and signatures of the long term space weathering of the regolith. All these features are keys to understand the formation and evolution of these exospheres. On the other hand, Mercury’s exosphere is denser and brighter than the Moon’s one. As a consequence, exospheric spatial structures have been identified, in association with the interaction between the solar wind and the magnetosphere or to specific source regions or suggesting some global exospheric dynamic around these objects. It has been also possible to reconstitute their time variations on several time scales, another way to constrain formation and evolution of these objects.

In this talk, I will underline what could be common and different on these two objects and will provide few clues how a comparative approach could be helpful to better understand surface bounded exospheres.