CONTRIBUTION OF MICROMETEOROID IMPACTS TO THE EXOSPHERES OF THE GALILEAN MOONS GANYMEDE AND EUROPA - ROZENN ROBIDEL

The exosphere is a thin atmosphere where the density is so low that atoms and molecules are unlikely to collide with each other. In the case of bodies with a substantial atmosphere (e.g. the Earth, Mars), the exosphere is the uppermost layer of the atmosphere, where it thins out and merges with outer space. In the case of airless bodies (e.g. the Moon, Mercury), the exosphere is the only atmosphere, in direct contact with the surface.
The exosphere of an airless body is transient and is continuously supplied by incoming sources (e.g. solar wind) and particles released from the surface through different processes (e.g. sputtering, micrometeoroid impact vaporization, photon- and electron-stimulated desorption…). The processes that release atoms and molecules in the exosphere are various according to the element (volatile or refractory) but also according to the body, its physical and chemical properties and its environment (e.g. surface, gravity, heliocentric distance, magnetic field, plasma environment…).
As micrometeoroids impact directly the surface of airless bodies, they release surface material into the exosphere by impact vaporization and ejection of dust grains. At Ganymede and Europa, micrometeoroid impact vaporization is often neglected as a source for neutral atmosphere models. However, micrometeoroid impacts result in eroding the surface and unveiling fresh material (the so-called impact-gardening). The contribution of micrometeoroid impacts is therefore important to understand the aging of surface and the relationship between endogenous and exogenous sources of surface material. Therefore, during my fellowship at ESA, I intend to study the contribution of micrometeoroid impacts on the formation of Ganymede and Europa exospheres, as a proxy to constrain their surface history. It is essential in the context of the upcoming missions ESA/JUICE and NASA/Europa Clipper, to explore the Jovian system in the early 2030s.