Europa, the smallest of the Galilean moons of Jupiter was thought to be a passive rocky moon as late as the mid fifties. Voyager flybys in the late 70’s revealed a flat fractured surface almost free of craters, which hints at a dynamic thermal history. Galileo flybys in the 90’s provided strong evidences of a liquid salty ocean under the icy crust, which could be favorable to the emergence and evolution of life independent from life on Earth. The JUICE and EUROPA missions at Europa are planed for the 2020’s to search for this ocean.
Europa has an O2 atmosphere, hundred of billion times more tenuous than that of Earth’s. It is produced by the bombardment of the icy surface by ions and electrons from the magnetosphere of Jupiter but its characteristics are still very poorly understood.
The modeling of the interaction of this atmosphere with the jovian plasma provides a way to constrain this neutral atmosphere, its column density and extension as well as its plasma sputtering origin.
I will review briefly the plasma-atmosphere interaction at Europa and present the results of a physical chemistry model of this interaction recently published. The main claim of this paper is that the interaction at Europa results in the ejection of neutrals from the atmosphere at a rate ~ 10 times larger than the ionization rate. The main neutral loss process is the symmetrical charge exchange of O2, which has received little attention so far by the Europa community.
This large neutral loss could be consistent with the indirect detection of large neutral clouds spreading along the orbit of Europa. The detection of these ejected neutrals is relevant for the future missions at Europa.