Plasma and Thermal Processing Leading to Latitudinal and Temporal Variability of the Trapped O ₂ at Europa and Ganymede

We describe the physical processes that affect the formation, trapping, and outgassing of O ₂ at Europa and Ganymede. Following Voyager measurements of their ambient magnetospheric plasmas, laboratory data indicated that the observed ions, which were mostly ejected from volcanic Io, would in turn impact and sputter their surfaces. This would decompose the ice and produce thin oxygen atmospheres. More than a decade later, Europa’s O ₂ atmosphere was inferred from observations of the O aurora, and “condensed” O ₂ bands were observed in Ganymede’s icy surface at 5773 and 6275 Å. More than another decade later, their atmospheres were shown to have a dusk/dawn enhancement, confirmed by recent Juno data. Although the incident plasma produces these observables, processes that occur within the topmost surface are still not well understood. Here, we note that the incident plasma particles produce a nonequilibrium defect density locally in the surface ice grains. Defect diffusion within these grains leads to the formation of voids and molecular products, some of which are volatile. Although some volatiles are released into the satellite atmospheres, others are trapped at defect sites or trapped in voids, which create gas bubbles whose lifetimes (in steady state) are limited by the plasma-induced destruction rate. Here, we discuss how trapping competes with the annealing of the radiation damage. We describe the differences observed at Europa and Ganymede and roughly determine the observed trend with latitude of O ₂ bands observed on Ganymede’s trailing hemisphere. This understanding is used to discuss the relative importance of “condensed” O ₂ and O ₂ adsorbed on regolith grains as atmospheric sources, accounting for dusk/dawn enhancements and temporal variability reported in “condensed” O ₂ band depths. Since plasma-induced damage and thermal annealing timescales drive oxidant variability on icy moons (likely also Callisto, Dione, and Rhea), they can help determine volatile downwelling, a potentially metabolic source for their oceans, and upwelling of other trapped oxidants (e.g., CO ₂ ), suggestive of ongoing geologic activity.