Spatial and Temporal Variability of Surface Oxidants at Europa and Ganymede

We describe physical processes affecting the formation, trapping, and outgassing of molecular oxygen (O₂)at Europa and Ganymede. Following Voyager measurements of their ambient plasmas, laboratory data indicated that the observed ions were supplied by and would in turn impact and sputter their surfaces (Lanzerotti et al 1978), decomposing the ice (Brown et al 1982) and producing thin O₂ atmospheres (Johnson et al 1982). More than a decade later Europa’s ambient O₂ was inferred from observations of the O aurora (Hall et al 1995,1998)  and condensed O₂ bands at 5773 & 6275 Å were observed in Ganymede’s icy surface (Spencer et al 1995; Calvin et al. 1996). More than another decade later, the O₂ atmosphere was shown to have a dusk/dawn enhancement (Roth et al. 2016; Leblanc et al. 2017; Oza et al. 2019), confirmed by Juno data (Addison et al. 2024). Although the incident plasma produces these observables, processes within the surface are still not well understood. Here we note that incident plasma produces a non-equilibrium defect density in the surface grains. Subsequent diffusion leads to the formation of voids and molecular products, some of which are volatile (Johnson and Quickenden 1997). Although some volatiles are released into their atmospheres, others are trapped at defects or in voids forming gas bubbles, which might be delivered to their subsurface oceans. Here we discuss how trapping competes with annealing of the radiation damage. We describe differences observed at Europa and Ganymede and roughly determine the trend with latitude of O₂ bands on Ganymede’s trailing hemisphere (Trumbo et al. 2021). This understanding is used to discuss the importance of condensed and adsorbed O₂ as atmospheric sources, accounting for dusk/dawn enhancements and temporal variability reported in condensed O₂ band depths. Since plasma and thermal annealing timescales affect the observed O₂ variability on all of the icy moons, understanding the critical physical processes of O₂ can help determine the evolution of other detected oxidants often suggested to be related to geologic activity and venting.