The Origin and Fate of O 2 in Europa's Ice: An Atmospheric Perspective

The early prediction and subsequent detection of an $\mbox{O}_{2}$ atmosphere on Europa, coupled with the discovery that Europa has an ocean under its ice mantle, has made this moon a prime astrobiologic target, soon to be visited by the JUICE and Europa Clipper spacecraft. In spite of the considerable number of observational, modeling, and laboratory efforts, understanding the physics leading to the observed morphology of Europa’s near-surface $\mbox{O}_{2}$ atmosphere has been problematic. This is the case as the observed emissions depend on the local incident plasma ion flux, the local temperature and composition of the regolith, as well as on the near-surface electron temperature and density. Here we rely heavily on earlier reviews briefly summarizing the observational, laboratory and simulation efforts. Although it is agreed that radiolysis of the surface ice by the incident Jovian plasma is the ultimate source of observed O2, a recent, simple model of thermal desorption from a regolith permeated with $\mbox{O}_{2}$ has changed the usual paradigm. In that model, the observed orbital dependence of the local source of the near-surface O2 atmosphere is suggested to be due to the release of $\mbox{O}_{2}$ likely trapped on the ice grains at dangling bonds by the solar flux with a smaller contribution due to direct sputtering. This assumes that Europa’s icy regolith is permeated with trapped $\mbox{O}_{2}$ , which could also affect our understanding of the suggestion that the radiolytic products in Europa’s regolith might be a source of oxidants for its underground ocean.