Abstract
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.
Highlights
The photolytic production of O2 and H2 from H2O sublimated from the surfaces of Jupiter’s icy satellites, along with the preferential escape of H2, led to the suggestion that the bodies could have accumulated a thin O2 atmosphere (Yung and McElroy 1977; Kumar and Hunten 1982)
It is agreed upon that O2, as well as its concomitant radiolytic product H2, are produced and ejected by the incident plasma, but a clear, quantitative description of the spatial and temporal distribution of the oxygen aurora based on Hubble Space Telescope (HST) observations remains elusive
The O2 is seen to trap in the near surface region of the irradiated laboratory ice samples, which on Europa would be the surface of an ice grain exposed to the incident plasma
Summary
The photolytic production of O2 and H2 from H2O sublimated from the surfaces of Jupiter’s icy satellites, along with the preferential escape of H2, led to the suggestion that the bodies could have accumulated a thin O2 atmosphere (Yung and McElroy 1977; Kumar and Hunten 1982). It is agreed upon that O2, as well as its concomitant radiolytic product H2, are produced and ejected by the incident plasma, but a clear, quantitative description of the spatial and temporal distribution of the oxygen aurora based on HST observations remains elusive This is unfortunate as Europa is a high priority target for exploration by both NASA and ESA. The production of oxygen in Europa’s surface ice is especially interesting, as it has been suggested to be important for potential biologic activity by subduction of oxygen rich material into its underground ocean (e.g., Chyba 2000; Johnson et al 2003; Orlando et al 2005; Greenberg 2008; Vance et al 2016; Russell et al 2017) This possibility depends critically on the state of the O2 produced in Europa’s icy regolith: that is, on how O2 binds in and diffuses through the porous icy regolith and ice mantle. We first give a brief review on the production of O2, summarize the observational data, and, discuss our recent understanding
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