The Origin and Fate of O 2 $\mbox{O}_{2}$ in Europa’s Ice: An Atmospheric Perspective

Abstract

The early prediction and subsequent detection of an 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 nearsurface 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 O 2 , a recent, simple model of thermal desorption from a regolith permeated with O 2 has changed the usual paradigm. In that model, the observed orbital dependence of the local source of the near-surface O 2 atmosphere is suggested to be due to the release of 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 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.

The Optical Aurorae of Europa, Ganymede, and Callisto

“…For Europa, the spatial distribution of emissions exhibits some systematic trends, but is still not fully understood. A dusk-dawn asymmetry is observed in both the UV and optical auroral data ( Roth et al 2016;de Kleer & Brown 2019), which is consistent with simulations ( Oza et al 2019) and suggests a thermal role in the production of the O 2 atmosphere ( Oza et al 2018;Johnson et al 2019). For Ganymede, the aurora appears to behave analogously to Earth's auroral ovals, whereby electrons are accelerated into the near-surface region along field lines at the open-closed field line boundary (Feldman et al 2000;Eviatar et al 2001;McGrath et al 2013).…”

Callisto's Atmosphere: The Oxygen Enigma

“…Instead, they will permeate or become trapped in the porous, radiation-altered regolith and, assuming reactions therein are negligible, eventually thermally desorb back into the atmosphere, where their accumulation is limited by gas-phase ionizing and dissociative processes. Thus, over time, a small ab initio source flux (Figure 5) can eventually lead to a steady-state, global, and even collisional atmosphere, where molecules are continuously returning to and thermally desorbing from the regolith (e.g., Carberry Mogan et al, 2020Mogan et al, , 2021Mogan et al, , 2022Johnson et al, 2019).…”

“…Having examined a number of additional sources (Section 5.1), we suggest that the upward flux of O 2 from the regolith principally supplies the inferred atmospheric column. O 2 can accumulate in Callisto's porous regolith as a result of being weakly bound to defect sites on grain surfaces, as suggested to account for Europa's dusk/dawn asymmetry (Johnson et al., 2019); being stably trapped in bubbles in grains (e.g., Johnson & Jesser, 1997), consistent with observations of O 2 on Callisto's surface (Spencer & Calvin, 2002); and/or being recycled therein via reactions (e.g., Shematovich, 2006; Shematovich & Johnson, 2001; Shematovich et al., 2005). Therefore, a better understanding of the production and fate of the O 2 in the regolith is required in order to place better constraints on the mechanisms for generating Callisto's O 2 atmosphere.…”

“…Johnson and Jesser (1997) described how O 2 can form and subsequently become trapped in porous regolith and/or radiation‐damaged ice at Ganymede. Formation and trapping of O 2 has recently been explored at Europa (Johnson et al., 2019). Hence, it is expected that the phenomenon could also occur at Callisto as well.…”

“…Upon release from the surface, since O 2 is too heavy and slow to escape the atmosphere and reactions therein are infrequent, it will eventually return to the surface. However, O 2 is too volatile to freeze out at Callisto's surface temperatures, so it will permeate the porous regolith (e.g., Johnson et al., 2019). Since O 2 is a relatively inert molecule, reactions in the regolith are infrequent, so that it will eventually thermally desorb back into the atmosphere.…”