Callisto's Atmosphere: The Oxygen Enigma

Observations of Callisto's atmosphere have indicated an O 2 component should exist, but the evolution from its initial source to its inferred steady‐state abundance is not well understood. Herein we constrain the production of O 2 via radiolysis within Callisto's exposed ice patches and determine the corresponding O 2 column density. To do so, for the first time we simulate the thermal and energetic components of the Jovian magnetospheric plasma irradiating Callisto's atmosphere and estimate energy deposited therein by the impinging charged particles along their trajectories to the surface. We then calculate O 2 source fluxes corresponding to the energy of the impacting plasma fluxes, which is coupled with estimated atmospheric lifetimes to determine the steady‐state abundance of O 2 . Our results suggest that production of O 2 via radiolysis within the exposed ice on Callisto's surface does not produce a sufficiently dense atmosphere relative to the column densities inferred from observations by about 2–3 orders of magnitude. To resolve this discrepancy between estimated and observed abundances, we provide the first estimates for other potential sources of atmospheric O 2 . We also make similar estimates for the production of H 2 in Callisto's atmosphere relative to constraints provided in the literature, and the conclusion is the same: a sufficiently dense atmosphere is not produced. Thus, we have shown that a better understanding of the production and fate of radiolytic products in Callisto's regolith is required in order to place firmer constraints on the generation mechanisms of its atmosphere in preparation for future observations. Molecular oxygen (O 2 ) has been inferred to exist in Callisto's atmosphere from observations taken across more than two decades by three separate instruments each using distinct measurement techniques. Exposure of Callisto's icy surface by the ions and electrons trapped in Jupiter's magnetic field is expected to produce O 2 , which subsequently releases into an atmosphere around Callisto. However, prior to this study, the amount of O 2 produced through such pathways has not been explicitly quantified. Here, for the first time, we determine the amount of O 2 produced by the irradiation of the exposed ice on Callisto's surface by Jupiter's plasma after depositing energy in the atmosphere. Our results show that this source of O 2 does not produce nearly enough compared to what has been inferred from observations. Therefore,