As the most geologically primitive of the icy Galilean satellites, Callisto has the least well understood atmosphere, limiting our understanding of the evolution of the objects in this important system, soon to be the subject of multiple new spacecraft observations. There is no consensus on the state of water ice on its surface nor how that correlates with the production of atmospheric water vapor. Similarly, although radiolysis is likely the primary source of O 2 in Callisto's atmosphere (Cunningham et al., 2015), the concomitant H 2 component has not yet been identified. Forthcoming exploration of Callisto by ESA's JUpiter ICy moons Explorer (JUICE; e.g., Galli et al., 2022), NASA's Europa Clipper, and CNSA's planned Gan De can help resolve such issues. With

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