Data underlying the publication: Callisto's atmosphere: First evidence for H2 and constraints on H2O

We explore the parameter space for the contribution to Callisto's H corona observed by the Hubble Space Telescope (Roth et al. 2017a) from sublimated H₂O and radiolytically produced H₂ using the Direct Simulation Monte Carlo (DSMC) method. The spatial morphology of this corona produced via photo- and magnetospheric electron impact-induced dissociation is described by tracking the motion of and simulating collisions between the hot H atoms and thermal molecules including a near-surface O₂ component. Our results indicate that sublimated H₂O produced from the surface ice, whether assumed to be intimately mixed with or distinctly segregated from the dark non-ice or ice-poor regolith, cannot explain the observed structure of the H corona. On the other hand, a global H₂ component can reproduce the observation, and is also consistent with enhanced electron densities observed at high altitudes by Galileo's plasma-wave instrument (Gurnett et al. 1997, 2000), providing the first evidence of H₂ in Callisto's atmosphere. The range of H₂ surface densities explored, under a variety of conditions, that are consistent with these observations is ∼(0.4-1)×10⁸ cm^-3. The simulated H₂ escape rates and estimated lifetimes suggest that Callisto has a neutral H₂ torus. We also place a rough upper limit on the peak H₂O number density (<∼10⁸ cm^-3), column density (<∼10¹⁵ cm^-2), and sublimation flux (<∼10¹² cm^-2 s^-1), all of which are 1-2 orders of magnitude less than that assumed in previous models. Finally, we discuss the implications of these results, as well as how they compare to Europa and Ganymede.