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.