Callisto's Atmosphere: First Evidence for H2, Constraints on Source Rates, and Implications for Future Observations

We explore the parameter space for the contribution to Callisto's H corona observed by the Hubble Space Telescope [1] from sublimated H2O and radiolytically produced H2 using the Direct Simulation Monte Carlo (DSMC) method [2]. 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 [3, 4], providing the first evidence of H2 in Callisto's atmosphere. The simulated H2 escape rates and estimated lifetimes suggest that Callisto has a neutral H2 toroidal cloud. We also place rough upper limits on the peaks of H2O sublimation flux and the corresponding density, both of which indicate previous models of Callisto's atmosphere overestimated the abundance of H2O by at least 1-2 orders of magnitude. Finally, we discuss how these results compare to Europa and Ganymede.

References

[1] Roth, L. et al. (2017): Detection of a hydrogen corona at Callisto. Journal of Geophysical Research: Planets.

[2] Bird (1994): Molecular gas dynamics and the direct simulation of gas flows.

[3] Gurnett, D. A. et al. (1997): Absence of a magnetic-field signature in plasma-wave observations at Callisto. Nature.

[4] Gurnett, D. A. et al. (2000): Plasma densities in the vicinity of Callisto from Galileo plasma wave observations. Geophysical Research Letters.