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 from sublimated H₂O and radiolytically produced H₂ using the Direct Simulation Monte Carlo method. The spatial morphology of this corona produced via photoelectron 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 nonice 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 capable of producing the enhanced electron densities observed at high altitudes by Galileo's plasma-wave instrument, 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.