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. Plain Language Summary The surface and atmosphere of Callisto, the outermost Galilean moon of Jupiter, are not well understood. Although water ice is a significant fraction of its bulk composition, there is no consensus on the amount of surface ice nor how that correlates with the amount of atmospheric water vapor produced via sublimation. Similarly, although irradiation of the icy surface by the plasma trapped in Jupiter's magnetic field is expected to release O2 and H2 as well as directly eject H2O into the atmosphere, only near‐surface O2 and trace extended H components have been observed by the Hubble Space Telescope, while H2O and H2 have not. By simulating the motion of these four species in Callisto's atmosphere, we estimated the contributions to the extended H atmosphere via dissociation of H2O and H2. Using sublimation rates suggested in the literature, H2O produces too much H near the subsolar point and too litt