Callisto's Atmosphere: First Evidence for H 2 and Constraints on H 2 O

We explore the parameter space for the contribution to Callisto's H corona observed by the Hubble Space Telescope from sublimated H 2 O and radiolytically produced H 2 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 O 2 component. Our results indicate that sublimated H 2 O 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 H 2 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 H 2 in Callisto's atmosphere. The range of H 2 surface densities explored, under a variety of conditions, that are consistent with these observations is ∼(0.4–1) × 10 8 cm −3 . The simulated H 2 escape rates and estimated lifetimes suggest that Callisto has a neutral H 2 torus. We also place a rough upper limit on the peak H 2 O number density (≲10 8 cm −3 ), column density (≲10 15 cm −2 ), and sublimation flux (≲10 12 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. 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 O 2 and H 2 as well as directly eject H 2 O into the atmosphere, only near‐surface O 2 and trace extended H components have been observed by the Hubble Space Telescope, while H 2 O and H 2 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 H 2 O and H 2 . Using sublimation rates suggested in the literature, H 2 O produces too much H near the subs