Callisto's Atmosphere: First Evidence for H2 and Constraints on H2O. Issue 11 (17th November 2022)

• Record Type: Journal Article
• Authors: Carberry Mogan, Shane R.; Tucker, Orenthal J.; Johnson, Robert E.; Roth, Lorenz; Alday, Juan; Vorburger, Audrey; Wurz, Peter; Galli, Andre; Smith, H. Todd; Marchand, Benoit; Oza, Apurva V.
• Is Part Of: Journal of geophysical research. Volume 127:Issue 11(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-11-17
• Subjects: Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2022JE007294
• Languages: English
• ISSNs: 2169-9097

Abstract

Abstract: We explore the parameter space for the contribution to Callisto's H corona observed by the Hubble Space Telescope from sublimated H2 O 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 H2 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 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) × 10 8 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 H2 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. 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 H2 O into the atmosphere, only near‐surface O2 and trace extended H components have been observed by the Hubble Space Telescope, while H2 O 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 H2 O and H2 . Using sublimation rates suggested in the literature, H2 O produces too much H near the subsolar point and too little closer to the terminator to reproduce the observation. On the other hand, a more global tenuous H2 component can explain the Hubble observation, as well as earlier observations made by the Galileo spacecraft of a highly extended ionosphere. This provides the first evidence for H2 in Callisto's atmosphere. Key Points: We provide the first evidence of H2 in Callisto's atmosphere: it is the primary source of the observed H corona and extended electrons Sublimated water vapor cannot produce the morphology of the observed H corona, constraining upper limits on peak densities and source rates Our results suggest the role of H2 versus H2 O as the primary source of the H coronae observed at Europa and Ganymede should be reexamined