Diagram showing the molecular hydrogen in Callisto’s atmosphere.
Molecular hydrogen (H2) in Callisto’s atmosphere. Callisto is the grey semicircle in the middle of the figure and the atmospheric H2 density is represented in colors about the planet as indicated by the color bar. The black profile indicates the line-of-sight H2 column density in Callisto’s atmosphere. The Sun is to the right in this figure. Credit: Carberry Mogan et al. [2022], Figure 5a
Editors’ Highlights are summaries of recent papers by AGU’s journal editors.
Source: Journal of Geophysical Research: Planets

The presence of subsurface water reservoirs in the Galilean moons of Jupiter is a major topic in planetary exploration, especially as it links to the potential of the moons to support life. In particular, the atmosphere of the outermost Galilean moon of Jupiter, Callisto, is not well understood as only few observations from the Galileo mission and the Hubble Space Telescope are available.

Observations from these missions have only been able to detect a hydrogen (H) corona and near-surface molecular oxygen (O2). However, given the amount of water ice at the surface of Callisto and the interaction of Jupiter magnetic field with the surface, water vapour and molecular hydrogen (H2) are expected to be present in the atmosphere, but they have not been detected so far.

Carberry Mogan et al. [2022] demonstrate via modelling that Callisto possesses a global tenuous H2 atmosphere, which is responsible for the hydrogen observations made by both the Galileo mission and the Hubble Space Telescope. It is demonstrated that the hydrogen observations are not produced via water sublimation, as suggested before.

This research provides the first indication for H2 in Callisto’s atmosphere and constitutes a large step in our understanding of the Galilean moons and their potential habitability. It also provides the limits for both water vapour and molecular hydrogen in Callisto’s atmosphere, which are important inputs for the forthcoming ESA’s JUpiter ICy moons Explorer (JUICE) and NASA’s Europa Clipper missions.

Citation: Carberry Mogan, S. R., Tucker, O. J., Johnson, R. E., Roth, L., Alday, J., Vorburger, A., et al. (2022). Callisto’s atmosphere: First evidence for H2 and constraints on H2O. Journal of Geophysical Research: Planets, 127, e2022JE007294. https://doi.org/10.1029/2022JE007294

—Beatriz Sanchez-Cano, Associate Editor, JGR: Planets; and Anni Määttänen, Editor, JGR: Planets 

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