[P-PS01] Outer Solar System Exploration Today, and Tomorrow

[PPS01-09] Evidence for H₂ in Callisto's atmosphere

*Shane Carberry Mogan¹、Orenthal J Tucker²、Robert E Johnson^1,3、Lorenz Roth⁴、Juan Alday⁵、Audrey Vorburger⁶、Peter Wurz⁶、Andre Galli⁶、Howard Todd Smith⁷、Apurva Oza⁸ (1.New York University, New York, USA、2.NASA Goddard Space Flight Center, Greenbelt, USA、3.University of Virginia, Charlottesville, USA、4.KTH Royal Institute of Technology, Stockholm, Sweden、5.Open University, Milton Keynes, England、6.University of Bern, Bern, Switzerland、7.Johns Hopkins University Applied Physics Lab, Laurel, USA、8.NASA Jet Propulsion Laboratory, Pasadena, USA)

キーワード：Callisto, JUICE, Atmosphere, Hubble Space Telescope, Galilean Satellites, Galileo

We explore the parameter space for the composition of Callisto's atmosphere with contributions to its H corona from sublimated H₂O and radiolytically produced H₂ using the Direct Simulaton Monte Carlo (DSMC) method [1]. The 2D spatial morphology of the observed H corona produced by photon and magnetospheric electron induced dissociation at eastern elongation [2] is described by tracking the motion of and collisions between the hot H atoms and thermal molecules in Callisto's atmosphere. Sublimated H₂O produced from the surface ice, whether assumed to be intimately mixed with or distinctly segregated from the dark, non-ice or ice-poor regolith, cannot explain the structure of the H corona. The observation instead suggests the presence of a roughly global source, which we suggest is H₂ as we originally proposed in an earlier study [3]. Such a source is also roughly consistent with the Galileo plasma-wave observations [4, 5]. This provides the first direct evidence of H₂ in Callisto's atmosphere. Comparison with the morphology of the observed H corona allows us to estimate the surface source rate of H₂ (~2x10²⁸ s^-1, ~67 kg/s) and to place a rough upper limit on the peaks in H₂O density (<~10⁸ cm^-3) and sublimation flux (<~10¹² cm^-2 s^-1). The latter is 1-2 orders of magnitude less than that assumed in previous models [3, 6-8]. These constraints, along with the concomitant H₂ escape rates are used to obtain estimates for a neutral H₂ torus co-rotating with Callisto. With a closest approach of 200 km the Neutral gas and Ion Mass spectrometer on board JUICE should be able to measure the neutral gas densities for H₂O, H₂, and H presented here on the day-side, with detections of H₂O becoming more difficult with increasing distance from the subsolar point. In addition, we show that collisions in the atmosphere can have a significant affect on the density distribution of nascent light species, such as that of the H, which in some of the instances considered here inflated by more than an order of magnitude as a result of such interactions, and thus need to be considered when predicting JUICE atmospheric measurements. Based on these results, the role of H₂ versus H₂O as a source of the H corona needs re-examining at Europa [9] and Ganymede [10-11].

References:

[1] Bird (1994): Molecular gas dynamics and the direct simulation of gas flows.

[2] Roth, L. et al. (2017): Detection of a hydrogen corona at Callisto. Journal of Geophysical Research: Planets.

[3] Carberry Mogan, S. R. et al. (2021): A tenuous, collisional atmosphere on Callisto. Icarus.

[4] Gurnett, D. A. et al. (1997): Absence of a magnetic-field signature in plasma-wave observations at Callisto. Nature.

[5] Gurnett, D. A. et al. (2000): Plasma densities in the vicinity of Callisto from Galileo plasma wave observations. Geophysical Research Letters.

[6] Liang, M.C. et al. (2005): Atmosphere of Callisto. Journal of Geophysical Research: Planets.

[7] Hartkorn, O. et al. (2017): Structure and density of Callisto’s atmosphere from a fluid-kinetic model of its ionosphere: Comparison with Hubble Space Telescope and Galileo observations. Icarus.

[8] Vorburger, A. et al. (2015): Monte-Carlo simulation of Callisto’s exosphere. Icarus.

[9] Roth, L. et al. (2017): Detection of a hydrogen corona in HST Lyman-alpha images of Europa in transit of Jupiter. The Astronomical Journal.

[10] Barth, C. A. et al. (1997): Galileo ultraviolet spectrometer observations of atomic hydrogen in the atmosphere of Ganymede. Geophysical Research Letters.

[11] Feldman, P. D. et al. (2000): HST/STIS ultraviolet imaging of polar aurora on Ganymede. The Astrophysical Journal.