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[16.15] The Atmosphere of Callisto

Y. Yung, M-C. Liang (Caltech), B. Lane (MIT), R. Pappalardo (University of Colorado), M. Allen (Caltech)

During the Galileo flybys of Callisto in 1999, a CO₂ atmosphere and an ionosphere were detected. Using the Caltech/Jet Propulsion Laboratory one-dimensional KINETICS model, we have successfully simulated the observed electron density within a factor of 2, while satisfying the observational constraints on carbon and oxygen atoms. We conclude that photoionization of CO₂ alone is insufficient to produce the observed electron density. An atmosphere 20-100 times denser than the CO₂ atmosphere must be introduced, as suggested by Kliore et al. [2002]. We show that an O₂-rich atmosphere is highly probable. However, the atomic oxygen produced from O₂ photodissociation is 2 orders of magnitude greater than the upper limit given by Strobel et al. [2002]. The introduction of reactive hydrogen chemistry assuming a surface abundance of H₂O of ~2x10⁹~cm^-3 (4x10^-8~mbar) is required to reduce the excess atomic O abundance. The calculated atomic O column density is >5x10¹²~cm^-2, which is about the observed upper limit, suggesting we should be able to detect O in the atmosphere of Callisto.

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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.