During the Galileo flybys of Callisto in 1999, a CO 2 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 CO 2 alone is insufficient to produce the observed electron density. An atmosphere 20-100 times denser than the CO 2 atmosphere must be introd…
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