Callisto's ionosphere was first detected during Galileo flybys; its high electron density of 7–17 cm−3 cannot be explained by the photoionization of the atmospheric carbon dioxide alone. Hence, it is suspected that the atmosphere of Callisto is actually dominated by molecular oxygen (in amounts 10–100 times greater than ). However, oxygen has not yet been directly detected in the atmosphere of Callisto. Observations with the Hubble Space Telescope (HST) placed an upper limit on its possible concentration in the atmosphere, based on lack of detection, which is still compatible with the ionospheric measurements. At the same time, HST was able to detect condensed oxygen trapped on the surface of Callisto.
Callisto's ionosphere was first detected during Galileo flybys; its high electron density of 7–17 cm−3 cannot be explained by the photoionization of the atmospheric carbon dioxide alone Hence it is suspected that the atmosphere of Callisto is actually dominated by molecular oxygen (in amounts 10–100 times greater than ) However oxygen has not yet been directly detected in the atmosphere of Callisto Observations with the Hubble Space Telescope (HST) placed an upper limit on its possible concentration in the atmosphere based on lack of detection which is still compatible with the ionospheric measurements At the same time HST was able to detect condensed oxygen trapped on the surface of Callisto