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Atomic hydrogen has also been detected in Callisto's atmosphere via recent analysis of 2001 Hubble Space Telescope data. Spectral images taken on 15 and 24 December 2001 were re-examined, revealing a faint signal of scattered light that indicates a hydrogen geocorona. The amount of hydrogen present in Callisto's atmosphere varies depending on the stage of its orbit.

Callisto's leading hemisphere is less reflective than its trailing hemisphere, which could indicate that one of its sides has a higher surface temperature. On this darker, warmer side, sublimation rates of its icy surface would also be higher, causing more water vapor to be released into the atmosphere. This process would ultimately produce more hydrogen, which might explain the aforementioned variance in Callisto's atmospheric hydrogen levels.