A sublimated water atmosphere on Ganymede detected from Hubble Space Telescope observations

Ganymede’s atmosphere is produced by charged particle sputtering and sublimation of its icy surface. Previous far-ultraviolet observations of the O i 1,356 Å and O i 1,304 Å oxygen emissions were used to infer sputtered molecular oxygen (O 2 ) as an atmospheric constituent, but an expected sublimated water (H 2 O) component remained undetected. Here we present an analysis of high-sensitivity spectra and spectral images acquired by the Hubble Space Telescope revealing H 2 O in Ganymede’s atmosphere. The relative intensity of the oxygen emissions requires contributions from the dissociative excitation of water vapour, indicating that H 2 O is more abundant than O 2 around the subsolar point. Away from the subsolar region, the emissions are consistent with a pure O 2 atmosphere. Eclipse observations constrain atomic oxygen to be at least two orders of magnitude less abundant than these other species. The higher H 2 O/O 2 ratio above the warmer trailing hemisphere compared with the colder leading hemisphere, the spatial concentration in the subsolar region and the estimated abundance of ~10 15 molecules of H 2 O per cm 2 are consistent with sublimation of the icy surface as source.