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Ganymede's atmosphere is produced by charged particle sputtering andsublimation of its icy surface. Previous far-ultraviolet observations of theO{\small I\,}1356-{\AA} and O{\small I\,}1304-{\AA} oxygen emissions were usedto infer sputtered molecular oxygen (O$_2$) as an atmospheric constituent, butan expected sublimated water (H$_2$O) component remained undetected. Here wepresent an analysis of high-sensitivity spectra and spectral images acquired bythe Hubble Space Telescope revealing H$_2$O in Ganymede's atmosphere. Therelative intensity of the oxygen emissions requires contributions fromdissociative excitation of water vapor, indicating that H$_2$O is more abundantthan O$_2$ around the sub-solar point. Away from the sub-solar region, theemissions are consistent with a pure O$_2$ atmosphere. Eclipse observationsconstrain atomic oxygen to be at least two orders of magnitude less abundantthan these other species. The higher H$_2$O/O$_2$ ratio above the warmertrailing hemisphere compared to the colder leading hemisphere, the spatialconcentration to the sub-solar region, and the estimated abundance of$\sim$10$^{15}$ H$_2$O/cm$^{2}$ are consistent with sublimation of the icysurface as source.

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