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 the O{\small I\,}1356-{\AA} I\,}1304-{\AA} oxygen emissions were used to infer sputtered molecular (O$_2$) as an atmospheric constituent, but expected sublimated water (H$_2$O) component remained undetected. Here we present analysis high-sensitivity spectra spectral images acquired Hubble Space Telescope revealing H$_2$O in atmosphere. The relative intensity requires contributions from dissociative excitation vapor, indicating that more abundant than O$_2$ around sub-solar point. Away region, are consistent with a pure Eclipse constrain atomic be at least two orders magnitude less these other species. higher H$_2$O/O$_2$ ratio above warmer trailing hemisphere compared colder leading hemisphere, spatial concentration estimated abundance $\sim$10$^{15}$ H$_2$O/cm$^{2}$ surface source.