A Stable H2O Atmosphere on Europa’s Trailing Hemisphere From HST Images

Previous studies of the global intensities of the oxygen emissions at 1,356 Å and 1,304 Å revealed molecular oxygen (O2) in Europa's atmosphere. Here we investigate the relative changes of the two oxygen emissions when Europa emerges from eclipse as well as the radial profiles of the relative emissions across the sunlit disk in Hubble Space Telescope observations taken in 1999, 2012, 2014, and 2015 while the moon was at various orbital positions. The eclipse observation constrains the atomic oxygen (O) column density to 6×1012 cm−2 or lower. We then find that the OI1356‐Å/OI1304‐Å ratio systematically decreases towards the disk center on the trailing hemisphere. The observed emission ratio pattern and the persistence of it from 1999 to 2015 imply a stable H2O abundance in the central sunlit trailing hemisphere with an H2O/O2 ratio of 12–22. On the leading hemisphere, the emissions are consistent with a pure O2 atmosphere everywhere across the moon disk. Plain Language Summary Observations by the Hubble Space Telescope in far‐ultraviolet light of Jupiter's icy moons were used in the past to detect their oxygen atmospheres. Results of a new analysis of images and spectra of the moon Ganymede have recently shown that the same observations also contain information that water vapor is abundant in the atmosphere in addition to oxygen. We use the same analysis here for Europa and find a water vapor atmosphere as well, but only above the orbital trailing hemisphere of the moon. Key Points Eclipse observations limit the abundance of atomic oxygen in Europa's atmosphere to a low level The OI1356‐Å/OI1304‐Å ratio is lower in the central part of the trailing hemisphere than elsewhere on Europa Modeling of the emissions suggest H2O to be more abundant than molecular oxygen in the trailing sub‐solar region