A Stable H 2 O Atmosphere on Europa’s Trailing Hemisphere From HST Images

Abstract Previous studies of the global intensities of the oxygen emissions at 1,356 Å and 1,304 Å revealed molecular oxygen (O 2 ) 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 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 H 2 O abundance in the central sunlit trailing hemisphere with an H 2 O/O 2 ratio of 12–22. On the leading hemisphere, the emissions are consistent with a pure O 2 atmosphere everywhere across the moon disk.