A Stable H₂O 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 (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.