Presentation #513.07 in the session Fire and Ice: Europa and Aurorae.
Auroral emissions at UV and optical wavelengths are one of the few remotely-detectable signatures of the tenuous atmospheres of the galilean satellites. We present observations of the optical oxygen aurorae of Europa and Ganymede from observations made during Jupiter eclipse over four nights per target between 1998 and 2021 with Keck/HIRES. These observations resulted in the first simultaneous measurements of the atomic oxygen emissions at 5577 A and 6300 A at both targets. The emission lines are produced by electron impact dissociative excitation of molecules in these satellites’ atmospheres, and the ratio between the emission lines constrains the parent molecule. We interpret the observations using a model that permits O2, H2O, and atomic O as potential atmospheric species. We find that O2 dominates the atmospheric composition, and place stringent upper limits on atomic O. We find weak evidence for H2O on Europa at an H2O/O2 relative abundance of 0.25, but rule out H2O on Ganymede above an H2O/O2 abundance of 0.06. These conclusions are supported by the additional detection of new near-infrared oxygen lines at our targets; see presentation by Milby et al. These data collectively demonstrate the value of optical auroral measurements for providing robust constraints on atmospheric make-up.