The Optical Aurorae of Europa, Ganymede, and Callisto

Abstract

The tenuous atmospheres of the Galilean satellites are sourced from their surfaces and produced by a combination of plasma-surface interactions and thermal processes. Even though they are thin, these atmospheres can be studied via their auroral emissions, and most work to date has focused on their aurorae at UV wavelengths. Here we present the first detections of the optical aurorae of Ganymede and Callisto, as well as detections of new optical auroral lines at Europa, based on observations of the targets over 10 Jupiter eclipses from 1998 to 2021 with Keck/HIRES. We present measurements of O i emission at 6300/6364, 5577, 7774, and 8446 Å and place upper limits on hydrogen at 6563 Å. These constitute the first detections of emissions at 7774 and 8446 Å at a planetary body other than Earth. The simultaneous measurement of multiple emission lines provides robust constraints on atmospheric composition. We find that the eclipse atmospheres of Europa and Ganymede are composed predominantly of O2, with average column densities of (4.1 ± 0.1) × 1014 cm−2 and (4.7 ± 0.1) × 1014 cm−2, respectively. We find weak evidence for H2O in Europa's bulk atmosphere at an H2O/O2 ratio of ∼0.25, and place only an upper limit on H2O in Ganymede's bulk atmosphere, corresponding to H2O/O2 < 0.6. The column density of O2 derived for Callisto is (4.0 ± 0.9) × 1015 cm−2 for an assumed electron density of 0.15 cm−3, but electron properties at Callisto's orbit are very poorly constrained.