The Far‐Ultraviolet Oxygen Airglow of Europa and Ganymede

Far-UV spectra of Europa and Ganymede, acquired by the Hubble Space Telescope Goddard High Resolution Spectrograph, indicate that, in addition to faintly reflected sunlight, both satellites emit O I 1304 Å and O I 1356 Å airglow radiation. The observed brightnesses of the reflected solar C II 1335 Å feature indicate that the disk-averaged albedos of Europa and Ganymede are about 1.5% and 2.6%, respectively. Airglow emissions from both satellites are characterized by the flux ratio F(1356 Å)/F(1304 Å) of roughly 1-2, diagnostic of dissociative electron impact excitation of O2. Inferred O2 vertical column densities are in the range (2.4-14) × 1014 cm-2 for Europa and (1-10) × 1014 cm-2 for Ganymede. The observed double-peaked profile of Ganymede's O I 1356 Å feature indicates a nonuniform spatial emission distribution that suggests two distinct and spatially-confined emission regions, consistent with the satellite's north and south poles.