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 Angstrom and O I 1356 Angstrom airglow radiation. The observed brightnesses of the reflected solar C II 1335 Angstrom 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 Angstrom)/F(1304 Angstrom) of roughly 1-2, diagnostic of dissociative electron impact excitation of O-2. Inferred O-2 vertical column densities are in the range (2.4-14) x 10(14) cm(-2) for Europa and (1-10) x 10(14) cm(-2) for Ganymede. The observed double-peaked profile of Ganymede's O I 1356 Angstrom 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.