Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.0408h&link_type=abstract
American Astronomical Society, DPS meeting #28, #04.08; Bulletin of the American Astronomical Society, Vol. 28, p.1071
Astronomy and Astrophysics
Astronomy
3
Scientific paper
Far-UV spectra of Europa and Ganymede acquired by the Hubble Space Telescope High Resolution Spectrograph during 1996 June and July indicate that, in addition to faintly reflected sunlight, both satellites emit excess Oi1304 Angstroms and Oi1356 Angstroms radiation. Fifteen HST orbits were devoted to acquiring spectra of Europa's leading and trailing hemispheres, as well as Ganymede's trailing hemisphere. The new measurements of Europa's far-UV spectrum in the 1200--1500 Angstroms spectral range confirm those reported previously (Hall et al. Nature 373 677 1995). The observed brightnesses of the reflected solar Cii1335 Angstroms feature indicate that Europa's leading and trailing hemisphere disk-averaged albedos are less than about two percent. The 1335 Angstroms albedo of Ganymede's trailing hemisphere is about a factor of two larger. In addition to the faint reflected sunlight, all three spectra clearly show Oi1304 Angstroms and Oi1356 Angstroms line emission features, with the characteristic brightness ratio B(1356 Angstroms)/B(1304 Angstroms)~2 diagnostic of dissociative electron impact excitation of O_2. The measured instrumental line-shape of Ganymede's Oi1356 Angstroms feature implies that the emission originates from two distinct and spatially confined locations on the disk. Preliminary analysis indicates that these locations are consistent with the polar regions of the satellite, and suggests the existence of far-UV auroral emissions excited by precipitation of charged particles in the polar-cap regions of Ganymede's newly-discovered magnetic field.
Feldman Paul D.
Hall Doyle T.
McGrath Melissa A.
Strobel Darrell F.
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