HUBBLE-SPACE-TELESCOPE UV SPECTRAL OBSERVATIONS OF IO PASSING INTO ECLIPSE

Time-resolved spectra of Io have been obtained with the Faint Object Spectrograph on the Hubble Space Telescope in January 1992 at times centered on the passage of Io into Jupiter's shadow. Two different eclipse observations covered 1100-1600 angstrom and 2250-3.300 angstrom. In the far-UV range, emission lines of atomic sulfur and oxygen from Io's atmosphere (similar to those previously detected with IUE) have been observed from lo in sunlight, and the spatial extent of the emitting region has been resolved for the first time: this is 0.5-1 Io radii (R(Io)) above the surface. The emission lines are typically 1 kR in brightness while lo is in sunlight, and decrease to a few hundred Rayleighs within 20 min or less of Io's passing into shadow. If the emissions are produced in Io's ionosphere, the decrease in shadow appears consistent with the collisional slowing and recombination of photoelectrons in 100-1000 s, with recombination an important quenching process if the dominant ion is molecular (i.e., SO2+). By contrast, the impact of corotating torus electrons is expected to continue when lo is in shadow. If impact by torus plasma dominates the emission, the decrease in shadow may be due to surface SO2 condensation, with the residual emission in shadow due either to plasma impact of gas above the hot volcanic calderas or electron impact on S and O. In the near-UV range, we have not detected any airglow emissions from lo's atmosphere in shadow, with the main limitation being a high level of scattered light from Jupiter. We derive a 3sigma upper limit to the 2560 angstrom So emission feature of 1 kR, which is close to what is expected from electron impact on SO2 based on the observed brightness of the FUV S and O lines in shadow. A high signal-to-noise spectrum of Io's albedo in sunlight reveals a spectral shape similar to laboratory spectra of SO2 frost reflectivity, and the relative albedo spectrum changed as lo passed into eclipse and part of the disk was in shadow. No specific SO2 gas absorption features appear in the albedo spectrum, although there could be substantial gas absorption near 2800 angstrom if the individual lines are narrow and saturated. Finally, we present preliminary models for the near-UV spectrum of Io as functions of SO2 frost areal coverage and SO2 gas density.