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A.H. Bouchez, M.E. Brown, N.M. Schneider (Caltech)
Galileo images of Io in eclipse showed glows from three separate atmospheric components (Geisler et al., submitted to Science). Red, green, and blue emission appeared in bright knots above known volcanic plumes, a diffuse green emission was seen in patches across the surface, and a limb brightened red glow was seen above the northern limb. Interpretation of these results has been hampered by the lack of spectral identifications for these lines; the Galileo images gave only broad-band colors. In order to identify these emissions, we obtained high resolution spectra of Io in eclipse with the Keck telescope. We found 5 separate atomic emission lines causing the eclipse glows. The red emission is caused by [OI] 6300 and 6363 lines and the green is a combination of [OI] 5577 and Na 5889 and 5895 emission. No emissions were seen in the blue, which suggests that the blue emission is likely a broad molecular band from either SO or SO2. Several fundamental characteristics of Io's atmosphere are apparent from these observations. First, volcanic plumes appear to be a major source of atomic oxygen and sodium and of SO or SO2 (depending on the source of the blue emissions). Sodium appears to also have a separate source more widely distributed in patches over the surface. Finally, atomic oxygen, which appears highly limb-brightened at the northern limb, must have a source more distributed than the SO or SO2 of the plumes. We attempt to synthesize these characteristics into a coherent picture of Io's tenuous atmosphere.