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. 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. 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.
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.