Eclipse Spectroscopy of Io’s Atmosphere

High resolution optical (4193–6619 Å) spectra of Io in eclipse show auroral emission in five lines: [Oi] 6300, 6363, 5577 Å and Na 5889, 5896 Å. We conclude that Io's diffuse red emissions imaged by the Galileo Solid State Imager (P. E. Geissler et al. 1999, Science 285, 870–874) are due to impact or dissociative excitation of oxygen, while diffuse green emissions are caused by the impact or dissociative excitation of sodium. No emission lines were detected in the blue region of the spectra, suggesting that the intense emission observed above Io's equatorial plumes is due to a molecular species such as SO_2 or SO