Eclipse spectroscopy of Io's atmosphere

High resolution optical (4193-6619 Angstrom) spectra of Io in eclipse show auroral emission in five lines: [O-I] 6300, 6363, 5577 Angstrom and Na 5889 , 5896 Angstrom, We conclude that Io's diffuse red emissions imaged by the Galileo Solid State Imager (P, E. Geissler ef al. 1999, Science 285, 870-87 4) are due to impact or dissociative excitation of oxygen, while diffuse gr een emissions are caused by the impact or dissociative excitation of sodium . No emission lines were detected in the blue region of the spectra, sugges ting that the intense emission observed above Io's equatorial plumes is due to a molecular species such as SO2 or SO. (C) 2000 Academic Press.