Detection of Atomic Chlorine in Io’s Atmosphere with theHubble Space TelescopeGHRS

We report the detection of atomic chlorine emissions in the atmosphere of Iousing Hubble Space Telescope observations with the Goddard High ResolutionSpectrograph (GHRS). The Cl I 1349 A dipole allowed and Cl I] 1386 A forbiddentransition multiplets are detected at a signal to noise ratio (SNR) of 6 and10, respectively, in a combined GHRS spectrum acquired from 1994 through 1996.Oxygen and sulfur emissions are simultaneously detected with the chlorine whichallows for self-consistent abundance ratios of chlorine to these otheratmospheric species. The disk averaged ratios are: Cl/O = 0.017 +/- 0.008, Cl/S= 0.10 +/- 0.05, and S/O = 0.18 +/- 0.08. We also derive a geometric albedo of1.0 +/- 0.4 % for Io at 1335 A assuming an SO2 atmospheric column density of1x10^{16} cm^{-2}.Comment: 23 pages, 5 figures, accepted for publication in ApJ, August 1, 200