Detection of Atomic Chlorine in Io's Atmosphere with the Hubble Space Telescope GHRS

We report the detection of atomic chlorine emissions in the atmosphere of Io using Hubble Space Telescope observations with the Goddard High Resolution Spectrograph (GHRS). The Cl I l1349 dipole allowed and Cl I] l1386 forbidden transition multiplets are detected at a signal-to-noise ratio of 6 and 10, respectively, in a combined GHRS spectrum acquired from 1994 through 1996. Oxygen and sulfur emissions are simultaneously detected with the chlorine, which allows for self-consistent abundance ratios of chlorine to these other atmospheric 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 of 1.0% +- 0.4% for Io at 1335 A, assuming an SO2 atmospheric column density of 1 x 1016 cm-2.