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 \lambda 1349 dipole allowed and Cl i ] \lambda 1386 forbidden transition multiplets are detected at a signal to noise ratio ( SNR ) 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 \pm 0.008 , Cl/S = 0.10 \pm 0.05 , and S/O = 0.18 \pm 0.08 .
We also derive a geometric albedo of 1.0 \hskip { 3.0 pt } \pm \hskip { 3.0 pt } 0.4 \% for Io at 1335 Å assuming an SO _ { 2 } atmospheric column density of 1 \times 10 ^ { 16 } cm ^ { -2 } .