High-resolution infrared spectroscopy of Io and possible surface materials

We have obtained new spectra of Io in the 3.5- to 4.2- and 4.5- to 5.4-μm regions with a resolution (λ/Δλ) of roughly 500. The Io spectra cover a range of longitudes, and times from 1983 through 1985. Laboratory spectra of various materials have also been obtained. In this wavelength region are located several strong bands of SO 2, known to be a major component of Io's surface, as well as the bands of several potential surface materials. In the Io spectra we identify several new features attributable to SO 2, and have obtained strengths of the 33S, 34S, and 18O isotopic bands, which appear normal. We place limits on the amounts of Na 2SO 3, and Na 2SO 4 present. Finally, we use the data to place limits on the SO 2 gas abundance in Io's atmosphere.