NATO ASI Series Volume 156, 1985, pp 805-815

Sulfur Dioxide Ice on IO

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The 4-μm band system in the spectrum of Io, as well as the ultraviolet absorptions, are caused by SO2 ice or frost distributed over a major fraction of the satellite. The spectral contribution of any adsorbed gas component of the surface cannot be discriminated from the ice absorption band with the data now available. The 4-μm band is strongest on the leading hemisphere of Io and weakest on the trailing. No temporal changes are seen in the six-year interval in which the infrared data have been studied. Individual spectral features attributed to SO2 with various combinations of the 32S, 34S, 16O, and 18O isotopes are seen in the 4-μm region.

Infrared spectra of Io during one of its eclipse by Jupiter in 1983 revealed particles or flakes of SO2 snow in the volcanic eruption plume above the Loki volcano. Calculations of the erupption rate of SO2 from Loki from our observations are consistent with independent determinatons of the deposition rate made from other data. Spectroscopic studies in combination with photometric observations in the thermal infrared are providing a satisfactory way in which to monitor the activity of the volcanoes on Io from ground-based observations.