SO2 Frost: UV‐visible feflectivity and Io surface coverage

Abstract

The discovery of SO2 gas in Io's atmosphere and possible frost on Io's surface prompted us to measure the spectral reflectance of laboratory SO2 frost in the range 0.24 ‐ 0.85 µm. The frost samples were grown in suitable vacuum at 130°K and bi‐directional reflectance spectra were obtained by illuminating at 20° and observing at 60° from the sample normal. The spectrum of typical SO2 frost has very low reflectivity (2 ‐ 5%) at 0.30 µm, a steep absorption edge rising at 0.32 µm, maximum reflectivity (75 ‐ 80%) at 0.4 µm, and uniformly high reflectance throughout the visible and near infrared. These results, when compared with Io's full‐disk, Earth‐based reflectance spectrum, suggest that SO2 frost is not the dominant component of Io's optical surface layer. Various calculated UV‐visible spectra for mixtures of SO2 frost with other plausible surface components (sulfur allotropes and sodium sulfide) provide a good match to Io's spectrum and limit optically thick SO2 frost coverage to less than about 20% of Io's average projected surface area; this limit varies from a high of ∼ 25% on the leading hemisphere to a low of ∼ 10% on the sub‐Jupiter hemisphere.