SO₂ Frost: UV‐visible feflectivity and Io surface coverage

Abstract

The discovery of SO 2 gas in Io's atmosphere and possible frost on Io's surface prompted us to measure the spectral reflectance of laboratory SO 2 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 SO 2 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 SO 2 frost is not the dominant component of Io's optical surface layer. Various calculated UV‐visible spectra for mixtures of SO 2 frost with other plausible surface components (sulfur allotropes and sodium sulfide) provide a good match to Io's spectrum and limit optically thick SO 2 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.