The reflection spectrum of liquid sulfur: Implications for Io

The spectral reflectance from 0.38 to 0.75 μm of a column of liquid sulfur has been measured at several temperatures between the melting point (∼118° C) and 173°C. Below 160°C the spectral reflectance was observed to vary reversibly as a function of temperature, independent of the previous thermal history of the column. Once the temperature exceeded 160°C, the spectrum would not change given a subsequent decrease in temperature. The spectral reflectance of the liquid-sulfur column at all temperatures was very low (10–19%). Combining this information with Voyager spectrophotometry of Jupiter's satellite Io, it is concluded that liquid sulfur at any temperature on Io's surface would be classified as a “black area” according to the standards used by the Voyager imaging team in their spectrophotometric analysis ( L. Soderblom, T. V. Johnson, D. Morrison, E. Danielson, B. L. Smith, J. Veverka, A. Cook, C. Sagan, P. Kupferman, D. Pieri, J. Mosher, C. Avis, J. Gradie, and T. Clancy (1980). Geophys. Res. Lett. 7, 963–966).