Observational tests for sulfur allotropes on Io

One of the intrinsic properties of particulate sulfur allotropes is a change in UV-visible reflectivity with temperature change of the material. The surface of Io experiences temperature changes during eclipse which are sufficient to cause a detectable change in the spectral reflectivity of sulfur; thus, if the surface of Io is composed primarily of sulfur allotropes, a change in reflectivity at certain wavelengths should be observable shortly after eclipse reappearance. We observed four eclipse reappearances during July and August of 1983 and saw no posteclipse brightening effects in filter bands selected for sensitivity to color changes in sulfur. Our model of the brightness change for S₈ (“yellow” sulfur) implies that this material covers less than 50% of Io's surface. Negative posteclipse brightening observations were also obtained with a filter chosen for the high contrast between SO₂ frost and the average albedo of the surface of Io at that wavelength. We conclude that no significant condensation of optically thick SO₂ occurred on the surface of Io during these eclipses.