Galileo Search for SO2-Frost Condensation on Io's Nightside

New, high-quality Galileo images of Io's dayside and Jupiterlit nightside, examined for diurnal changes in violet albedo patterns and diurnal changes in hemispheric color, do not provide any evidence for significant nighttime condensation of the satellite's thin SO2atmosphere. In particular, the Galileo images do not show the major change in Io's average hemispheric color between day and night that was reported by Burattiet al.(1995,Icarus118, 418–422) in a study of Voyager data. The lack of detectable nighttime frost deposition in the Galileo images is not surprising, given the tenuous, “patchy” nature of Io's SO2atmosphere and the associated difficulty in producing optically thick nighttime frost layers.