Phoebe - Voyager 2 observations

Voyager 2 images of Phoebe obtained over a period of 24 hours provide information on the size, rotation rate, surface markings, and photometric properties of this mysterious object. Phoebe is approximately equidimensional: its longest diameter, about 230 km, is only 10 percent greater than the shortest-diameter (about 210 km). A prograde-rotation period of 9.4 + or - 0.2 hours was determined from both the disk-integrated light curve and by tracking individual markings. Because of the limited resolution of the images (11 pixels across the disk) crater counts cannot be made. The geometric albedo of Phoebe is longitudinally variable from 0.046 to 0.060 (clear filter, lambda = 0.47 micron). The most prominent surface markings are brighter patches at high northern and southern latitudes that have reflectances as much as 50 percent greater than the dark, bland areas. These patches are scattered and do not appear to constitute polar caps. The Voyager color data agree with earlier ground-based spectra that show that Phoebe has a flatter spectrum than does the dark side of Iapetus; this observation is not consistent with simple contamination of Iapetus by debris from Phoebe.