Radii, Albedos, and 20-MICRON Brightness Temperatures of Iapetus and RHEA

From infrared flux measurements at a wavelength of 20 we deduce that the radius of lapetus is 850 j 100 km and that the remarkable light variations are caused by an albedo difference between the leading and trailing hemispheres. The visual albedo is 0.04 0.01 on the leading side and 0.28 0.05 on the trailing side. For Rhea we find a radius of 725 j 100 km and an albedo of 0.57 0.07.