On the Nature of Iapetus

Abstract

For solar system bodies with little or no atmosphere, a linear relation exists between the depth of the negative polarization branch and the reciprocal of the geometric albedo. Considered in this fashion, the polarization data for Iapetus imply an albedo difference by about a factor of 6 between its leading and trailing hemispheres, and are consistent with the amplitude of the light curve for Iapetus. The brightness of the satellite at any particular maximum or minimum is unpredictable by a certain factor because of inadequately understood variations in solar phase angle and rotation. Although other measurements of the Iapetus albedos give results different from those in the presented paper, it is still clear that the two hemispheres have very different albedos.