The Two Faces of Iapetus

Combined photometry and radiometry of Iapetus can be used to investigate the nature of its surface and, in particular, the distribution of albedo that is responsible for the large variations in its visible and infrared brightness as it rotates. We present new 20-μm radiometric observations made in 1971-1973 and discuss these together with the photometric studies by Widorn (in 1949), Mills (in 1971), Noland et al. (in 1972-1973), and Franklin and Cook (in 1972-1974). The linear phase coefficient varies as the satellite rotates from 0.028 to 0.068 mag deg ^-1. When corrected for this effect, the photometric variations suggest an albedo distribution characterized by a dark area covering most of the leading hemisphere and a bright trailing hemisphere and bright south polar cap. A combined analysis of the photometry and radiometry yields a radius of 800 to 850 km and mean geometric albedos for the light and dark faces of about 0.35 and 0.07, respectively. The average phase integral of the bright hemisphere is between 1.0 and 1.5. We offer no explanation for the unique photometric properties of this satellite.