Iapetus: Unique Surface Properties and a Global Color Dichotomy from Cassini Imaging

Iapetus Revealed The striking appearance of Saturn's moon Iapetus—half black and half white—has puzzled astronomers for over three centuries. Now Spencer and Denk (p. 432 , published online 10 December) present an explanation for this asymmetry: A thermally controlled runaway migration of water ice triggered by exogenic deposition of dark material on the moon's leading darker side, which faces the direction of motion of the moon in its orbit around Saturn. This mechanism is unique to Iapetus because it rotates slowly enough to allow large temperature variations to arise, it is small enough to allow long-range migration of water, and there is a source of dust to trigger the process. In a related paper, Denk et al. (p. 435 , published online 10 December) present data derived from the Cassini Imaging Science Subsystem that reveal that both dark and bright materials on the leading side of Iapetus are redder than their trailing-side counterparts. This asymmetry results from the deposition of dust and debris from other moons in the saturnian system—the very same process that initiates the thermal segregation proposed above.