Formation of Iapetus’ Extreme Albedo Dichotomy by Exogenically Triggered Thermal Ice Migration (original) (raw)

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.

Abstract

The extreme albedo asymmetry of Saturn’s moon Iapetus, which is about 10 times as bright on its trailing hemisphere as on its leading hemisphere, has been an enigma for three centuries. Deposition of exogenic dark material on the leading side has been proposed as a cause, but this alone cannot explain the global shape, sharpness, and complexity of the transition between Iapetus’ bright and dark terrain. We demonstrate that all these characteristics, and the asymmetry’s large amplitude, can be plausibly explained by runaway global thermal migration of water ice, triggered by the deposition of dark material on the leading hemisphere. This mechanism is unique to Iapetus among the saturnian satellites because its slow rotation produces unusually high daytime temperatures and water ice sublimation rates for a given albedo.

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Supplementary Material

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