December 10, 2009--
That's no shadow over Saturn's moon Iapetus: The so-called leading hemisphere (left) is much darker than the opposite side, due to a combination of migrating ice and dark red dust, astronomers have found.
Possibly solving a 300-year-old mystery, two recent studies suggest a likely mechanism behind Iapetus' two-faced nature. To start, infalling dust from an external source--maybe another moon or Saturn's newest known ring
--darkens the leading side of Iapetus, which therefore absorbs more sunlight and heats up enough to evaporate ice near the equator.
The evaporating ice recondenses on the colder, undusted poles and on the trailing hemisphere. Meanwhile, loss of ice on the leading hemisphere makes the surface even darker, triggering a feedback loop that keeps Iapetus' extreme contrasts in place.
Image courtesy NASA/JPL/Space Science Institute