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What causes the dark and bright hemispheres of Iapetus?
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Saturn
Saturn and its spectacular rings, in a natural-color composite of 126 images taken by the Cassini spacecraft on October 6, 2004. The view is directed toward Saturn's southern hemisphere, which is tipped toward the Sun. Shadows cast by the rings are visible against the bluish northern hemisphere, while the planet's shadow is projected on the rings to the left.
The distinct brightness contrast on Iapetus, one of Saturn’s major moons, is primarily caused by the accumulation of dark material on its leading hemisphere.
This dark material consists of particles originating from a dust ring created by impacts on the outer moon Phoebe. As this dark matter collects on the leading hemisphere, it absorbs more sunlight, which increases the surface temperature. This heating causes the water ice in that region to undergo sublimation over geologic time. The resulting water vapor then migrates and condenses onto the colder trailing hemisphere, where it freezes, maintaining the bright, icy appearance of that side.1