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Iapetus: Saturn's Disappearing Moon

Iapetus, one of Saturn's moons, has a peculiar surface: one half is very dark, the other very light. This contrast led its discoverer, Cassini, to note that Iapetus was only visible on one side of Saturn. The reason for this dramatic difference between its hemispheres is still a mystery. Iapetus is the third largest of Saturn's moons and is mostly made of water ice.