Saturn's outermost main moon is unusual because its leading hemisphere - the half of the moon that permanently faces in the direction it orbits around Saturn – is much darker than its trailing hemisphere.
It is thought the moon may have been contaminated with a dark material originating from another of Saturn's moons, though it is uncertain exactly where Iapetus gets its dark hemisphere from.
Photo: Iapetus taken by the Cassini probe (NASA/JPL/Space Science Institute)
Sir Patrick Moore looks at Cassini-Huygens's first images of some of Saturn's moons.
Sir Patrick Moore has a look at the Cassini-Huygens probe's first images of some of Saturn's icy moons - Phoebe, Mimas, Tethys and Iapetus - and discusses the mission with his guests.
Iapetus ( /aɪˈæpɨtəs/;Greek: Ιαπετός), occasionally Japetus ( /ˈdʒæpɨtəs/), is the third-largest moon of Saturn, and eleventh largest moon in the Solar System. It was discovered by Giovanni Domenico Cassini in 1671. Iapetus is best known for its dramatic 'two-tone' coloration, but recent discoveries by the Cassini mission have revealed several other unusual physical characteristics, such as an equatorial ridge that runs about halfway around the moon.
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