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Global Dichotomy

The leading hemisphere of Iapetus is covered by extremely dark material, whereas the trailing hemisphere is covered with bright material. This dichotomy puzzled the discoverer, astronomer Giovanni Cassini, who noted that the satellite was visible only from one side of its orbit. Two models have been proposed to explain this. The first proposes that dark material from Phoebe, a dark exterior moon, falls onto Iapetus from orbit. The second model says that the dark material erupted from the interior of Iapetus into a low area in the leading hemisphere.

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