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From the Cassini probe of Saturn:

The international Cassini spacecraft has taken new images of Saturn's two-faced moon Iapetus, possibly offering clues to why the moon has a dark hemisphere and another that is bright, scientists said Thursday.

Researchers at NASA's Jet Propulsion Laboratory hope Cassini's observations of the mysterious moon help determine where the dark material comes from.
(via CNN)

Oh.... Make up your own jokes!

But I know where the dark material comes from....