Iapetus, one of Saturn’s over 60 moons, has two faces.  One side is bright white, almost like snow, yet the other half is a dark black.  No one really knows what the dark stuff is.  However:

Close inspection indicates that the dark coating typically faces the moon’s equator and is less than a meter thick. A leading hypothesis is that the dark material is mostly dirt leftover when relatively warm but dirty ice sublimates. An initial coating of dark material may have been effectively painted on by the accretion of meteor-liberated debris from other moons. This and other images from Cassini’s Iapetus flyby are being studied for even greater clues.

On a random note, don’t you hate it when people pronounce it “ying-yang?”  That’s always bugged me.

via io9