Iapetus

Splendid picture of Saturn's moon Iapetus put together from Cassini imagery. Half of Iapetus is as dark as coal, the other half as bright as ice. Nobody is sure why, but the dark material seems to be carbon. One theory is that the leading edge of Iapetus is bombarded with dirty ice cast off by Saturn's other moons, which sublimates, leaving dark carbon compounds behind. But that's just a guess.