Screaming Penguin - Source Does Matter

Yeah I know its good to have two halves, sure, in this case though Iapetus has two different havles. One is bright "as snow" and the other is dark "as tar".

There are various theories as to why this is. The most interesting (IMHO) is that Iapetus was "painted" by a clound of orbiting organic dust. Cassini has made the closest ever approach to Iapetus and is still underway to Titan.

For more see the linked BBC article.    Probe passes 'moon of two halves'