More than 60 moons are known to orbit Saturn, but few of them are as visually striking as Iapetus. Named formally after the Greek mythological Titan, Iapetus is sometimes referred to as the "painted," or "yin-yang" moon, due to puzzling variations in its surface composition.
The moon's trailing hemisphere, shown here, is so bright and reflective as to resemble snow (click here for a much, much closer look); but if you direct your gaze eastward towards the moon's leading hemisphere, you'll see a material, dark as freshly poured asphalt, start to dominate its surface.
The composition of this dark material is unknown, but infrared spectra suggest it comprises some dark form of carbon. Also unclear is how this soot-colored substance came to paint Iapetus in the first place. According to NASA:
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.
[Photo by The Cassini Imaging Team, via NASA]