Saturns moon Iapetus is a vast frozen world, much of it is covered in ice while the rest of it is a mysterious dark material is of unknown composition. But infrared spectroscopy indicates that it could contain some form of carbon. Iapetus has a strange equatorial ridge that makes it look like a walnut. To help better understand Iapetus NASA sent the Cassini to orbit within 2,000 kilometers of the moon in 2007. It is thought that the dark parts of the moon are less than a meter thick and was formed when dirty ice sublimated.