Coming within 1,000 miles of the yin-yang moon, Cassini helped discern where this distinctive coloration comes from. Iapetus is tidally locked with Saturn, meaning only one side of the moon ever faces the gas giant. That means that as it orbits, another side is always facing forward. Like bugs on a car windshield, debris from Saturn's rings and moons smacks onto that face of the moon, creating the dark coloration. In turn, the darker side heats up more easily, so its ice is constantly sublimating and moving over to the white side, where it deposits and adds to its brightness.