What is Iapetus?
Iapetus is Saturn’s third-largest moon, after Titan, which is larger than Luna, and Rhea, which is slightly larger than Iapetus. Iapetus is slightly smaller than our own Moon, with a diameter of roughly 1,500 km, compared to the Moon’s 1,737 km. Iapetus has a range of physical peculiarities that make it somewhat more interesting than any typical rocky moon. The first, and most striking, is a dramatic two-tone coloration of black and white, which has given it the nickname “the Yin-Yang moon.” Like many moons, Iapetus is tidally locked to its planet, meaning the same side always faces it. In the direction of its motion around Saturn, Iapetus is as dark as charcoal, on its trailing side, bright as snow. This created quite a bit of confusion for the early astronomers who observed it, who saw the moon blinking in and out of existence as its white and dark sides oscillated from our point of view. But Iapetus’ two-tone appearance isn’t the only unusual thing about it. Closer observations by t
