Iapetus

Iapetus is Saturn's outermost large moon, orbiting Saturn every 79.3 days at an average distance of 2,213,000 miles (3,561,000 km). Iapetus orbits Saturn about three times farther out than Titan, and with a high orbital inclination (about 16 degrees) to Saturn's equatorial plane.

Iapetus is about 1450 km in diameter (900 mi), and although it is about 40% the size of our Moon, it is only about 3% of the Moon's mass. Therefore, Iapetus is a lightweight moon with low density.

Iapetus displays a very strange surface. One side of Iapetus is as dark as fresh asphalt, and the other side is as bright as snow. Iapetus' rotation is tidally locked with Saturn, and keeps the same face toward the ringed planet. This means that Iapetus also keeps the same face "looking forward" as it revolves around Saturn in its orbit. The dark hemisphere of Iapetus is the forward-facing side, and the bright hemisphere is the backward-looking side. This has led to speculation that the dark material on Iapetus's surface has been "splattered on", like paint, from an external source.

Another unusual feature of Iapetus is a very prominent ridge that runs nearly halfway around the moon's equator. This ridge makes Iapetus appear somewhat like a walnut. It is not clear how the ridge formed.