Iapetus is the third-largest natural satellite of Saturn, eleventh-largest in the Solar System, and the largest body in the Solar System known not to be in hydrostatic equilibrium. Iapetus is best known for its dramatic 'two-tone' coloration. Discoveries by the Cassini mission in 2007 revealed several other unusual features, such as a massive equatorial ridge running three-quarters of the way around the moon. The low density of Iapetus indicates that it is mostly composed of ice, with only a small (~20%) amount of rocky materials. Unlike most of the large moons, its overall shape is neither spherical nor ellipsoid, but has a bulging waistline and squashed poles; also, its unique equatorial ridge (see below) is so high that it visibly distorts Iapetus's shape even when viewed from a distance. These features often lead it to be characterized as walnut-shaped.
Iapetus (moon) - Wikipedia, the free encyclopedia