Iapetus is the Saturnian moon with a prominent equatorial ridge that gives it a walnut-like appearance. The ridge follows much of the moon's equator and dramatically distorts its otherwise rounded shape.
Cassini spacecraft images revealed the ridge clearly on December 31, 2004, although Voyager 2 images had provided earlier hints of unusual equatorial peaks. Some sections rise more than 20 kilometers above the surrounding terrain, making the structure one of the tallest mountain systems known in the Solar System.
The ridge is heavily cratered, suggesting that it is ancient. Its origin remains unresolved: proposed explanations include a fossil equatorial bulge from a faster-spinning young Iapetus, material accumulated from an ancient ring, and debris from an impact-generated satellite.
Iapetus is also famous for its striking two-tone surface. Its leading hemisphere is darkened by material associated with Phoebe, while much of the rest of the moon is bright ice. The walnut shape and dramatic color contrast make Iapetus one of Saturn's most distinctive moons.