Iapetus (astronomy)

Iapetus (astronomy), large moon of the planet Saturn. Iapetus orbits Saturn at a distance of about 3.56 million km (about 2.21 million mi), making it the most distant large moon of Saturn. Iapetus completes an orbit about once every 79.3 Earth days. It rotates once on its axis for every orbit of Saturn that it makes and so keeps the same hemisphere facing the planet at all times. The moon’s orbit is tilted about 15° to Saturn’s equator and is slightly elliptical, or oval-shaped.

Iapetus is thought to be made almost entirely of water ice with some rock. The moon has an irregular, somewhat oval shape. Its estimated widest diameter is about 1,468 km (912 mi) across, less than half the diameter of Earth’s moon. One of the most striking features of Iapetus is a ridge or chain of mountains that runs almost the entire length of its bulging equator, giving the moon a walnutlike appearance. The mountainous ridge was discovered by the Cassini space probe in 2004 and is about 1,300 km (800 mi) long, 20 km (12 mi) wide, and 13 to 20 km (8 to 12 mi) high, reaching twice the height of Mt. Everest.

In a paper published in 2007, a group of planetary scientists theorized that Iapetus’s odd shape dates back to the moon’s early history about 4.5 billion years ago. Short-lived radioactive isotopes in the material that formed the moon provided internal heat at a time when Iapetus spun much faster on its axis. The high rotation rate caused the moon to bulge out at its equator in an oblate shape. Over time, the gravitational pull of Saturn slowed down the moon’s rotation and the internal radioactive heat died away. Iapetus’s outer crust froze at about the time the moon’s rotation rate fell, preserving a bulging shape at the equator. The surface area of the cold moon also shrank, a process that may have created the ridge around the equator as excess material piled up.

Astronomers have known for centuries that the moon was unusual because of its color. Iapetus’s leading hemisphere—the side of the moon that faces in the direction of the moon’s orbit—is covered with very dark reddish material. Its trailing hemisphere is ten times brighter than the leading hemisphere. The Voyager space probes in 1980 and 1981, and the Cassini space probe beginning in 2004, have provided detailed views. The Voyager probes did not find any bright craters in the dark hemisphere. The material is either so thick that no crater has blasted through it, or it is continually renewed, covering up any white ice excavated by crashing asteroids. The dark reddish material is similar to, but not identical to, dark material on Saturn’s moons Phoebe and Hyperion. The origin of the dark material on Iapetus is not known. It may have erupted from the interior of the moon or have been deposited from a source in space.

Iapetus has four large craters called Othon, Ogier, Charlemagne, and Basan in the trailing hemisphere. They each measure about 100 km (about 60 mi) across. The dark, heavily cratered region on the leading hemisphere is called Cassini Regio, after the moon’s discoverer. Iapetus has no atmosphere.

Iapetus was discovered in 1671 by Italian astronomer Giovanni Cassini. The moon was named after a character in Greek mythology who was a Titan and the son of Uranus and Gaea. Individual features on Iapetus are named after persons and places from British author Dorothy Sayers’s translation of the French medieval epic The Song of Roland.