Iapetus (moon)

Iapetus () is the outermost of Saturn's large moons. With an estimated diameter of 1,469 km (913 mi), it is the third-largest moon of Saturn and the eleventh-largest in the Solar System. Named after the Titan Iapetus from Greek mythology, the moon was discovered in 1671 by Giovanni Domenico Cassini.

A relatively low-density body composed mostly of ice, Iapetus is home to several distinctive and unusual features, such as a striking difference in coloration between its dark leading hemisphere and its bright trailing hemisphere, as well as a massive equatorial ridge that runs three-quarters of the way around the moon.

History

Discovery

Iapetus was discovered by Giovanni Domenico Cassini, an Italian-born French astronomer, in October 1671. It was the first moon that Cassini discovered; the second moon of Saturn to be discovered after Christiaan Huygens spotted Titan 16 years prior in 1655; and the sixth extraterrestrial moon to be discovered in human history.

Cassini discovered Iapetus when the moon was on the western side of Saturn, but when he tried viewing it on the eastern side some months later, he was unsuccessful. This was also the case the following year, when he was again able to observe it on the western side, but not the eastern side. Cassini finally observed Iapetus on the eastern side in 1705 with the help of an improved telescope, finding it two magnitudes dimmer on that side.

Cassini correctly surmised that Iapetus has a bright hemisphere and a dark hemisphere, and that it is tidally locked, always keeping the same face towards Saturn.