Iapetus ("eye AP i tus" ) is the seventeenth of Saturn's known satellites and the third largest:
orbit: 3,561,300 km from Saturn diameter: 1460 km mass: 1.88e21 kg
In Greek mythology Iapetus was a Titan, the son of Uranus, the father of Prometheus and Atlas and an ancestor of the human race.
Discovered by Cassini in 1671.
With a density of only 1.1, Iapetus must be composed almost entirely of water ice.
The leading and trailing hemispheres of Iapetus are radically different. The albedo of the leading hemisphere is between .03 and .05, as dark as lampblack, whereas the trailing hemisphere's albedo is .5, almost as bright as Europa. This difference is so striking that Cassini noted that he could see Iapetus only on one side of Saturn and not on the other.
One explanation of this is that the leading hemisphere is dusted with a coating of material knocked off of Phoebe. However, the color of the leading half of Iapetus and that of Phoebe don't quite match. Another possibility is that some active process within Iapetus is responsible. The puzzle is compounded by the fact that the dividing line between the two sides is inexplicably sharp.
All of Saturn's moons except for Iapetus and Phoebe are very nearly in the plane of Saturn's equator. Iapetus is inclined almost 15 degrees.