Iapetus was discovered in 1671 by Giovanni Cassini, but he could see the moon only when it was on the west side of Saturn. He kept trying to observe Iapetus when it was on Saturn’s east side, but was unsuccessful for over 30 years. Using a better telescope in 1705, he was finally able to see Iapetus to the east of Saturn, much dimmer than when it was on the west. He theorized (correctly) that one hemisphere of Iapetus always faces Saturn, and that one of its hemispheres is much darker than the other.
We now know this is the case with several of Saturn’s moons, but the effect is most pronounced on Iapetus.
Iapetus has an equatorial ridge with some of the highest mountains in the Solar System. The average height of this ridge is about 42,000 feet above the surrounding plains, with some peaks rising as much as 65,000 feet.
You can view these links to learn more about Iapetus and the other moons of Saturn:
