Iapetus, the third largest moon of Saturn, was discovered by Giovanni Cassini, October 25, 1671. It is unique for several reasons. First, it has a dark region, known as Cassini Regio, which is contrasted with the brightness from ice on the rest of the moon. Ice covers most of the moon.
Second, Iapetus has an inclined orbit, more than any of the regular, i.e. largest, moons. The rings of Saturn would be clearly visible because of its inclined orbit.
Third, it has an equatorial ridge that runs through the center of Cassini Regio. Mountains in this ridge rise over 12 miles of the surrounding plains, making them among the tallest mountains in the solar system. Mons Olympus on Mars is the tallest in the solar system.
The source of the dark material and why the equatorial ridge exists only in Cassini Regio are unknown.
The spacecraft Cassini passed by at a range of 1,227 km (762 miles) on September 10, 2007.
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