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Iapetus is Saturn's third-largest moon, after Titan, which is larger than Luna, and Rhea, which is slightly larger than Iapetus. Iapetus is slightly smaller than our own Moon, with a diameter of roughly 1,500 km, compared to the Moon's 1,737 km.
Iapetus has a range of physical peculiarities that make it somewhat more interesting than any typical rocky moon. The first, and most striking, is a dramatic two-tone coloration of black and white, which has given it the nickname "the Yin-Yang moon." Like many moons, Iapetus is tidally locked to its planet, meaning the same side always faces it. In the direction of its motion around Saturn, Iapetus is as dark as charcoal, on its trailing side, bright as snow. This created quite a bit of confusion for the early astronomers who observed it, who saw the moon blinking in and out of existence as its white and dark sides oscillated from our point of view.
But Iapetus' two-tone appearance isn't the only unusual thing about it. Closer observations by the Cassini orbiter have found it to possess a huge equatorial ridge that follows its equator almost perfectly. With an average height of 13 km, it extends for more than 200 km, with peaks in parts at 20 km. In light of the fact that the Moon's tallest peaks are only about 7 km, and only exist in isolated spires or small ridges like on Earth, Iapetus' planet-wide equatorial ridge is quite bizarre. Various theories exist for its origin, none confirmed. Some conspiracy theorists have even taken it for evidence that Iapetus is artificial!
Iapetus was observed closely by the space probe Cassini on 10 Sep 2007, flying about 1,640 km (1,000 mi) from its surface and taking spectacular high-resolution images of features such as the equatorial ridge and the interface between its light and dark regions.
Iapetus is one of Saturn's outer satellites, which is unusual because it is so large. Other large satellites are located much more closely to the planet. Iapetus also has a highly eccentric orbit, traveling far above and below Saturn's rings as it makes its way around the planet. Because of this, Iapetus is the only major moon from which the rings of Saturn would be clearly visible. The other moons orbit along with the ring particles, meaning that the rings are seen edge-on and are not very pronounced. Because of its great view, we can safely say that Iapetus will become prime real estate sometime in the late 21st or early 22nd century. Start saving!