25 October: Iapetus

On this date in 1671 Giovanni Cassini discovered Iapetus, one of Saturn's moons. 10 things you might not know about Iapetus:

1. Iapetus is the third-largest natural satellite of Saturn, and the eleventh-largest in the Solar System.
2. Cassini called it one of the four Sidera Lodoicea (Stars of Louis) after King Louis XIV (the other three being Tethys, Dione and Rhea). Other astronomers called it Saturn V, and it became Saturn VIII after additional moons were discovered. It was John Herschel who suggested that the moons of Saturn be named after the mythical brothers and sisters of Kronus (Kronus being the Greek equivalent of the Roman god Saturn). Iapetus is the father of Atlas and Prometheus. As the father of Prometheus, the ancient Greeks regarded Iapetus as the father of the human race.
3. Iapetus has a mean radius of 457 miles (736 kilometres) - that's 8.7x smaller than Earth, and isn't much more dense than liquid Water. Therefore it is believed to be three quarters ice and one quarter rock.
4. Iapetus orbits at 2,213,000 miles (3,561,000 kilometres) from Saturn, and is tidally locked with the planet, that is, the moon always presents the same face toward Saturn.
5. Thanks to its distant, inclined orbit, Iapetus is the only large moon from which you could get a good view of Saturn's rings.
6. One hemisphere of Iapetus is much darker than the other. The difference was noticed by Cassini back in 1671. He noted that he could only see Iapetus when it was on the west side of Saturn. He correctly concluded from this that Iapetus had one darker side was tidally locked with Saturn. Why this should be isn't known but one current theory is that the moon's slow rotation leads to the dark areas being heated by the sun and volatile substances move to the colder areas. This feature has led to Iapetus being known as the yin and yang of the Saturn moons.
7. Another feature is a giant ridge around about three quarters of the middle which make the moon look a little like a giant Walnut. The ridge consists of mountains which are 6-miles (10-km) high. The jury is out on why this should be, too, but one theory is that the ridge consists of debris from a dead moon.
8. Geological features on Iapetus get their names from the French epic poem The Song of Roland. Two of the craters are called Charlemagne and Baligant, and the northern bright region is called Roncevaux Terra. The one exception is Cassini Regio, the dark region, which is named after Giovanni Cassini.
9. Iapetus is in resonance with Saturn's largest moon, Titan. As the two moons pass each other in orbit, they speed up and slow down as they pass each other in a complex set of variations.
10. In Arthur C. Clarke's novel 2001: A Space Odyssey, the monolith is located on Iapetus.

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