October 25 – Discovery of Saturn’s moon, Iapetus (1671)

Iapetus, the third largest moon of Saturn, was discovered on October 25th, 1671, by Giovanni Cassini, and is a weird old place for a couple of reasons. Firstly, it looks like two different moons, depending on whether you view it from the leading or trailing hemisphere, and secondly because of a pronounced ridge around the equator that gives the moon the appearance of a walnut.
The colour difference is really obvious. It was first suggested by Cassini himself, who noticed that he could only see Iapetus when it was on Saturn’s western side. Various theories have been put forward to explain this two-tone look, with the latest being to invoke thermal segregation , as a result of Iapetus’ very long rotation period (79 Earth days). This might cause one side to be brighter than the other, with Iapetus’ tidally locked rotation being the reason it will always look darker from Earth when on one side of the planet.

Iapetus from the Cassini spacecraft (image credit: NASA)
Iapetus from the Cassini spacecraft (image credit: NASA)

The equatorial ridge has proved equally baffling, with two formation theories currently being pondered: (i) the result of much faster rotation at some point in the past, and (ii) the collapse of a ring.

Cassini view of Iapetus' equatorial ridge (image credit: NASA)
Cassini view of Iapetus’ equatorial ridge (image credit: NASA)

Iapetus was named after one of the Titans of Greek mythology, as per John Herschel’s suggestion that they be given the names of the mythological siblings of Kronos (the Greek equivalent of Saturn). Iapetus is sometimes credited with being a distant ancestor of the human race, and the story goes that each of his four sons (Atlas, Prometheus, Epimetheus and Menoetius) was responsible for a particular fault in mankind. Thanks, Iapetus.

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