23 December: Saturn's moon Rhea

On this date in 1672, astronomer Giovanni Cassini discovered Saturn's moon Rhea. 10 things you might not know about it:

1. Rhea is the second largest moon of Saturn, with a mean radius of 475 miles (764 kilometres). The largest of Saturn’s moons is Titan, which is about three times bigger.
2. It’s 327,500 miles (527,000 kilometres) away from Saturn and takes four and a half days to orbit the planet.
3. When Cassini discovered it, along with three other moons, (Tethys, Dione and Iapetus) he named them all Sidera Lodoicea (the stars of Louis) to honour King Louis XIV. It wasn’t until 1847 that John Herschel suggested using the names of Titans from Greek mythology for the moons of Saturn. In Greek mythology, Rhea was the wife of Kronos, the Greek equivalent of the Roman god Saturn. Rhea was called the mother of the gods because she gave birth to several Greek gods, including Zeus (whose Roman equivalent is Jupiter).
4. It has a low density for its size which suggests it is made mostly of ice mixed with some rock. As NASA puts it, “a dirty snowball”.
5. The temperature on Rhea is 99 K (−174 °C) in direct sunlight and between 73 K (−200 °C) and 53 K (−220 °C) in the shade.
6. In 2010 it was discovered that Rhea’s atmosphere contained Oxygen – the first world other than Earth where this had been found. It is a very thin atmosphere, however, actually an exosphere.
7. There is evidence, too, that Rhea may be the first moon discovered to have a ring system. This is based on lines of material detected around Rhea’s equator which was thought to have been caused by debris from the rings falling out of orbit. The Cassini probe, however, couldn’t find any ring material.
8. Some of Rhea’s surface is heavily cratered with large craters than 25 miles (40 kilometres) across while other areas have smaller ones. There are also canyons which look like bright wispy areas, but are actually hundreds of metres high. They look bright because they are caused by subsidence causing darker material to fall off and expose fresh ice underneath. This is evidence for past tectonic activity on Rhea.
9. The moons of Saturn are thought to have formed through co-accretion, where discs of material surrounding the planets gradually coalesced into moons. There is a theory, however, that early giant moons might have collided at some point and the collision formed Rhea, along with Iapetus and Titan.
10. In the film Independence Day: Resurgence, humanity builds a defence outpost on Rhea.

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