Rhea

Rhea is one of Saturn’s moons. It was discovered in 1672 by the Italian astronomer Giovanni Domenico Cassini.

Rhea is the second largest of Saturn’s moons and the ninth-largest moon in the Solar System. Saturn has 62 moons with confirmed orbits, 53 of which have names. 24 of these are regular satellites that orbit Saturn with only small inclinations to the planet’s equatorial plane. 37 of the remaining 38 are irregular satellites that orbit at high inclinations and a much greater distance from Saturn. These are probably minor planets captured by Saturn’s gravity or debris from their break-up. The final irregular satellite is unclear.

Saturn is the Roman god of agriculture and harvest. The first of the plant’s moons to be discovered were named after his brothers and sisters. When all of these names had been used, other ones were taken from additional characters in Greek or Roman mythology or from the mythologies of other cultures. The irregular satellites are named after figures from Inuit, Gallic and Norse mythology. Rhea is named after the Titaness Rhea from Greek mythology. She was the daughter of Uranus and Gaia and known as the “mother of the gods”. The name was first suggested 1847 by William Herschel’s son John. Rhea is also designated Saturn V as the fifth moon going outward from the planet.

Rhea is predominantly a water-ice body. There is a large amount of ammonia in the ice. The presence of a rock core in Rhea was discounted after the Cassini mission in 2004. Instead, astronomers proposed that Rhea condensed from a concentric family of gas rings that had been cast off from a proto-Saturnian cloud. Rhea’s surface is old and heavily cratered, like Jupiter’s moon, Callisto. Its brightness varies with its position relative to Saturn.

Rhea has two large basins caused by impacts on the anti-Cronian (facing away from Saturn) hemisphere with diameters between 400 and 500km. This hemisphere shows tectonic fractures similar to those on Dione that have exposures of bright ice on escarpment slopes. Impact craters in Rhea’s medium-latitude regions have diameters larger than 40 km, while those in polar or equatorial regions are usually smaller. A tenuous oxygen and carbon dioxide atmosphere around Rhea was discovered by NASA in 2010.

As with most of Saturn’s other moons, Rhea rotates synchronously around the planet and keeps the same hemisphere towards Saturn and the same hemisphere facing forward during the orbit. Rhea takes 4.52 Earth days to rotate around Saturn and to complete one spin on its own axis. Along with Saturn and its other satellites, it takes 29.4 Earth years to orbit the sun.

Spacecraft missions Voyager 1 and 2 in 1980 and 1981 provided the first images of Rhea. The Cassini spacecraft mission to Saturn sent back detailed images of the moon. The closest Cassini flyby was at 69 km in January 2011. Image processing was able to produce details down to a few metres on size.