Dione

Dione is one of Saturn’s moons. It was discovered in 1684 by the Italian astronomer Giovanni Domenico Cassini.

Dione is the 12th largest and the densest of Saturn’s moons. 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 its 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 the planet’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 planet’s moons to be discovered were named after his brothers and sisters. Once 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. Dione is named after the mother of Aphrodite (Venus) in Greek mythology. She was a Titaness and a female form of Zeus (Jupiter). The name was first suggested 1847 by William Herschel’s son John.

The composition of Dione is mainly water-ice with a large amount of silicate rock at its core. The surface consists of heavily cratered areas and plains that are moderately or lightly cratered. Some of the craters are over 100 km in diameter, although craters in the plains are about 30 km diameter. There is a large basin in the southern polar region that is named Evander and is 350 km wide.

There are ice cliffs created by tectonic fractures on its trailing hemisphere (the hemisphere that faces away from Saturn). These cliffs were once called “wispy features”. Most of the heavily cratered areas on Dione are on its trailing hemisphere. Detailed imaging has revealed a variety of tectonic features on the moon, such as horst, grabens, scarps and ridges that indicate extensional, compressional and shear stresses.

The presence of charged particles around Dione has led to suggestions from astronomers that the moon may have a thin atmosphere that includes oxygen.

Dione orbits synchronously around Saturn, meaning that the same hemisphere always points at the planet. The orbit around Saturn takes 2.73 Earth days, the same period as it own axial rotation. Along with Saturn and its other moons, Dione takes 29.4 years to orbit the sun.

Spacecraft missions Pioneer 11 in 1979, Voyager 1 in 1980 and Voyager 2 in 1981 took the first photographs of Dione. The Cassini spacecraft mission to Saturn provided detailed images of the moon. The closest Cassini flyby was at 99 km above Dione.