Dione, is the fourth-largest moon of Saturn, and 15th largest moon in the Solar System.
Discovery
Dione was discovered by Giovanni Domenico Cassini, an Italian/French astronomer, in 21st March 1684. He found Dione using a large aerial telescope he set up on the grounds of the Paris Observatory.
Naming
It is named after the titan Dione of Greek mythology.
Dione, a female Titan, a daughter of Oceanus and Tethys, and, according to others, of Uranus and Ge, or of Aether and Ge. She was beloved by Zeus, by whom she became the mother of Aphrodite. When Aphrodite was wounded by Diomedes, Dione received her daughter in Olympus, and pronounced the threat respecting the punishment of Diomedes. Dione was present, with other divinities, at the birth of Apollo and Artemis in Delos.
Stats
Diameter: 1,123 km
Semi-major axis: 377,396 km
Orbital Period: 2.74 days
Orbit
Dione takes as long to rotate on its axis as it does to make one orbit of Saturn; and therefore always keeps the same hemisphere pointed to Saturn.
Physical characteristics
Dione is the densest of Saturn's moons with the exception of Titan. It is composed mainly of water ice, but must contain a larger amount of rocky material than Saturn's other ice moons, Tethys and Rhea.
Though somewhat smaller and denser, Dione is otherwise very similar to Rhea. They both have similar albedo features and varied terrain, and both have dissimilar leading and trailing hemispheres.
Dione's leading hemisphere is heavily cratered and is uniformly bright. Its trailing hemisphere, meanwhile, contains an unusual and distinctive surface feature: a network of bright ice cliffs.
The ice cliffs
When the Voyager space probe photographed Dione in 1980, it showed what appeared to be wispy features covering its trailing hemisphere. The origin of these features was mysterious, as all that was known was that the material has a high albedo and is thin enough that it does not obscure the surface features underneath.
One hypothesis was that shortly after its formation Dione was geologically active, and some process such as ice volcanism resurfaced much of its surface, with the streaks forming from eruptions along cracks in Dione's surface that fell back to the surface as snow or ash. Later, after the internal activity and resurfacing ceased, cratering continued primarily on the leading hemisphere and wiped out the streak patterns there.
This theory was proven wrong by the Cassini probe flyby of December 13, 2004, which produced close-up images. These revealed that the 'wisps' were in fact not ice deposits at all, but rather bright ice cliffs created by tectonic fractures (chasmata); Dione has been revealed as a world riven by enormous fractures on its trailing hemisphere.
Atmosphere
NASA's Cassini spacecraft revealed that Dione has a thin atmosphere during a recent close flyby of Dione.
Dione lacks a strong gravitational fields to prevent atmospheric particles from escaping into space. Dione's thin atmosphere exists only because it's constantly being recharged.
Saturn is surrounded by a belt of highly energetic particles, akin to the Van Allen belts around Earth. Dione is located in this belt, and the reason it possesses an atmosphere is that these hot and very fast particles continuously splatter on the moon's surface.
When the particles hit Dione, they cause the moon's surface ice to break apart chemically, releasing molecules that become the moon's atmosphere.
Life?
Unknown but unlikely.
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