Rhea is the second largest moon of Saturn and the nineth largest moon in the Solar System.
Discovery
It was discovered in 1672 by Giovanni Domenico Cassini. Cassini named the four moons he discovered (Tethys, Dione, Rhea and Iapetus) Sidera Lodoicea (the stars of Louis) to honor King Louis XIV.
Naming
Rhea is named after the Titan Rhea of Greek mythology.
Rhea was the Titaness daughter of Uranus, the sky, and Gaia, the earth, in Greek mythology. She was known as "the mother of gods". In earlier traditions, she was strongly associated with Gaia and Cybele, the Great Goddess, and was later seen by the classical Greeks as the mother of the Olympian gods and goddesses, though never dwelling permanently among them on Mount Olympus.
Stats
Diameter: 1,527 km
Semi-major axis: 527,108 km
Orbital Period: 4.52 days
Orbit
Rhea 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.
Possible ring system?
On March 6, 2008, NASA announced that Rhea may have a tenuous ring system. This would mark the first discovery of rings about a moon. The rings' existence was inferred by observed changes in the flow of electrons trapped by Saturn's magnetic field as Cassini passed by Rhea.
Dust and debris could extend out to Rhea's Hill sphere, but were thought to be denser nearer the moon, with three narrow rings of higher density. The case for a ring was strengthened by the subsequent finding of the presence of a set of small ultraviolet-bright spots distributed along Rhea's equator (interpreted as the impact points of deorbiting ring material).
However, when Cassini made targeted observations of the putative ring plane from several angles, no evidence of ring material was found, suggesting that another explanation for the earlier observations is needed.
Surface features
Rhea has a rather typical heavily cratered surface, with the exceptions of a few large fractures (wispy terrain) on the trailing hemisphere (the side facing away from the direction of motion along Rhea's orbit) and a very faint "line" of material at Rhea's equator that may have been deposited by material deorbiting from its rings.
Its surface can be divided into two geologically different areas based on crater density. The first area contains craters which are larger than 40 km in diameter. The second area, in parts of the polar and equatorial regions, has only craters under that size. This suggests that a major resurfacing event occurred some time during its formation.
Atmosphere
On November 27, 2010, NASA announced the discovery of a tenuous atmosphere—exosphere. It consists of oxygen and carbon dioxide in proportion of roughly 5 to 2.
The surface density of the exosphere is from 105 to 106 molecules in a cubic centimeter depending on local temperature. The main source of oxygen is radiolysis of water ice at the surface by ions supplied by the magnetosphere of Saturn. The source of the carbon dioxide is less clear, but it may be related to oxidation of the organics present in ice or to outgassing of the moon's interior.
Life?
Models suggest that Rhea could be capable of sustaining an internal liquid water ocean through heating by radioactive decay.
If there are thermal vents on the floor of this ocean as there are on Earth, it is remotely possible that similar organisms to those which live around the vents on Earth could also survive there.
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