Rhea

Rhea, or Saturn V, is the second largest moon of Saturn (and the largest moon without an atmosphere) and the ninth most massive moon in all the solar system. Recent evidence from the Cassini-Huygens mission indicates that Rhea, unique among natural satellites, might have a ring system.

Discovery and naming
Giovanni Domenico Cassini discovered Rhea, along with the moons Tethys, Dione, and Iapetus, in the latter half of the seventeenth century, during the reign of "Sun-King" Louis XIV of France. Cassini named these moons the "Sidera Lodoicea" in honor of the king. Later generations (especially after the French Revolution) would not retain such regal flattery.

Sir John Herschel, son of the astronomer William Herschel, suggested the current names of the seven largest satellites of Saturn, including Rhea. Titan received a generic name, and the other six received names of the Titans of mythology. Rhea was the wife of Kronos or Cronus and mother of Zeus, the eventual king of the gods.

Orbital and rotational characteristics
Rhea is in a relatively circular orbit around Saturn, at a mean distance of 527,040 km. It makes one orbit around Saturn in 4.52 days. Rhea is in tidal lock with Saturn and hence its rotation is synchronous with its orbit.

Physical characteristics
Rhea is a low-density body consisting mainly of water ice.

Surface
Rhea has multiple craters on its surface. These craters, like those of Callisto, are almost flat, and their walls and central mountains are of relatively low altitude.

The trailing hemisphere of Rhea has multiple wisp-like formations that might be mountain ranges.

Interior
The density of Rhea at first led astronomers to believe that Rhea had a rocky core consisting of one-third the total mass of the body, and a mantle of water ice. But recent evidence suggests an undifferentiated interior. Specifically, the moment of inertia for Rhea is 0.3911 ± 0.0045 kg m², which suggests that Rhea's rock and ice are evenly mixed, with compression of the ice from Ice I to Ice II toward the core.

Ring system
NASA announced on March 6, 2008 their tentative hypothesis that Rhea has its own ring system. In its first rendezvous with Rhea, the Cassini orbiter's instruments, including its Magnetospheric Imaging Instrument (MIMI) detected brief interruptions in the normal flow of electrons from Saturn's magnetic field, before the orbiter passed into Rhea's electronic shadow. The same interruptions, faithfully in reverse, occurred on Rhea's far side. These data, and the previously unexplained depletion of high-energy electrons downstream from Rhea that Voyager 1 had found in 1980, strongly suggest that Rhea has rings.

Observation and Exploration
The first spacecraft to explore Rhea was Visiting mission::Voyager 1. The Cassini orbiter has made two close rendezvous with Rhea during its first four years of operation, on November 26, 2005, and August 30, 2007. Mission planners have fixed a date for another rendezvous with Rhea on March 2, 2010, during the two-year extended mission.

Related Links

 * Rhea by Wikipedia

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