Tethys or Saturn III is the tenth innermost confirmed moon of Saturn and the fourth (or perhaps fifth) moon of Saturn to be discovered. It is also one of the seven moons of Saturn of dwarf planet size.
Discovery and naming
Giovanni Domenico Cassini discovered Tethys, along with the moons Dione, Rhea, 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. Tethys was the wife of Ocean and the mother of all rivers.
Tethys actually lies within Saturn's E ring and might therefore be subject to a continuous shower of particles from this ring. This, some authorities suggest, might also be responsible for Tethys' relatively high albedo.
Tethys is in tidal lock with Saturn.
Tethys has a density of 960 kg/m³, less than that of water. For that reason, astronomers now believe that Tethys is composed entirely of water ice. Furthermore, its physical dimensions are most consistent with a homogeneous interior.
Astronomers currently suggest that the surface of Tethys is geologically old, on account of its many impact craters. But the surface might have been re-formed in the past. The craters of Tethys also have bright, reflective floors.
One of the two most prominent features of the surface of Tethys is the great chasm called Ithaca Chasma. This extends for three-fourths of the circumference of Tethys and measures 65 km wide and several kilometers deep. Most observers suggest that Tethys was once fluid and later froze, with the crust freezing before the interior.
The other feature is the Odysseus basin, a 400-kilometer-wide impact crater that dominates Tethys' western hemisphere. Today the floor of this basin conforms to the spherical shape of Tethys. This basin has no central mountain, and its walls are relatively low.
Problems for uniformitarian theories posed by Tethys
Tethys is in a circular orbit and therefore could not be subject to tidal heating. Yet if it had been a solid object at the time of the impact that made the Odysseus basin, it would have shattered. To explain this paradox, Chen and Nimmo suggest that Tethys once had a differentiated interior of "a silicate core, a liquid water ocean, a viscous icy mantle and a rigid icy crust." This would require an eccentricity in the distant past as high as 0.02. Chen and Nimmo also believe that Tethys was once in a 3:2 orbital resonance with Dione, but they tend to doubt that this would have had a significant effect.
The obvious problem with this theory is how Tethys could have changed its orbit to one that was nearly circular.
Another problem is what sort of impact could have left the Odysseus basin without itself sending Tethys into a more eccentric orbit, and what became of the impactor. Arnett suggests that the two Trojan moons that share Tethys's orbit might be remnants of that or another collision.
Observation and Exploration
Visiting mission::Voyager 1 and Visiting mission::Voyager 2 both photographed Tethys from a distance, though Voyager 2 made a much closer rendezvous. However, the orbiter of the Visiting mission::Cassini-Huygens mission made an even closer rendezvous (at a distance of 1500 km) on September 23, 2005. No further rendezvous are planned at present.
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