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| magnitude = 10.2 | single_temperature = | note = no }} Tethys or Saturn III is a mid-sized moon of Saturn about across. It was discovered by G. D. Cassini in 1684 and is named after the titan Tethys of Greek mythology. Tethys has a low density of 0.98 g/cm3, the lowest of all the major moons in the Solar System, indicating that it is made of water ice with just a small fraction of rock. This is confirmed by the spectroscopy of its surface, which identified water ice as the dominant surface material. A small amount of an unidentified dark material is present as well. The surface of Tethys is very bright, being the second-brightest of the moons of Saturn after Enceladus, and neutral in color. Tethys is heavily cratered and cut by a number of large faults/graben. The largest impact crater, Odysseus, is about 400 km in diameter, whereas the largest graben, Ithaca Chasma, is about 100 km wide and more than 2000 km long. These two largest surface features may be related. A small part of the surface is covered by smooth plains that may be cryovolcanic in origin. Like all other regular moons of Saturn, Tethys formed from the Saturnian sub-nebula—a disk of gas and dust that surrounded Saturn soon after its formation. Tethys has been approached by several space probes including ''Pioneer 11'' (1979), ''Voyager 1'' (1980), ''Voyager 2'' (1981), and multiple times by ''Cassini'' since 2004. == Discovery and naming == Tethys was discovered by Giovanni Domenico Cassini in 1684 together with Dione, another moon of Saturn. He had also discovered two moons, Rhea and Iapetus earlier, in 1671–72. Cassini observed all of these moons using a large aerial telescope he set up on the grounds of the Paris Observatory. Cassini named the four new moons as ''Sidera Lodoicea'' ("the stars of Louis") to honour king Louis XIV of France. By the end of the seventeenth century, astronomers fell into the habit of referring to them and Titan as ''Saturn I'' through ''Saturn V'' (Tethys, Dione, Rhea, Titan, Iapetus). Once Mimas and Enceladus were discovered in 1789, the numbering scheme was extended to ''Saturn VII'' by bumping the older five moons up two slots. The discovery of Hyperion in 1848 changed the numbers one last time, bumping Iapetus up to ''Saturn VIII''. Henceforth, the numbering scheme would remain fixed. The modern names of all seven satellites of Saturn come from John Herschel (son of William Herschel, discoverer of Mimas and Enceladus). In his 1847 publication ''Results of Astronomical Observations made at the Cape of Good Hope'', he suggested the names of the Titans, sisters and brothers of Kronos (the Greek analogue of Saturn), be used. Tethys is named after the titaness Tethys of Greek mythology. It is also designated Saturn III or S III Tethys. The correct adjectival form of the moon's name is ''Tethyan'', although other forms are also used. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Tethys (moon)」の詳細全文を読む スポンサード リンク
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