Tethys (pronounced /ˈtiːθɨs/, /ˈtɛθɨs/, or as Greek Τηθύς) is a moon of Saturn that was discovered by Giovanni Domenico Cassini in 1684.
Tethys is named after the titan Tethys of Greek mythology. It is also designated Saturn III or S III Tethys.
Cassini named the four moons he discovered in 1671–1684 (Tethys, Dione, Rhea and Iapetus) Sidera Lodoicea ("the stars of Louis") to honour king Louis XIV. Cassini found Tethys using a large aerial telescope he set up on the grounds of the Paris Observatory. 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 names of all seven satellites of Saturn then known 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, wherein he suggested the names of the Titans, sisters and brothers of Kronos (the Greek analogue of Saturn), be used.
The correct adjectival form of the moon's name is Tethyan, although other forms are also used.
At 1066 km in diameter, Tethys is the 16th largest moon in the Solar System, and is more massive than all known moons smaller than itself combined. Tethys is an icy body similar in nature to Dione and Rhea. The density of Tethys is 0.97 g/cm³, indicating that it is composed almost entirely of water-ice. The Tethyan surface is heavily cratered and contains numerous cracks caused by faults in the ice. Its surface is one of the most reflective (at visual wavelengths) in the solar system, with a visual albedo of 1.229. This very high albedo is the result of the sandblasting of particles from Saturn's E-ring, a faint ring composed of small, water-ice particles generated by Enceladus's south polar geysers.
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