Tethys (moon)

Lore & Background

Tethys was discovered by Giovanni Domenico Cassini in 1684 together with Dione, using a large aerial telescope at the Paris Observatory. Cassini originally named the four new moons Sidera Lodoicea to honor King Louis XIV of France. The modern name Tethys was suggested by John Herschel in 1847, following the convention of naming Saturn's moons after Titans. Tethys is also designated Saturn III.

The moon's surface is very bright, the second-brightest of Saturn's moons after Enceladus, and neutral in color. It is heavily cratered and cut by large faults and trench-like graben. The largest impact crater, Odysseus, is about 400 km in diameter, while the largest graben, Ithaca Chasma, is about 100 km wide and more than 2,000 km long. A small part of the surface is covered by smooth plains that may be cryovolcanic in origin. Tethys has two co-orbital moons, Telesto and Calypso, which orbit near its Lagrange points.

Tethys formed from the Saturnian sub-nebula—a disk of gas and dust that surrounded Saturn soon after its formation. It has been observed by several space probes, including Pioneer 11 (1979), Voyager 1 (1980), Voyager 2 (1981), and most extensively by Cassini–Huygens (2004–2017).

Reader's Guide

Tethys is notable for its extremely low density, the lowest among all major moons in the Solar System, which confirms it is composed almost entirely of water ice with only a small fraction of rock. Its surface is one of the most reflective at visual wavelengths, with a visual albedo of 1.229, largely due to sandblasting by particles from Saturn's E-ring. The moon's surface features include the massive Odysseus impact basin and the extensive Ithaca Chasma graben, which may be related. Tethys also exhibits a distinctive color pattern: its trailing hemisphere is redder and darker, while the leading hemisphere has a dark bluish equatorial band that gives it a 'Pac-Man'-like appearance in mid-infrared temperature maps. The moon's interior is likely homogeneous, and a subsurface ocean is considered unlikely. Tethys's legacy includes its role in the numbering scheme of Saturn's moons and its extensive study by the Cassini mission, which provided the most detailed observations of this icy world.

Did You Know?

• Tethys has the lowest density of all the major moons in the Solar System, at 0.98 g/cm³.
• The largest impact crater on Tethys, Odysseus, is about 400 km in diameter.
• The surface of Tethys is the second-brightest of Saturn's moons, after Enceladus.