Tethys () is the fifth-largest moon of Saturn, measuring about 1,060 km (660 mi) across. It was discovered by Giovanni Domenico 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 was confirmed by the spectroscopy of its surface, which identified water ice as the dominant surface material. A further, smaller amount of an unidentified dark material is present as well. The surface of Tethys is very bright, 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 and trench-like 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 2,000 km long; the two surface features may be related. A small part of the surface is covered by smooth plains that may be cryovolcanic in origin. Like the 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 and observed by several space probes, including Pioneer 11 (1979), Voyager 1 (1980) and Voyager 2 (1981), with Cassini–Huygens observing the moon the most, and in greatest detail, during its extensive mission to the Saturnian system (2004–2017).
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 by William Herschel, 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. It is also designated Saturn III or S III Tethys. The name Tethys has two customary pronunciations, with either a 'long' or a 'short' e: or . The conventional adjectival form of the name is Tethyan, again with either a long or a short e. Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of the dwarf planet symbols, proposed a Greek theta combined with the crook of the Saturn symbol as the symbol of Tethys (). This symbol is not widely used.
Orbit Tethys orbits Saturn at a distance of about 295,000 km (about 4.4 Saturn's radii) from the center of the planet. Its orbital eccentricity is negligible, and its orbital inclination is about 1°. Tethys is locked in an inclination resonance with Mimas; however, due to the low gravity of the respective bodies, this interaction does not cause any noticeable orbital eccentricity or tidal heating. The Tethyan orbit lies deep inside the magnetosphere of Saturn, so the plasma co-rotating with the planet strikes the trailing hemisphere of the moon. Tethys is also subject to constant bombardment by the energetic particles (electrons and ions) present in the magnetosphere.
Trojans Tethys has two co-orbital moons, Telesto and Calypso, orbiting near Tethys's Lagrange points L4 (60° ahead) and L5 (60° behind) respectively.
Physical characteristics
Tethys is the 16th-largest moon in the Solar System, with a radius of 531 km. Its mass is about 6.17×1020 kg (0.000103 Earth mass), which is less than 1% of the Moon. Despite its relatively small mass, it is more massive than all known moons smaller than itself combined. The density of Tethys is 0.98 g/cm3, indicating that it is composed almost entirely of water-ice. It is also the fifth-largest of Saturn's moons. It is not known whether Tethys is differentiated into a rocky core and ice mantle. However, if it is differentiated, the radius of the core would not exceed 145 km, and its mass would only be less than 6% of the total mass of the moon. Due to the action of tidal and rotational forces, Tethys has the shape of triaxial ellipsoid. The dimensions of this ellipsoid are consistent with it having a homogeneous interior. The existence of a subsurface ocean—a layer of liquid salt water in the interior of Tethys—is considered unlikely. The surface of Tethys 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. The radar albedo of the Tethyan surface is also very high. The leading hemisphere of Tethys is brighter by 10–15% than the trailing one. The high albedo indicates that the surface of Tethys is composed of almost pure water ice with only a small amount of darker materials. The visible spectrum of Tethys is flat and featureless, whereas in the near-infrared strong water ice absorption bands at 1.25, 1.5, 2.0 and 3.0 μm wavelengths are visible. No compound other than crystalline water ice has been unambiguously identified on Tethys. (Possible constituents include organics, ammonia and carbon dioxide.) The dark material in the ice has the same spectral properties as seen on the surfaces of the dark Saturnian moons—Iapetus and Hyperion. The most probable candidate is nanophase iron or hematite. Measurements of the thermal emission as well as radar observations by the Cassini spacecraft show that the icy regolith on the surface of Tethys is structurally complex and has a large porosity exceeding 95%.
Surface features
Color patterns
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