Saturn: Moons: Tethys
Tethys [pronounced TEE-this; or TE-this; adjective: Tethyan] is a small moon 1,066 kilometers (662 miles) in diameter that orbits 294,660 kilometers (183,100 miles) from Saturn. This cold, airless, and heavily scarred body is very similar to sister moons Dione and Rhea except that Tethys is not as heavily cratered as the other two. This may be because its proximity to Saturn causes more tidal warming than on the other two moons, and that warming kept Tethys partially molten longer, erasing or dulling more of the early terrain.
Odysseus Crater
Tethys has two overpowering features, a giant impact crater and a great valley. Odysseus Crater (named for a Greek warrior king in Homer's two great works, The Iliad and The Odyssey) dominates the Tethyan western hemisphere. Odysseus Crater is 400 kilometers in-diameter (almost 250 miles). That diameter is nearly two-fifths of Tethys itself. Such an impact could have shattered a solid body, which suggests that the internal composition of Tethys was still partially molten. The crater's rim and central peak have largely collapsed, leaving a shallow crater, and this also suggests a terrain that was elastic enough to change shape. The subdued features of Odysseus are in contrast to many steep cliffs elsewhere. This again suggests that the ancient terrain that was still elastic enough to change shape.
Ithaca Chasma
The second major feature, a valley called Ithaca Chasma (named for the country ruled by Odysseus), runs roughly from the Tethyan north pole to its south pole. It is 100 kilometers wide, 3 to 5 kilometers deep, and extends 2000 kilometers (62, 2 to3, and 1200 miles, respectively). Ithaca Chasma may have been caused by expansion of internal liquid water as it froze into ice after the surface had already frozen. An alternate theory is that the impact that created the Odysseus Crater also generated forces that created Ithaca Chasma, especially since the chasm is on the opposite side of Tethys from the Odysseus Crater. The chasm and surrounding area are heavily cratered, indicating that it was formed long ago.
Tethys' density is 0.97 times that of liquid water, which suggests that Tethys is composed almost entirely of water ice plus a small amount of rock.
As with all but two of the major Saturnian moons, Tethys is tidally locked in phase with its parent -- one side always faces toward Saturn. Likewise, Tethys has gravitationally locked two smaller moons into its own subsystem -- Telesto and Calypso. Telesto is an irregular body 34 x 28 x 36 kilometers (21 x 17 x 22 miles) that orbits 60 degrees ahead of Tethys. Calypso is an irregular body 34 x 22 x 22 kilometers (21 x 11 x 11 miles) that orbits 60 degrees behind Tethys. These smaller moons are held in Lagrangian points (L4 and L5, respectively), where objects are stable with the larger controlling body. The fact that Tethys and other Saturnian moons have such objects implies that Lagrangian points might be stable for millions of years. This, in turn, implies that Lagrangian points around the Earth and the Moon might be more stable than previously thought to be.
Tethys has a high reflectivity (or geometric albedo) of 1.229 in the visual range, again suggesting a composition largely of water ice, which would behave like rock in the Tethyan average temperature of -187 degrees Celsius (-305 degrees Fahrenheit). Many of the crater floors on Titan are bright, which also suggest an abundance of water ice. Also contributing to the high reflectivity is that Tethys is bombarded by Saturn E-ring water-ice particles generated by geysers on Enceladus.
The Tethyan northern hemisphere is lighter colored and heavily reworked from ages of bombardment. For instance, near the prominent peaked crater Telemachus (Odysseus' son in The Odyssey) are the remnants of Teiresias Crater (named for a famous soothsayer of ancient Greece). The ancient Teiresias impact site is so badly overprinted and eroded by impact weathering and degradation that only a circular pattern of hummocks remains to indicate the old crater rim.
Closer to the equator, the terrain is darker and has fewer craters. This less-dense cratering again suggests past internal activity and resurfacing of the terrain.
Tethys appeared as a tiny dot to astronomers until the Voyager I and Voyager II encounters in 1980 and 1981. The Voyager images showed the major impact crater and the great chasm. Then, the Cassini spacecraft has added details including a great variety of colors at small scales suggesting a variety of materials not seen elsewhere.
Discovery and Origin of Names
Giovanni Cassini discovered Tethys in 1684. The name comes from the Greek goddess (or titan) Tethys, who was the daughter of Uranus and Gaia and the wife of Oceanus. She was said to be the mother of the chief rivers, the mother of three thousand daughters called the Oceanids, and the embodiment of the waters of the world.
Cassini referred to Tethys as one of the Sidera Lodoicea (Stars of Louis) after King Louis XIV (the other three were Iapetus, Dione, and Rhea). Other astronomers called the Saturn moons by number in terms of distance from Saturn. Thus, Tethys was Saturn III. John Herschel suggested that the moons of Saturn be associated with mythical brothers and sisters of Kronus, known to the Romans as Saturn. Geological features on Tethys generally get their names from The Odyssey by Homer. The International Astronomical Union now controls naming of astronomical bodies.
