5th Largest Moon of Saturn - Tethys (14th Moon outwards from Saturn)

Tethys, is the fifth largest moon of Saturn, and 16th largest moon in the Solar System.

Discovery

Tethys was discovered by Giovanni Domenico Cassini, an Italian/French astronomer, in 21st March 1684. He found Tethys and Dione using a large aerial telescope he set up on the grounds of the Paris Observatory.



Naming

Tethys is named after the titan Tethys of Greek mythology.

In Greek mythology, Tethys, daughter of Uranus and Gaia, was an archaic Titaness and aquatic sea goddess, invoked in classical Greek poetry but not venerated in cult. Tethys was both sister and wife of Oceanus. She was mother of the chief rivers of the world known to the Greeks, such as the Nile, the Alpheus, the Maeander, and about three thousand daughters called the Oceanids.

Stats

Diameter: 1,062 km

Semi-major axis: 294,619 km

Orbital Period: 1.89 days

Orbit

Tethys takes as long to rotate on its axis as it does to make one orbit of Saturn; and therefore always keeps the same hemisphere pointed to Saturn.

The Tethyan orbit lies deep inside the magnetosphere of Saturn, so the plasma co-rotating with the planet strikes the trailing hemipshere of the moon. Tethys is also subject to constant bombardment by the energetic particles (electrons and ions) present in the magnetosphere.

The co-orbital moons Telesto and Calypso are located within Tethys' Lagrangian points L4 and L5, 60 degrees ahead and behind Tethys in its orbit respectively.

Physical characteristics

The density of Tethys is 0.98 g/cm³, indicating that it is composed almost entirely of water-ice. The mass of rocky material cannot exceed 6% of the mass of this moon.

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 high albedo indicates that the surface of Tethys is composed of almost pure water ice with only a small amount of a dark material. The visible spectrum of the moon is flat and featureless. 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.

Life?

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 is about 145 km. Due to the action of tidal and rotational forces, Tethys has the shape of triaxial ellipsoid. The dimensions of this ellipsoid are consistent with this moon having a homogeneous interior.

The existence of a subsurface ocean — a layer of liquid water in the interior of Tethys — is considered unlikely.