Physical Characteristics
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. The density of Tethys is 0.98 g/cm³, indicating that it is composed almost entirely of water-ice. The mass of rocky material can not exceed 6% of the mass of this moon.
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.
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 by 10–15% brighter 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 a dark material. The visible spectrum of the moon is flat and featureless, while 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%.
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