Tethys or Saturn III is a mid-sized moon of Saturn about 1,060 km (660 mi) across. It was discovered by G. D. Cassini in 1684 and is named after titan Tethys of Greek
Tethys has a low density of 0.98 g/cm³ indicating that it is made of water ice with just a small fraction of rock.
This is confirmed by the spectroscopy of its surface, which identified water ice as the dominant surface material.
A small amount of an unidentified dark material is present as well.
The surface of Tethys is very bright, being second brightest among the moons of Saturn after Enceladus, and neutral in color.
(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
Cassini observed all these moons using a large aerial telescope he set up on the grounds of the Paris Observatory
Tethys orbits Saturn at a distance of about 295000 km (about 4.4 Saturn's radii) from the center of the planet.
The orbital eccentricity is negligible, while the orbital inclination is about 1 degree.
The moon is locked in an inclination resonance with Mimas, which, however, does not cause any noticeable orbital eccentricity and tidal heating
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
The surface of Tethys has a number of large-scale features distinguished by their color and sometimes brightness.
The trailing hemisphere gets increasingly red and dark as the anti-apex of motion is approached. This darkening is responsible for the hemispheric albedo asymmetry
The leading hemisphere also reddens slightly as the apex of the motion is approached, although without any noticeable darkening.
Such a bifurcated color pattern results in the existence of a bluish band between hemispheres following a great circle that runs through the poles.
This coloration and darkening of the Tethyan surface is typical for Saturnian middle-sized satellites.
Its origin may be related to a deposition of bright ice particles from the E-ring onto the leading hemispheres and dark particles coming from outer satellites on the
The darkening of the trailing hemispheres can also be caused by the impact of plasma from the magnetosphere of Saturn, which co-rotates with the planet
The western part of the leading hemisphere of Tethys is dominated by a large impact crater called Odysseus, whose 450 km diameter is nearly 2/5 of that of Tethys
The crater is now quite flat or more precisely, its floor conforms to Tethys's spherical shape.
This is most likely due to the viscous relaxation of the Tethyan icy crust over geologic time. Nevertheless the rim crest of Odysseus is elevated by approximately 5 km
above the mean satellite radius.
The central complex of Odysseus features a central pit 2–4 km deep surrounded by massifs elevated by 6–9 km above the crater floor, which itself is about 3 km below
the average radius
(Impact craters and chronology)
The majority of Tethyan impact craters are of a simple central peak type.
Those more than 150 km in diameter show more complex peak ring morphology.
Only Odysseus crater has a central depression resembling a central pit.
Older impact craters are somewhat shallower than young ones implying a degree of relaxation.
(Origin and evolution)
Tethys is thought to have formed from an accretion disc or subnebula; a disc of gas and dust that either existed around Saturn for some time after its...