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.

[14] Cassini observed all of these moons using a large aerial telescope he set up on the grounds of the Paris Observatory.

[14] 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.

[14] In his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope,[17] he suggested the names of the Titans, sisters and brothers of Kronos (the Greek analogue of Saturn), be used.

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.

[21] 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.

[27] The high albedo indicates that the surface of Tethys is composed of almost pure water ice with only a small amount of darker materials.

[29] 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[26] and has a large porosity exceeding 95%.

[31] Such a bifurcated color pattern results in the existence of a bluish band between hemispheres following a great circle that runs through the poles.

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.

[32] On the leading hemisphere of Tethys spacecraft observations have found a dark bluish band spanning 20° to the south and north from the equator.

[33] The band is almost certainly caused by the influence of energetic electrons from the Saturnian magnetosphere with energies greater than about 1 MeV.

These particles drift in the direction opposite to the rotation of the planet and preferentially impact areas on the leading hemisphere close to the equator.

[34] Temperature maps of Tethys obtained by Cassini have shown this bluish region is cooler at midday than surrounding areas, giving the satellite a "Pac-man"-like appearance at mid-infrared wavelengths.

[37] It is thought that Ithaca Chasma formed as Tethys's internal liquid water solidified, causing the moon to expand and cracking the surface to accommodate the extra volume within.

[38] There is another theory about the formation of Ithaca Chasma: when the impact that caused the great crater Odysseus occurred, the shock wave traveled through Tethys and fractured the icy, brittle surface.

[36] However, age determination based on crater counts in high-resolution Cassini images showed that Ithaca Chasma is older than Odysseus making the impact hypothesis unlikely.

The cratered terrain is the oldest unit likely dating back to the Solar System formation 4.56 billion years ago.

[41] The youngest unit lies within Odysseus crater with an estimated age from 3.76 to 1.06 billion years, depending on the absolute chronology used.

[43] The low temperature at the position of Saturn in the Solar nebular means that water ice was the primary solid from which all moons formed.

The conditions in the Saturnian sub-nebula likely favored conversion of the molecular nitrogen and carbon monoxide into ammonia and methane, respectively.

Models suggest that impacts accompanying accretion caused heating of Tethys's outer layer, reaching a maximum temperature of around 155 K at a depth of about 29 km.

This caused strong extensional stresses in Tethys's crust reaching estimates of 5.7 MPa, which likely led to cracking.

[49] Because Tethys lacks substantial rock content, the heating by decay of radioactive elements is unlikely to have played a significant role in its further evolution.

[59] The spacecraft obtained spatially resolved near-infrared spectra of Tethys showing that its surface is made of water ice mixed with a dark material,[27] whereas the far-infrared observations constrained the bolometric bond albedo.