|Discovered by||Giovanni Cassini|
|Discovered in||1684||Orbital characteristics|
|Semimajor axis||294,660 km|
|Orbital period||45h 18m 26s|
|Is a satellite of||Saturn|
|Mean radius||530 km|
|Mean density||0.99 g/cm3|
|Surface gravity||0.16 m/s2|
|Rotation period||45h 18m 26s (synchronous)|
Tethys is an icy body similar in nature to Dione and Rhea. The density of Tethys is 1.21 g/cm3, indicating that it is composed almost entirely of water-ice. Tethys's surface is heavily cratered and contains numerous cracks caused by faults in the ice. There are two different types of terrain found on Tethys, one composed of densely cratered regions and the other consisting of a dark colored and lightly cratered belt that extends across the moon. The light cratering of this second region indicates that Tethys was once internally active, causing parts of the older terrain to be resurfaced.
The exact cause of the darkness of the belt is unknown but a possible interpretation comes from recent Galileo probe images of Jupiter's moons Ganymede and Callisto, both of which exhibit light polar caps that are made from bright ice deposits on pole-facing slopes of craters. From a distance the caps appear brighter due to the thousands of unresolved ice patches in small craters present there. Tethys' surface may have been formed in a similar manner, consisting of hazy polar caps of unresolved bright ice patches with a darker zone in between.
The western hemisphere of Tethys is dominated by a huge impact crater called Odysseus, whose 400 km diameter is nearly 2/5 of that of Tethys itself. The crater is now quite flat (or more precisely, it conforms to Tethys' spherical shape), like the craters on Callisto, without the high ring mountains and central peaks commonly seen on the Moon and Mercury. This is most likely due to the slumping of Tethys' weak icy crust over geologic time.
The second major feature seen on Tethys is a huge valley called Ithaca Chasma, 100 km wide and 3 to 5 km deep. It runs 2000 km long, approximately 3/4 of the way around Tethys' circumference. It is thought that Ithaca Chasma formed as Tethys' internal liquid water solidified, causing the moon to expand and cracking the surface to accommodate the extra volume within. Earlier craters from before Tethys solidified were probably all erased by geological activity before then. There is another theory about the formation of Ithica Chasma: when the impact that caused the great crater Odysseus occurred, the shockwave traveled through Tethys and fractured the icy, brittle surface on the other side. Tethys' surface temperature is -187°C.