Tethys is a moon of Saturn.
How big is Tethys?
Tethys is about 1060 km wide.
What is its surface like?
The surface of Tethys has many craters, as well as faults caused by cracks on its icy surface.
There are two types of terrain on the surface. One is made of heavily cratered regions, and the other is made of a dark coloured, lightly cratered belt that extends across the surface of the moon. The light cratering of the region indicates that Tethys was once internally active, causing older terrain to resurface. The cause of the terrain's dark colour is unknown, but recent images of Ganymede and Callisto from the orbiter Galileo suggest that Tethys' surface might have been formed in a similar manner as these two moons, consisting of hazy polar caps of unresolved bright ice patches with a darker zone in between.
The largest crater on Tethys is called Odysseus. It is over 400 km wide, almost 2/5 of Tethys' surface! It is quite flat, due to the movement of Tethys' icy crust.
Another interesting feature of Tethys' surface is the Ithaca Chasma. It measures 100 km wide and 3 to 5 km deep. It runs 2000 km long, about 3/4 of the way around Tethys' circumference. Thwre are two theories as to how it formed. The first is that as Tethys' internal water solidified, the moon expanded and the surface cracked to accommodate the extra volume. The other theory is that as impact that created the crater Odysseus caused shockwaves that travelled through Tethys and cracked the ice on the other side.
How long is a day on Tethys?
One day on Tethys is equal to 1.89 Earth days, or just over 1 day and 21 hours. It takes the same amount of time to finish a single orbit around Saturn. This means that the same side of the moon is always facing toward Saturn
What is it made of?
Thethys is almost completely made of water-ice.
Who is it named after?
After they were discovered, Tethys, Dione, Rhea and Iapetus were named Sidera Lodoicea ("the stars of Louis") to honour king Louis XIV. Astromomeers used to call them and Titan as Saturn I through Saturn V. (Tethys being Saturn III). Once Mimas and Enceladus were discovered, in 1789, the numbering scheme was extended to Saturn VII.
The name Tethys was suggested by John Herschel, son of astromomer William Herschel. In 1847, he published Results of Astronomical Observations made at the Cape of Good Hope, in which he suggested the names the names of the Titans, sisters and brothers of Cronos (the Greek Saturn), be used.
In Greek mythology, Tethys was a Titaness and sea goddess who was both sister and wife of Oceanus. She was mother of the main rivers of the universe, such as the Nile, the Alpheus, the Maeander, and about three thousand daughters called the Oceanids.
How was it discovered?
Tethys was discovered by Giovanni Cassini on March 21, 1684.