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Tethys measures 1066 km across, and orbits at an altitude of 295,000 km above the center of Saturn.
The density of Tethys is 0.97 grams per cubic centimeter. Since water is 1 g/cm3, this means that Tethys is comprised almost entirely of water ice (if you brought it closer to the Sun, the whole moon would evaporate away). This ice is very reflective, and makes Tethys relatively bright.
There are two different regions of terrain on Tethys. One portion is ancient, with densly packed craters, while the other parts are darker and have less cratering. Scientists think that Tethys was once internally active, with cryovolcanism that resurfaced parts of its surface.
The western hemisphere of Tethys is dominated by a huge crater called Odysseus. It’s 400 km across, making it 2/5th the size of Tethys itself.
Tethys has two co-orbital moons: Telesto and Calypso. These have been captured into Tethy’s Lagrangian points. One orbits ahead of Tethys, and the other follows behind.