Tethys (pronounced Teeth-iss) is 660 miles in diameter. Like Mimas, Tethys has a very large impact crater, Odysseus, which is 2/5 of the diameter of Tethys (in fact, in comparison to Tethys, Odysseus is even larger than Herschel is, in comparison to Mimas). It is believed that Tethys must have been partially liquid at the time that Odysseus was created, or else the moon would have been completely shattered by the force of the impact. Originally, the crater must have been quite deep, but over time, the moon's gravity has forced it into a shallower curve which more nearly matches the moon's spherical shape.
      Another confirmation of the idea that Tethys might have once been at least partially liquid is a valley, Ithaca Chasma, which is 40 miles wide, 2 to 3 miles deep, and spans 3/4 of the moon's circumference. Tethys has a relatively low density, and is probably made almost entirely of water ice. It is thought that if it was once wholly or partially liquid, it might have significantly expanded when the liquid froze, and Ithaca Chasma may be a huge fissure created by the expansion of the surface.
      Because Tethys is fairly large, it has enough gravity that it can slightly alter the orbits of other objects that are orbiting near it and Saturn. Two of Saturn's other moons, Telesto and Calypso, are orbiting exactly in Tethys' orbit, but 60 degrees ahead and behind it, at the Lagrange points created by the gravitational interaction of Tethys and Saturn. A similar gravitational interaction between Jupiter and the Sun is the cause of the Trojan asteroids' similar orbits, and as a result, Telesto and Calypso are sometimes referred to as the Tethys Trojans.