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. The crater must originally 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 of 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.

     A July 2007 Cassini view of Tethys showing its largest crater, Odysseus, near its right limb, and its next largest, Melanthius, near the terminator (on the lower left). The shallow shape of Odysseus indicates that it was formed when the moon was more liquid than not, but the large number of craters inside it indicate a relatively large age -- probably the best part of 4 billion years ago. (Cassini Imaging Team, SSI, JPL, ESA, NASA, apod070909)

     The "trailing" edge of Tethys, opposite the crater Odysseus, which is on the "leading" edge. The large crater in the center of the image is called Penelope, after Odysseus' wife. (NASA/JPL/Space Science Institute, Planetary Photojournal)

     Jagged cliffs and cracks on Tethys imaged by the Cassini spacecraft in late September 2005, from a distance of less than 20 thousand miles. The gravitational effect of the moon on the orbit of the spacecraft during its near passage confirmed that the density of Tethys is essentially the same as that of pure water ice. (Cassini Imaging Team, SSI, JPL, ESA, NASA, apod0512012)

     One side of Tethys as photographed by the Cassini spacecraft in 2005. The high albedo of the nearly white surface is now thought to be created by ice particles from the E ring -- particles now known to be ejected by ice geysers from Enceladus. The great rift Ithaca Chasma running diagonally down the middle of the iamge is less well understood, but is thought to be due to compressional forces created when, early in its evolution a more nearly liquid Tethys gradually froze, and the resultant expansion of its buried water oceans cracked the surface. If correct this theory suggests the possibility that as in the case of Enceladus, there may still be watery oceans buried far beneath the surface of Tethys. But whereas Enceladus has extensive resurfacing caused by its buried oceans, Tethys has so many presumably ancient craters that any oceans buried beneath its surface must be much less extensive, and have insignificant effects on its current surface conditions. (Cassini Imaging Team, SSI, JPL, ESA, NASA, apod091208)