Tethys

Tethys (/ˈtiːθɪs, ˈtɛθɪs/) is the fifth-largest moon of Saturn, measuring about 1,060 km (660 mi) across. It was discovered by Giovanni Domenico Cassini in 1684, and is named after the titan Tethys of Greek mythology.

Tethys has a low density of 0.98 g/cm, the lowest of all the major moons in the Solar System, indicating that it is made of water ice with just a small fraction of rock. This was confirmed by the spectroscopy of its surface, which identified water ice as the dominant surface material. A further, smaller amount of an unidentified dark material is present as well. The surface of Tethys is very bright, the second-brightest of the moons of Saturn after Enceladus, and neutral in color.