Tethys. Satellite image of Tethys, one of Saturn's moons. Tethys, also known as Saturn III, was discovered by Giovani Cassini in 1684. It has a diameter of 1,060 kilometres and is believed to have a density close to that of water, indicating it is likely composed mainly of water ice. Tethys has many craters, with smaller craters lying on top of older, larger ones suggesting an ancient surface. This image was taken as part of the Cassini-Huygens mission, which launched in 1997 with the purpose of studying Saturn and its moons. Image taken on 28 October 2004 at a distance of 256,000 kilometres by a narrow angle camera aboard the Cassini orbiter.