Saturn's moon Tethys. Cassini spacecraft image of part of the surface of Tethys. This is the southern part of Ithaca Chasma, which is covered with impact craters and steep valleys. Tethys is composed almost entirely of pure water ice. The Cassini-Huygens spacecraft was launched in 1997 to investigate Saturn, its rings and its moons. Image obtained on 24th September 2005 by the Cassini spacecraft's narrow-angle camera, at a distance of 32,300 kilometres from Tethys.