Saturn's moon Tethys, composite Cassini spacecraft image. This is the moon's south pole, which is covered in impact craters. Tethys has a diameter of 1060 kilometres and is composed almost entirely of pure water ice. The high reflectivity (albedo) of the icy surface makes Tethys the brightest moon orbiting Saturn and one of the brightest objects in the solar system. The Cassini-Huygens spacecraft was launched in 1997 to investigate Saturn, its rings and its moons. Images obtained on 24th September 2005 by the Cassini spacecraft's narrow-angle camera.