Saturn and its moon Tethys, Cassini spacecraft image. Tethys is some 1070 kilometres in diameter and it orbits just less than 300,000 kilometres from Saturn. It is primarily composed of water ice. Two large craters are visible on its surface, Odysseus (upper right) and Melanthius (bottom centre). Saturn itself is a gas giant planet, some 120,500 kilometres in diameter. It is composed almost entirely of hydrogen, with a rocky core and traces of other gases. The extreme temperatures and pressures inside Saturn mean that the hydrogen in its interior is in a metallic state. This image was taken on 3rd December 2005, while Cassini was around 2.5 million kilometres from Saturn.
