Saturn and moons. Composite image of Saturn and some of its moons. The moons of the ringed planet are: Mimas (upper centre), Enceladus (upper left), Tethys (far right), Dione (centre), Rhea (centre left), Titan (lower right) and Hyperion (foreground). Saturn is mostly comprised of hydrogen and helium gas, whilst its moons are largely made of frozen water. The largest moon, Titan, has a diameter of about 5150 kilometres, making it larger than the planet Mercury. This image was made by combining Voyager 2 images of Saturn and its moons with an artwork of the surface of Hyperion.