Titan (tĪˈtən) [key], in astronomy, the largest of the named moons, or natural satellites, of Saturn. Also known as Saturn VI (or S6), Titan is 3,200 mi (5,150 km) in diameter, orbits Saturn at a mean distance of 759,209 mi (1,221,830 km), and has equal orbital and rotational periods of 15.9454 earth days. Titan was discovered by the Dutch physicist Christiaan Huygens in 1655. Titan was thought to be the largest satellite in the solar system until recently, when it was recognized that estimates of its size had included its thick atmosphere. Titan's solid surface is slightly smaller than that of Ganymede, the largest of Jupiter's satellites. Titan is composed of about half water ice and half rocky materials. It probably consists of several layers, with a 2,100 mi (3,400 km) rocky center surrounded by several layers of different forms of crystal ice that surround a liquid water and ammonia ocean. The solid surface is surrounded by an atmosphere consisting mostly of molecular nitrogen with small amounts of methane, ethane, and other hydrocarbons as well as some hydrogen. The thick, opaque atmosphere prevents the surface from being seen in visible light. In Titan's frigid atmosphere, methane rains onto the surface to form streams and lakes; grains of frozen hydrocarbons form dunes in some areas. Titan is the only natural satellite in the solar system with a significant atmosphere, although a much thinner one has been detected around Triton, a satellite of Neptune. Titan's atmosphere was first detected spectroscopically by the Dutch-American astronomer Gerard P. Kuiper in 1944. Some surface detail has been observed via the Hubble Space Telescope using infrared light; one prominent feature seen in this way is c.2,500 mi (4,000 km) across, about the size of the Australian continent. More significant studies have been conducted by the space probe Cassini, an orbiter that was launched in 1997 and has been studying Saturn and its rings and moons since 2004; it has infrared, microwave, and other imaging systems. In 2005 the space probe Huygens, which had been carried by Cassini, landed on Titan and returned photographs of its surface. Titan forms a satellite pair with Hyperion; that is, the two moons interact gravitationally.