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. The solid surface is surrounded by an atmosphere consisting mostly of molecular nitrogen with small amounts of methane, ethane, and other hydrocarbons. This thick, opaque atmosphere prevents the surface from being seen in visible light, although 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. 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. Titan forms a satellite pair with Hyperion; that is, the two moons interact gravitationally. In 2004 the space probe Huygens, which had been carried to Saturn by Cassini, landed on Titan and returned photographs of its surface.
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