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The largest satellite of Saturn, and the second-largest satellite in the Solar System, diameter 5150 km; also known as Saturn VI. Titan orbits Saturn in 15.945 days at a distance of 1 221 800 km. Its axial rotation period is the same as its orbital period. At opposition it is magnitude 8.3. It was discovered in 1655 by C. Huygens. Titan is probably composed of rock and ice in about equal parts. It is the only satellite in the Solar System to have a substantial atmosphere, the surface atmospheric pressure being higher than on Earth. The atmosphere is mainly nitrogen, with 2–10% methane, 0.2% hydrogen (molecular percentages), and traces of ethane, propane, ethyne, hydrogen cyanide, and carbon monoxide. The atmospheric pressure at Titan's surface is 1.5 bar and the temperature about −180°C. Photochemical reactions have generated dense orange clouds of hydrocarbons at a height of 200 km, while layers of atmospheric haze exist up to 500 km. The Voyager probes showed a north polar cap in the clouds of Titan with a slightly darker collar around it. Also, the northern hemisphere was distinctly darker than the southern. These are probably both seasonal effects. The satellite was closely studied by the Cassini–Huygens mission. Radar and spectroscopy revealed a complex surface of hilly terrain and extensive darker fields of dunes. There are a few raised regions, one called Xanadu which appears to show weathering and layered deposits, perhaps of organic matter. The Huygens probe successfully landed on Titan in 2005 in a region of high ground known as Adiri, east of which is a dark lower-lying area called Shangri-la. Adiri is etched by river valleys, fed perhaps by sporadic liquid methane rain. Weathering on Titan is not restricted to fluid erosion. Radar observations of Titan's surface by the Cassini craft show what appear to be thousand-kilometre-long dune fields of an unknown material, as well as lakes of liquid methane and other hydrocarbons in the polar regions.


Subjects: Astronomy and Astrophysics.

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