TITAN

Titan, moon of Saturn, photographed by Voyager 2 on Aug. 25, 1981, at a distance of 907,000 km largest moon of Saturn and the only satellite in the solar system known to have clouds and a dense atmosphere. It was discovered in 1655 by Christiaan Huygens. Titan orbits Saturn in an elliptical orbit of eccentricity 0.029 and with a semimajor axis of 1,221,860 km (759,264 miles), taking 15.945 Earth days for one revolution. The satellite is believed to make one rotation for each revolution, so that it always keeps the same hemisphere toward Saturn. The diameter of Titan is 5,150 km (3,200 miles), making it the second largest moon (after Ganymede) in the solar system. Its mass is 1.3457 1026 g; the resulting bulk density of 1.881 g per cubic cm implies that Titan's interior is a mixture of rocky and icy materials, the latter probably including solid ammonia and methane as well as solid water. Titan's surface temperature and atmospheric pressure, first determined by the Voyager 1 spacecraft in 1981, are 94 2 K (-179 2 C) and approximately 1.6 bars (1.6 times the atmospheric pressure at the surface of the Earth), respectively. Its atmosphere is composed primarily of nitrogen, as is the Earth's, with several percent methane. In addition to hydrogen, carbon monoxide, and carbon dioxide, minor constituents (all between 20 and 0.01 parts per 1,000,000) include the organic gases ethane, propane, acetylene, ethylene, hydrogen cyanide, diacetylene, methylacetylene, cyanoacetylene, and cyanogen. Titan is enveloped in a layer of deep-reddish haze, probably composed of an aerosol of still more complex organic solids that slowly settle through the atmosphere and accumulate on the surface. The amount producedpredominantly by solar ultraviolet light falling on the nitrogen/methane atmospherethroughout the history of Titan is the equivalent of a continuous layer of organic solids covering the entire surface to a depth of at least hundreds of metres. Above the surface and beneath most of the opaque organic haze lies a thick cloud of condensed methane. Some evidence suggests that an extensive ocean of liquid methane and ethane covers much of the surface. As a deep-frozen repository of the types of organic molecules that led to the origin of life on Earth 4 billion years ago, and as the most probable locale in the solar system for a liquid hydrocarbon ocean, Titan represents an extraordinary objective for future spacecraft exploration. in Greek mythology, any of the children of Uranus (Heaven) and Gaea (Earth) and their descendants. According to Hesiod's Theogony, there were 12 original Titans: the brothers Oceanus, Coeus, Crius, Hyperion, Iapetus, and Cronus and the sisters Thea, Rhea, Themis, Mnemosyne, Phoebe, and Tethys. At the instigation of Gaea the Titans rebelled against their father, who had shut them up in the underworld (Tartarus). Under the leadership of Cronus they deposed Uranus and set up Cronus as their ruler. But one of Cronus' sons, Zeus, rebelled against his father, and a struggle then ensued between them in which most of the Titans sided with Cronus. Zeus and his brothers and sisters finally defeated the Titans after 10 years of fierce battles (the Titanomachia). The Titans were then hurled down by Zeus and imprisoned in a cavity beneath Tartarus.