Titan is the largest of Saturn's satellites. It is the second largest satellite in the solar system, and the only one know to have a dense atmosphere.
It may be the most interesting body, from a terrestrial perspective, in the solar system. For almost two decades, space scientists have searched for clues to the primeval Earth. The chemistry in Titan's atmosphere may be similar to what occurred in Earth's atmosphere several billion years ago.
Because of its thick, opaque atmosphere, astronomers believed Titan was the largest satellite in the solar system. Their measurements were necessarily limited to the cloud tops. Voyager 1's close approach and diametric radio occultation show Titan's surface diameter is only 5,150 kilometers (3,200 miles) - - slightly smaller than Ganymede, Jupiter's largest satellite. Both are larger than Mercury. Titan's density appears to be about twice that of water ice; it may be composed of nearly equal amounts of rock and ice.
Titan's surface cannot be seen in any Voyager photos; it is hidden by a dense, photochemical haze whose main layer is about 300 kilometers (200 miles) above Titan's surface. Several distinct, detached haze layers can be seen above the opaque haze layer. The haze layers merge with the main layer over the north pole of Titan, forming what scientists first thought was a dark hood. The hood was found, under the better viewing conditions of Voyager 2, to be a dark ring around the pole. The southern hemisphere is slightly brighter than the northern, possibly the result of seasonal effects. When the Voyagers flew past, the season on Titan was the equivalent of mid-April and early May on Earth, or early spring in the northern hemisphere and early fall in the south.
Atmospheric pressure near Titan's surface is about 1.6 bars, 60 percent greater than Earth's. The atmosphere is mostly nitrogen, also the major constituent of Earth's atmosphere.
The surface temperature appears to be about 95 Kelvins (-289 degrees Fahrenheit), only 4 Kelvins above the triple-point temperature of methane. Methane, however, appears to be below its saturation pressure near Titan's surface; rivers and lakes of methane probably don't exist, in spite of the tantalizing analogy to water on Earth. On the other hand, scientists believe lakes of ethane exist, and methane is probably dissolved in the ethane. Titan's methane, through continuing photochemistry, is converted to ethane, acetylene, ethylene, and (when combined with nitrogen) hydrogen cyanide. The last is an especially important molecule; it is a building block of amino acids. However, Titan's low temperature may inhibit more complex organic chemistry.
Titan has no intrinsic magnetic field; therefore it has no electrically conducting and convecting liquid core. Its interaction with Saturn's magnetosphere creates a magnetic wake behind Titan. The big satellite also serves as a source for both neutral and charged hydrogen atoms in Saturn's magnetosphere.
The Cassini Project will provide more information about Titan. It's lauch is currently set for October, 1997 with arrival at Saturn in June of 2004 and the landing of the Huygens probe on Titan in late 2004. For more information connect to the Cassini Mission web pages at http://www.jpl.nasa.gov/cassini/ and http://www.jpl.nasa.gov/mip/cassini.html.