Crescent Titan viewed by Voyager 2 in August 1981 from ~200,000 kilometers. The continuous ring is due to the presence of a thick atmosphere around Titan. Sunlight is refracted or bent by the atmosphere along the edge of the disk, illuminating the crescent. The bluish fringe is due to scattering by molecules in the atmosphere.


Titan is the largest major satellite of Saturn and the only satellite in the solar system with a thick atmosphere. It is the second largest satellite, with a diameter of 5150 kilometers. Although similar in bulk properties to Ganymede and Callisto, Titan's thick nitrogen and methane atmosphere clearly indicates that it is fundamentally different. The atmospheric pressure at the surface of Titan is roughly 1.5 times as high as that on the Earth. Some have proposed that a methane or ethane rain falls from the skies of Titan, possibly forming lakes or oceans. Radar signals and HST images of the surface suggest that these seas are not very extensive, but these observations are not yet conclusive. Titan and Pluto are the last major unexplored planetary bodies in the solar system. The Cassini Orbiter will make extensive observations of Titan, Saturn, and its rings and satellites beginning in 2004.

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This global view of Titan (Voyager 1) is nearly featureless due to a thick photochemical haze. The haze is probably a byproduct of the breakdown of methane gas into hydrocarbons, and in some sense resembles the infamous Los Angeles smog. This haze prevents us from seeing Titan's surface and understanding its geologic history.

The Hidden Surface of Titan

Although Titan's thick haze is opaque at optical wavelengths, the atmosphere is more transparent at infrared wavelengths. The Hubble Space Telescope looked at Titan in the infrared and revealed the presence of dark and bright albedo regions. These markings could be volcanic material, highlands washed by methane rain, or seas of ethane and hydrocarbons. Despite these and other conjectures, the origins of these bright and dark markings are simply not understood. The Huygens probe (on board Cassini launched in November 1997) will land to the west of the large bright area. (Image courtesy of STScI.)

Titan images courtesy of NASA.

©Lunar and Planetary Institute, 1997

Supplemental Reading Materials

Beatty J. K., O'Leary B., and Chaikin A., eds. (1990) The New Solar System. Sky Publishing Corporation, Cambridge, Massachusetts; Cambridge University Press, New York. 326 pp.