The three major Neptunian satellites: Proteus, Triton, and Nereid.


Triton (diameter 2700 kilometers) is the largest major satellite of Neptune. It is slightly smaller than Earth's moon. Since 1990, several dozen objects have been discovered beyond the orbit of Neptune in a zone called the Kuiper Belt. Triton and Pluto may be the largest remaining examples of these objects. Triton, however, was captured by Neptune early in its history, and intense tidal heating melted most of the interior. Triton's complex geology reflects this violent history and subsequent evolution.

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Triton and Pluto

Triton and Pluto, shown in these global views, are similar in shape and density. Frosts of carbon dioxide, carbon monoxide, nitrogen and methane have been discovered on the surfaces of both bodies. Both Triton and Pluto have a pastel pinkish color. Triton and Pluto have had very different histories, however. Triton was captured by Neptune early in its history and bears the scars of this violent event. Pluto, however, has a major natural satellite, Charon (shown crossing in front of Pluto), and has been in solar orbit for all of its history. Its geologic history is unknown.

Volcanic Terrains

Large parts of Triton have been volcanically resurfaced. This area, centered on Leviathan Patera (the 80-kilometer-wide circular structure at center left), is characterized by smooth plains, volcanic calderas, and irregularly shaped pits. These features resemble those found in basaltic volcanic plains on Earth. On Triton, these features are formed by volcanic eruption of molten ices, probably ammonia-rich in composition. Also visible at upper left is a walled plain of uncertain origin.

Cantaloupe Terrain

The weirdest terrain on Triton is the so-called cantaloupe terrain, characterized by closed depressions 30 to 50 kilometers wide, separated by ridges. It has now been shown that these depressions resemble those formed by diapirs, which are blobs of material that rise from depth and penetrate through a surface layer. This suggests that Triton's crust is layered. Some of the smooth deposits at right may be volcanic in origin.

Plumes (or Geysers)

One of the biggest surprises about Triton was the discovery of atmospheric plumes in the spotted southern hemisphere of Triton. These plumes reach heights of 8 kilometers and are blown laterally by winds in the extremely thin atmosphere. They can be traced for several hundred kilometers. The origin of these plumes is still a matter of debate. They may be the result of solar heating of a thin frozen nitrogen layer, or of melting of volatiles near the surface by internal heat.

This stereo image should be viewed using red-blue stereo glasses. The plumes can be seen to shift between the exposures used to make the view. The plumes are the reddish features toward the left.

All images by Paul M. Schenk, Lunar and Planetary Institute, Houston, TX.

©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.

Moons and Rings (1991) Voyage Through the Universe series. Time-Life Books, Alexandria, Virginia. 144 pp.

Rothery D. (1992) Satellites of the Outer Planets. Clarendon Press, Oxford. 208 pp.

Cruikshank D. P., ed. (1995) Neptune and Triton. University of Arizona Press, Tucson. 1249 pp.

Schenk P. (1993) Diapirism on Triton: A record of crustal layering and instability. Geology, 21, 299-302.

Binzel R. P. (1990) Pluto. Scientific American, 262, 50-58.

Stern S. A. (1992) The Pluto-Charon system. Annual Reviews of Astronomy and Astrophysics, 30, 185.