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Neptune on Triton's horizon
Neptune on Triton's horizon (Click on the image for a larger view)

The largest of Neptune's eight known satellites, Triton is different from all other icy satellites Voyager has studied. About three-quarters the size of Earth's Moon, Triton circles Neptune in a tilted, circular, retrograde orbit (opposite to the direction of the planet's rotation), completing an orbit in 5.875 days at an average distance of 330,000 kilometers (205,000 miles) above Neptune's cloud tops. Triton shows evidence of a remarkable geologic history, and Voyager 2 images show active geyser-like eruptions spewing invisible nitrogen gas and dark dust particles several kilometers into space.

Triton has a diameter of about 2,705 kilometers (1,680 miles) and a mean density of about 2.066 grams per cubic centimeter (the density of water is 1.0 gram per cubic centimeter). This means Triton contains more rock in its interior than the icy satellites of Saturn and Uranus do.

The relatively high density and the retrograde orbit offer strong evidence that Triton did not originate near Neptune, but is a captured object. If that is the case, tidal heating could have melted Triton in its originally eccentric orbit, and the satellite might even have been liquid for as long as one billion years after its capture by Neptune.

While scientists are unsure of the details of Triton's history, icy volcanism is undoubtedly an important ingredient.

To understand what is happening on Triton, one must ask, "How cold is cold? How soft is soft? How young is young?" Water ice, whose melting point is 0 degrees Celsius (32 degrees Fahrenheit), deforms more easily and rapidly on Earth than rock does, but becomes almost as rigid as rock at the extremely low temperatures found on Triton, more than 4.5 billion kilometers from the Sun. Most of the geologic structures on Triton's surface are likely formed of water ice, because nitrogen and methane ice are too soft to support much of their own weight.

On the other hand, nitrogen and methane, which form a thin veneer on Triton, turn from ice to gas at less than 100 degrees above absolute zero. Most of the geologically recent eruptions at those low cryogenic temperatures are due to the nitrogen and methane on Triton.

Evidence that such eruptions occur was found in Voyager images of several geyser-like volcanic vents that were apparently spewing nitrogen gas laced with extremely fine, dark particles. The particles are carried to altitudes of 2 to 8 kilometers (1 to 5 miles) and then blown downwind before being deposited on Triton's surface.

An extremely thin atmosphere extends as much as 800 kilometers (500 miles) above Triton's surface. Tiny nitrogen ice particles may form thin clouds a few kilometers above the surface. Triton is very bright, reflecting 60 to 95 percent of the sunlight that strikes it (by comparison, Earth's Moon reflects 11 percent).

The atmospheric pressure at Triton's surface is about 14 microbars, a mere 1/70,000th the surface pressure on Earth. Temperature at the surface is about 38 kelvins (-391 degrees F), the coldest surface of any body yet visited in the solar system. At 800 kilometers (500 miles) above the surface, the temperature is 95 kelvins (-290 degrees F).

Despite remarkable differences between Triton and the other icy satellites in the solar system, photographs reveal terrain that is reminiscent of Ariel (a satellite of Uranus), Enceladus (a satellite of Saturn), and Europa, Ganymede and Io (satellites of Jupiter). Even a few reminders of Mars, such as polar caps and wind streaks, can be seen on Triton's surface.

Triton appears to have the same general size, density, temperature and chemical composition as Pluto (the only outer planet not yet visited by any spacecraft), and will probably be our best model of Pluto for a long time to come.

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