# Triton

Neptune I

Triton [TRY-tun] is the largest moon of Neptune, with a diameter of 2,700 kilometers (1,680 miles). It was discovered by William Lassell, a British astronomer, in 1846 scarcely a month after Neptune was discovered. Triton is colder than any other measured object in the Solar System with a surface temperature of -235° C (-391° F). It has an extremely thin atmosphere. Nitrogen ice particles might form thin clouds a few kilometers above the surface. The atmospheric pressure at Triton's surface is about 14 microbars, 1/70,000th the surface pressure on Earth.

Triton is the only large satellite in the solar system to circle a planet in a retrograde direction -- in a direction opposite to the rotation of the planet. It also has a 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 has led some scientists to suggest that Triton may have been captured by Neptune as it traveled through space several billion years ago. 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.

Triton is scarred by enormous cracks. Voyager 2 images showed active geyser-like eruptions spewing nitrogen gas and dark dust particles several kilometers into the atmosphere.

## Triton Statistics

    Discovered by ............................... William Lassell
    Date of discovery ...................................... 1846

    Mass (kg) .......................................... 2.14e+22
    Mass (Earth = 1) ................................. 3.5810e-03
    Equatorial radius (km) ................................ 1,350
    Equatorial radius (Earth = 1) .................... 2.1167e-01
    Mean density (gm/cm^3) ................................. 2.07

    Mean distance from Neptune (km) ..................... 354,800
    Rotational period (days) ........................... -5.87685
    Orbital period (days) .............................. -5.87685
    Mean orbital velocity (km/sec) ........................ -4.39

    Orbital eccentricity ................................. 0.0000
    Orbital inclination .................................. 157.35°

    Escape velocity (km/sec) ............................... 1.45

    Visual geometric albedo ................................. 0.7
    Magnitude (Vo) ........................................ 13.47
    Mean surface temperature ............................. -235°C

## Animations of Triton

- Movie showing geysers on Triton - AVI, 2.7M. (Courtesy NASA/JPL)
- Flight over Triton - AVI, 4.4M. (Courtesy NASA/JPL)
- Flight over Triton (Larger Version) - AVI, 5M. (Courtesy NASA/JPL)

## Views of Triton

Triton (GIF, 466K; JPEG, 161K; caption)
Voyager 2 obtained this color image of Neptune's large satellite Triton during its close flyby on Aug. 25, 1989. The large south polar cap at the bottom of the image is highly reflective and slightly pink in color; it might consist of a slowly evaporating layer of nitrogen ice deposited during the previous winter. From the ragged edge of the polar cap northward, the satellite's face is generally darker and redder in color. This coloring may be produced by the action of ultraviolet light and magnetospheric radiation upon methane in the atmosphere and surface. Running across this darker region, approximately parallel to the edge of the polar cap, is a band of brighter white material that is almost bluish in color. The underlying topography in this bright band is similar; however, to that in the darker, redder regions surrounding it. (Courtesy NASA/JPL)

South Pole (GIF, 298K)
This high resolution image of Triton was obtained by Voyager 2 on August 25, 1989. It shows the moon's south polar cap. One of the unusual aspect of this image is the dark streaks in the images. Perhaps they were made by geyser-like eruptions of nitrogen. The geyser eruptions could have carried darker materials from the crust. The light regions probably consist of layers of nitrogen. (Credit: Calvin J. Hamilton)

Plain of Ice (GIF, 406K)
This view of Triton shows a plain of ice. It was probably formed by eruptions of water or a water-ammonia slurry. It seems to fill the remains of an ancient impact basin. (Credit: Calvin J. Hamilton)

Triton in 3D (JPEG, 95K; caption)
This is a computer-generated perspective rendering of one of the caldera-like depressions on Triton, as it would appear if viewed from the northeast. The image was obtained Aug. 24, 1989, when the Voyager spacecraft was about 181,800 kilometers (112,900 miles) from Triton. The caldera floor shown in this image is approximately 200 kilometers (120 miles) in diameter. It is extremely flat and probably was formed by the volcanic eruption of ice lavas of very low viscosity. The bench visible in the foreground may be a remnant of earlier flooding to a level about 200 meters (600 feet) higher than the present caldera floor. (Courtesy NASA/JPL)

Surface of Triton (GIF, 327K; JPEG, 89K)
This is one of the most detailed views of the surface of Triton taken by Voyager 2 on its flyby of the large satellite of Neptune early in the morning of August 25, 1989. The picture was taken from a distance of 40,000 kilometers (25,000 miles), the frame is about 220 kilometers (140 miles) across and shows details as small as 750 meters (0.5 miles). Most of the area is covered by a peculiar ladnscape of roughly circular depressions separated by rugged ridges. This type of terrain, which covers large tracts of Triton's northern hemisphere, is unlike anything seen elsewhere in the solar system. The depressions are probably not impact craters: They are too similar in size and too regularly spaced. Their origin is still unknown, but may involve local melting and collapse of the icy surface. A conspicuous set of grooves and ridges cuts across the landscape, indicating fracturing and deformation of Triton's surface. The rarity of impact craters suggests a young surface by solarsystem standards, probably less than a few billion years old. (Courtesy NASA/JPL)

Dark Areas on Triton (GIF, 99K; JPEG, 7K)
Voyager 2 acquired this black and white image of Triton during the night of August 24-25, 1989. Triton's limb cuts obliquely across the middle of the image. The field of view is about 1,000 km (600 miles) across. Three irregular dark areas, surrounded by brighter material, dominate the image. Low-lying material with intermediate albedo occupies the central area, and fresh craters occur along the right margin. Sub-parallel alignment of linear patches of dark material shown in the lower and left part of the image suggests that the patches are structurally controlled. (Courtesy NASA/JPL)

Neptune Proteus Nereid

##### Author: Calvin J. Hamilton.