Neptune

Neptune is the eighth planet from the Sun and about four times the size of Earth. Astronomers consider Neptune to form with Uranus a subgroup of the Jovian planets (Jupiter, Saturn, Uranus, and Neptune). Neptune and Uranus are similar in size, mass, periods of their rotation, the overall features of their magnetic fields, and ring systems. However they differ in the structure of their atmospheres (perhaps the more conspicuous features of Neptune's clouds are caused by its significant internal energy source, which Uranus lacks), the orientations of their rotation axes, and in their satellite systems.

Neptune's large satellite Triton, which has a very thin nitrogen atmosphere with clouds, plumes, and haze, an extremely cold surface with nitrogen, methane, carbon monoxide, and carbon dioxide ices which interact with the atmosphere, and a fairly high mean density, make it seem more like Pluto than the other satellites of Neptune and those of Saturn and Uranus. Not enough is known about Pluto to explore these similarities; this probably awaits future missions to Pluto, especially the New Horizons mission that NASA hopes to launch in 2006.

Triton's mass

Triton's mass was found to be 2.141.10²² kg (0.00358 Earth mass), and its radius 840 mi (1,352 km); from this, its mean density was found to be 2.07 grams/cm³. This makes Triton the densest satellite yet found beyond Jupiter, although Saturn's satellite Titan is considerably larger and more massive.

Triton's atmosphere

Triton has a thin atmosphere of mainly molecular nitrogen with a trace (.01%) of methane. Radio observations during the occultation of Voyager 2 by Triton found a value of 1.5 Pascal for the atmospheric surface pressure (the pressure in Earth's atmosphere about 75 mi [120 km] above sea level). Clouds are observed in the lower few miles of Triton's atmosphere, and haze is found as high as 31 mi (50 km) above its surface. Four mysterious plumes were observed to rise as high as eight km above the surface and then extend up to 93 mi (150 km) horizontally. The clouds and plumes showed the presence of winds in Triton's atmosphere. The mechanism which powers the plumes is uncertain; a "greenhouse" mechanism (which traps sunlight in a transparent solid nitrogen layer and then vaporizes some nitrogen), geothermal heat, and also "dust devils" in Triton's atmosphere have all been suggested.

Triton's surface

Triton's surface is very bright; its average albedo is about 0.8, while those for individual regions vary from 0.6 to nearly 1.0. Yellowish, pinkish, and peach-colored regions are seen on Triton's generally whitish background; these may show the presence of organic compounds produced by reactions of molecular nitrogen and methane in the presence of solar radiation, high energy electrons, or of cosmic rays. Voyager 2 infrared observations gave a -391°F (-235°C) for Triton's surface, which varied very little across it. This is the coldest temperature observed for the surface of any solar system satellite or planet. Nitrogen and methane ice were both identified on the surface; at 1.5 Pascal atmospheric surface pressure, the atmosphere is in vapor equilibrium with nitrogen surface ice. Triton's surface has few craters (compared with the outer satellites of Saturn and Uranus), and it lacks craters larger than 19 mi (30 km) in diameter. The crater density is considerably lower than that on the most heavily cratered regions on Uranus's satellite Miranda. Many regions show evidence of resurfacing. Many darker streaks are seen on Triton's surface; they may be debris that has been deposited by earlier plumes.

Triton's surface can be subdivided into four distinctive terrains including (1) an old, highly fractured terrain (dark plains crossed by intersecting networks of fractures in the crustal ice, (2) a smooth, volcanic plains terrain where icy lava have resurfaced vast tracts (includes irregular depressions that are likely volcanic caldera), (3) two polar ice terrains covered with nitrogen snow and frost (constant change due to tilt of axis of Triton and highly inclined orbital path), and (4) dark streak terrain formed by upon polar ice terrain due to volcanic activity spewing out dark methane particles.

Earth-based observations made in 1991 and 1992 made in near-infrared wavelengths with the United Kingdom Infrared Telescope Facility on Mauna Kea, Hawaii showed evidence of carbon monoxide and carbon dioxide ices on Triton's surface as well as those of nitrogen and methane. They confirmed the 391°F temperature of Triton's surface and found that there is less than 10% carbon monoxide ice dissolved in the nitrogen ice on Triton's surface.

This summarizes the present state of our knowledge about Triton. In size, mass, and mean density, Triton appears to be a larger and more massive variant of Pluto, since their mean densities are both nearly 2.1 grams/cm³. A plausible model for Triton's interior is one with a rocky core of about 621 mi (1,000 km) radius surrounded by a 217 mi (350 km) thick water ice mantle, above which there is a crust of nitrogen, methane, carbon monoxide, and carbon dioxide ices which is only a few miles thick.

Returning to Neptune, very little is now known about its interior except that its powerful internal energy source and strong magnetic field imply a field-generating region in Neptune's interior which extends through most of it, and that most of Neptune's interior is a fluid having a high internal temperature. Neptune's mean density indicates that it probably has a small rocky core surrounded by a hot fluid shell; this is surrounded by a second shell comprised of gaseous and icy water, ammonia and carbon compounds. Neptune's oblateness (polar flattening) of 0.0171 indicates that its interior is considerably denser than that of Uranus with oblateness of 0.023. The precession of Triton's orbit gives some information about Neptune's internal structure. However, it is too far from Neptune (14.4 Neptune radii) for detailed mapping of Neptune's interior and gravity field. Accurate orbits are needed for the nearest newly discovered satellites and rings to do this; they are not yet available. Even if such orbits are determined, the information from them may not allow a unique model to be selected for Neptune's interior; this cannot yet be done for Uranus, where much more information about its close satellites and rings is available.

The Hubble Space Telescope observed Neptune in 1994 from Earth orbit and detected cloud features in its atmosphere which appeared to be different from those observed by Voyager 2 in 1989. The main change noticed was that the Great and Small Dark Spots seemed to have disappeared by 1994. These spots were therefore of much shorter duration than Jupiter's Great Red Spot, to which these large dark spots in Neptune's atmosphere have sometimes been compared.