Neptune has six small satellites, irregular in shape and mostly rock, and one, Triton, that is farthest out and much larger (2,700 km [1,678 mi] in diameter) than the others. Triton moves in a retrograde circular orbit, i.e., it moves in a clockwise direction (looking down from north) compared to the counterclockwise rotation of Neptune and its other satellites. Triton's axis of rotation tilts 157° relative to its parent's axis. We cite these unusual conditions as evidence, either of its capture or of its assembly after a collision, possibly influenced by interactions with Pluto as that small body periodically orbits inside Neptune's orbit.
The full view (left) of Triton reveals it is very different from most of the giant planets' satellites. This satellite has a brownish pink color, has a polar cap (right), and a structured surface. The color may be due to frozen nitrogen and/or the effects of methane.
Triton's density (2.06 gm/cc) indicates it is a mix of rock (predominant)
and ice. But its surface shows not only rocky features but evidence of water
ice (?), and frozen methane and nitrogen (both would be solid at a surface
temperature of -235° C [35° K]) in deposits that have built an ice
cap, covering much of its southern hemisphere. Triton also has a very thin
atmosphere (0.014 millibar) extending to 800 km (497 mi). The south polar
ice cap appears mottled, with dark areas, perhaps showing rocks exposed along
wind streaks where gases have sublimated:
19-71:
The south polar ice cap appears mottled, with dark areas perhaps showing rocks exposed along wind streaks where gases has sublimed:
Triton's surface contains ridges, fault valleys, occasional impact craters and, of notable interest, ice "volcanoes," in which liquid nitrogen and/or methane vaporizes in plumes or geyser-like jets at the surface, carrying along particulates that form deposits on the ice cap. Ice "lavas" composed of water ice and some ammonia and methane produce flow features on the surface as well.
Fractured terrains on Triton can take on a "texture" similar to the surface of a cantaloupe (left) or appearing as a series of oval features controlled by ridges (right):
19-72:
Nereid, an irregular shaped satellite (maximum dimension = 300 km [200 miles] that is the outermost of the group, had been observed by telescope from Earth. Largest (418 km; 261 miles) of the six satellites discovered by Voyager is the nearly round Proteus (below):
The last planet in our solar system, Pluto, is not much different in diameter (about 2,340 km [1,454 mi]) than Triton. No space probe has visited it yet, although NASA has proposed one (Pluto Express) for early next century. First discovered (though its existence had been predicted) in 1930, by Dr. Clyde Tombaugh, through the large telescope at the Lowell Observatory in Flagstaff, AZ, the best pictures (from HST) today show it is a mix of very dark (organic?) and light zones of unknown nature. Its density suggests mostly rock with some ices (nitrogen, methane, and water), from which an extremely tenuous atmosphere has evaporated.
Pluto's orbit is so eccentric that at times its path lies inside Neptune's. Its orbital period is 1.5 times that of Neptune, with which it locks in a 3:2 resonance. Pluto has a relatively large satellite, Charon, whose diameter (1,260 km [783 mi]) makes it just half the size of its parent (one opinion considers the two to be a double planet), with which it is in synchronous rotation.
Primary Author: Nicholas M.
Short, Sr. email: nmshort@epix.net
Contributor Information
Last Updated: September '99
Site Curator: Nannette Fekete