The planet Neptune is surrounded by a system of 8 moons. Most of these are tiny, with diameters of less than 500 miles. One of these moons, however, is quite large and warrants further investigation. This is the moon Triton. It was named after the Greek god Triton, who was the son of Poseidon (Neptune). He is usually portrayed as having the head and body of a man and the tail of a fish. This moon of Neptune was discovered by the British astronomer William Lassell in 1846. Since then it has been visited by only one spacecraft, Voyager 2, in 1989.
The Backwards Moon
Triton orbits Neptune in what is known as a retrograde orbit. This means that it orbits Neptune a direction opposite the planet's rotation. It is the only large moon in the Solar System to do this. Astronomers are not quite sure of the reason for this retrograde orbit. Some believe that it condensed this way from the original material of the early solar system. Others think that Triton may have been formed elsewhere and then captured by Neptune's gravity. In fact, many astronomers have noticed that the surface features of Triton, as well as its size, are very similar to what they believe the planet Pluto to look like. Some even wonder if there is some connection between Triton's features and the fact that Pluto actually crosses Neptune's orbit from time to time. Just what that connection might be is anyone's guess at this point.
Features of Triton
Triton is the coldest known object in the Solar System. Its surface temperature averages only -391° F (-235° C). This is caused by the moon's high albedo. Very little sunlight is absorbed by the surface. Triton's axis of rotation is tilted 157 degrees with respect to Neptune's axis. This causes the moon's polar and equatorial regions to be alternately pointed towards the Sun. This causes extreme seasonal changes as Triton's orientation changes. This uneven heating and cooling could account for some of the moon's curious surface features. Triton has an extremely thin atmosphere composed mainly of nitrogen and methane. There are very few craters visible on the surface, indicating that the moon is very young and probably highly active. There are high ridges and deep valleys all over the moon's surface. Perhaps the most interesting discovery to be made by Voyager 2 was Triton's ice volcanoes. Voyager photographed a plume of frozen material being ejected from the moon's surface. Astronomers believe this material to be composed of liquid nitrogen or methane.
