Neptune’s biggest moon, Triton, has a diameter of 2,700 km, making it the 7th biggest moon in the solar system, as well as its 16th biggest object overall. Triton also contains more than 99.5% of all the mass that is known to orbit the planet Neptune, and has more mass than all the other known planetary satellites in the solar system that are smaller than it combined. Below are some more interesting facts about the Neptune’s huge natural satellite that was discovered by William Lassell on October 10th, 1846:
– Triton is named after a sea-god
Somewhat fittingly, the moon was named after Triton, the son of the Greek sea-god Poseidon whose Roman equivalent was Neptune. However, the name Triton was only officially adopted many years after its discovery, and it was known simply as “the satellite of Neptune” until the discovery of Neptune’s second moon, Nereid, in 1949.
– Triton has a retrograde orbit
While many moons in the solar system have retrograde orbits, Triton is unique in this regard because it is the largest moon to orbit its parent body in the wrong direction. By way of comparison, some of Jupiter’s and Saturn’s moons have retrograde orbits, but they are situated much further away from their planets, and are also much smaller, with the largest among these moons, Phoebe (a moon of Saturn), being only 8% as big, and 0.03% as massive as Triton.
– Triton will crash into Neptune
Since Triton is already orbiting Neptune at a distance that is smaller than the Earth-Moon distance, it is almost certain that tidal forces will cause the moon’s orbit to decay further, and at an increasing rate as the orbits decays. Computer modelling suggests that in about 3.6 billion years, Triton will cross Neptune’s Roche limit, which is the distance at which an object orbiting a massive body will break apart. In practical terms, the Roche limit is reached when tidal forces overcome the gravitational forces that hold an orbiting body together. It is expected that when Triton reaches this point, it will either collide with Neptune’s atmosphere, forming a complex ring system, or it will simply break up and fall into Neptune.
– Triton is likely a captured Kuiper Belt object
Since moons that have retrograde orbits cannot form in the same region as their primaries, the only explanation for Triton’s orbit is that it was captured from the Kuiper belt, a ring-shaped reservoir of small icy and/or rocky objects that remained after the formation of the solar system. The Kuiper belt extends from just inside the orbit of Neptune to a distance of about 50 astronomical units from the Sun and since Triton is almost identical in composition, size, mass, and temperature to Pluto (a known Kuiper belt object), it is almost certain that Triton was captured by Neptune in the distant past.
– Triton has few impact craters
Only 179 impact craters have been positively identified on the 40% of Triton’s surface that has been mapped, which suggests that the surface of the moon is continually undergoing a process of modification. In fact, studies have shown that on cosmic time scales, Triton’s surface is almost “brand new”, with an estimated age of between 50 million and only 6 million years old. Moreover, Triton’s surface is almost as smooth as a billiard table, with the highest known elevation being only about 1,000 metres.
– Triton has ice volcanoes
Although Triton has a crust of ice, the processes that produce the observed ice volcanoes are almost identical to those that produce hot, lava volcanoes on Earth. Triton’s entire known surface is crisscrossed by rift valleys and pressure ridges, which suggests an ongoing process of volcanic and tectonic activity, but instead of hot lava, the volcanoes on Triton spew water ice and ammonia as the result of endogenic geological processes, as opposed to fractures caused by violent impacts.
– Triton also has nitrogen geysers
Apart from ice volcanoes, Triton also features geysers of sublimated nitrogen, somewhat like the geysers on Earth that spout hot water. While there are not many nitrogen geysers on Triton, all are located close to the subsolar point, which suggests that sunlight is heating reservoirs of subsurface frozen nitrogen to the point where it sublimates, before bursting through the solid ice sheet that overlays the reservoir. One such geyser was observed to squirt nitrogen gas and entrap dust to a height of 8,000 metres, and based upon this, it is estimated that each eruption could last for up to one earth-year, releasing the sublimated nitrogen gas of about 100 million cubic meters of frozen nitrogen.
– Triton features unique cantaloupe terrain
– Triton has a nearly perfectly circular orbit
Triton’s orbit around Neptune is almost perfectly circular, with an eccentricity that is negligible. While there is some uncertainty how this came about in the relatively short history of the solar system, it is thought that factors such as gas drag from a substantial debris disc may have contributed significantly to the circularizing of Triton’s orbit.
– Tritons surface is completely frozen over
Although only about 40% of the moon’s surface is mapped, the percentage that is known is completely frozen over by a layer of frozen nitrogen, and it is extremely unlikely that the remaining 60% of Triton’s surface would be any different. The ice layer consists primarily of nitrogen ice, with water ice and frozen carbon dioxide completing the mix. The ice layer gives Triton a high albedo; it reflects between 60% and 95% of the light that falls onto it, while our own Moon only reflects about 11%-12% of light.