Neptune's Moon Triton
Written by Fraser Cain
The planet Neptune was first observed on September 23, 1846 by Berlin Observatory astronomer Johann Gottfried Galle, based on calculations made by Urbain Le Verrier and John Couch Adams. And just 17 days after that, astronomer William Lassell discovered Neptune's largest moon, Triton, orbiting the planet on October 10, 1846.
We now know that Triton is the 7th largest moon in the Solar System, measuring 2,700 km across. In fact, if Triton orbited the Sun, and not Neptune, it would be considered a dwarf planet because it has enough mass to pull itself into a sphere.
Triton is unusual for several reasons, but one of the biggest is its retrograde orbit. The moon orbits Neptune backwards compared to the orbit of all the other moons. This means that Triton didn't form in orbit around Neptune, but formed somewhere else and was captured later on by Neptune's gravity. It's possible that Triton was a Kuiper Belt Object that fell into Neptune's gravity field. Or maybe Neptune stole it from another planet.
Triton consists of a crust of frozen methane over an icy mantle that contains rock and metal. In fact, the heavy core makes up two-thirds of its mass, and helps give the moon a density above 2 grams per cubic centimeter. While other icy objects are much closer to water's 1 gram/cubic centimeter.
Another surprising feature of Triton is the fact that it's geologically active. When NASA's Voyager 2 made a flyby of the Moon in 1989, it saw geysers of liquid nitrogen blasting out of two distinct features on the surface of Triton. Compared to other objects in the Solar System, the surface of Triton is very young. This means that some process on the moon is constantly resurfacing it. These nitrogen geysers can send plumes of liquid nitrogen 8 km above the surface of the moon, and probably account for its young appearance.
Triton is doomed. The moon is tidally locked to Neptune, keeping one face turned towards the planet at all times. But its orbit is decaying. In approximately 3.6 billion years from now, it will pass below Neptune's Roche limit and be torn apart. After that, Neptune will have a huge ring like Saturn, until those particles crash into the planet as well. It would be something to watch.
We have written several articles about Triton. Here are more details about how it might have been captured by Neptune from another planet. And here's another article that explores how old Triton's surface might be.
Want more information about Neptune? Here's NASA's World Book on Neptune, and here's NASA's Solar System Exploration Guide.
We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.
Filed under: Astronomy
