College College Space and Planetary Sciences TSS

Triton: A Planet Disguised as a Moon

Triton is one of the most unique moons in our solar system. It orbits around Neptune, but in a weird way. This moon is also geologically active and even has seasons. Could Triton be an ancient planet simply captured by Neptune's gravity?

There are plenty of interesting moons within the solar system, but in my opinion Triton, a moon of Neptune, is possibly the most interesting of them all. Neptune has only been visited once by a spacecraft, specifically Voyager 2 in 1989, so the gas giant is still largely wrapped in mystery. The same goes for Triton as it was analyzed up close by Voyager 2 as well. There are many interesting features about Triton, and I am sure with more observation, scientists will discover many more interesting things about Triton and the Neptune system.

Triton as captured by Voyager 2. Image from NASA / Jet Propulsion Lab / U.S. Geological Survey

Triton was first observed by William Lassell in 1846, which was only 17 days after Neptune’s discovery. This moon turned out to be the only prominent Neptunian moon as the other 12 moons are small, some even not big enough to become spherical in shape (1). The first oddity about Triton is how it orbits around Neptune. Typically, moons in our solar system orbit in the direction of its planet’s spin; however, Triton is the only moon that does the opposite. This is especially strange since the other moons around Neptune are completely different from Triton and orbit in the correct way. One explanation for this unusual orbital behavior is that Triton could have been a dwarf planet in the distant past before it was captured by Neptune’s gravity. In fact, Triton is still moving towards Neptune albeit very slowly; in the distant future Triton may end up being torn apart by getting too close Neptune, which would give Neptune a set of rings. A recent study in 2017 suggests that Triton was part of a binary planetary system, like Pluto and Charon. The second planet/moon that was in the binary system with Triton was most likely ejected when Triton was captured by Neptune (2).

Geysers on Triton. Image from NASA-JPL.
Fractures in the ice. Image from NASA-JPL.









Another particularly interesting aspect about Triton is the fact that it has active geology. As Voyager 2 flew by Neptune and Triton, it captured views of plumes erupting from geysers and fractures within the surface ice. This is very similar to the other geologically active moons: Enceladus and Europa. Scientists think that these geysers are erupting plumes of water since the density of Triton makes it possible to have a subsurface ocean. The fractures in the ice come from the constant gravitational tugging caused by Triton’s tidally locked nature (3). This significant geological activity and the presence of water could make Triton a viable candidate for life even though it is so far away from the sun (4).

Triton also has an atmosphere and seasons! Normally, moons don’t have seasons or even an atmosphere, so this is especially interesting. The atmosphere is seasonal on Triton and currently the moon is experiencing summer in the southern hemisphere, the hemisphere we can easily observe. Summer on Triton makes the atmosphere thicker than in the winter since the frozen nitrogen, methane, and carbon monoxide become trapped in Triton’s ice sublimates (5).

In conclusion, Triton is quite a unique moon in our solar system. I think people tend to forget that it exists since there has been so little research and observations. Based on what we know, it is plausible that at some point Triton was a planet that unfortunately got captured by Neptune and could not escape.



3) Tidally Locked: when the same side of a moon always faces its planet.



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