Triton and Pluto sport similar bulk compositions and densities. The duo also show similar atmospheres. In addition, both remote frozen worlds travel in unusual orbits. Pluto sports a highly eccentric orbit that sometimes carries it closer to the Sun than Neptune. Also, Pluto orbits in the opposite direction around our Sun than do the other major planets in our Solar System. Similarly, Triton orbits its big blue planet in a direction counter to that of Neptune. Because of the rather strange attributes of both Triton’s and Pluto’s orbits, as well as the similarities of their bulk properties and atmospheres, astronomers have long thought that there is some sort of historical bond between the two. Indeed, it was once believed that Pluto is really an escaped moon of Neptune. However, this particular theory is now considered unlikely. It is much more probable that long ago Triton, like Pluto, orbited our Sun independently, but was unfortunately snared by its adoptive parent-planet. In contrast, Pluto was left to wander free from any planet’s gravitational embrace.
Triton’s surface is primarily coated with frozen nitrogen. It also possesses a mostly water-ice crust, an icy mantle, and a large core composed of rock and metal. This hefty core accounts for two-thirds of Triton’s total mass. Triton’s mean density is about 2.061grams per centimeter cubed, indicating that it has a composition of about 15-35% water ice.
Triton is one of only a handful of moons in our Solar System that is geologically active. As a result, it has a youthful surface that is pockmarked by few impact craters. Heavily cratered surfaces indicate an old surface, while few surface craters suggest a young surface that has been recently resurfaced (on geological time scales). Intricate icy volcanic (cryovolcanic) and tectonic terrains strongly suggest a complicated geological past. Regions of Triton’s surface display geyers that shoot out sublimated hydrogen gas. These eruptions contribute to a thin nitrogen atmosphere that is less than 1/70,000 the pressure of Earth’s atmosphere at sea level.
Triton is one of the coldest denizens of our Solar System. Indeed, it is so frigid that most of its nitrogen atmosphere is condensed as frost. This gives Triton’s surface an extremely bright, mirror-like appearance that reflects approximately 70% of the sunlight that is able to reach it. Like Earth’s own bewitching large Moon, Triton is locked in synchronous rotation with its planet. This means that one side of Triton always faces Neptune, while the other side is always turned away. However, because of Neptune’s rather strange orbital inclination, both of the moon’s poles take turns facing our Star. Spacecraft images show oval pits and mounds created as the result of cryovolcanic flows, as well as smooth volcanic plains. Triton’s young surface sparkles brightly with a new ice-coating.
There are two types of mechanisms proposed to explain how Triton was captured by Neptune. In order to be gravitationally snared by a planet, a wandering body must lose enough energy to be slowed down to a speed that is less than that needed to escape from these gravitational ties that bind. An early theory, explaining how Triton may have been sufficiently slowed down, suggests that there was an ancient collision with another object–either a moon or proto-moon circling Neptune, or another object that (as ill-luck would have it) just happened to be wandering by Neptune at a bad time. Of the two, the collision with a Neptunian moon or proto-moon is considered the most likely scenario. However, a more recent hypothesis proposes that, before it was snared by Neptune, Triton was a member of a binary system. When this binary wandered close to Neptune, it interacted in such a way that the binary was broken in two, with one member of the binary shot yowling into space–while the other, Triton, was fortunately captured by Neptune. This event would have to have been both brief and gentle, in order to save Triton from collisional disruption. Such events are thought to have been frequent during the formation of Neptune–or somewhat later as it migrated outward in our ancient Solar System. Simulations conducted in 2017 suggest that, after Triton’s capture and before its orbital eccentricity decreased, it probably did collide with at least one other Neptunian moon, as well as triggering collisions between other moons.