Useful Notes / The Moons of Neptune

Like Neptune itself, the Roman name of Poseidon, Neptune's moons are named after Greek deities and beings associated with water. Only Triton and Proteus have been imaged with enough detail to discern geographical features and only 14 have been discovered so far.

Like with Saturn, the Neptunian system is dominated by a single large moon in form of Triton but unlike both it and its brother Uranus it lacks any mid-sized ones. And most of the ones it does have (Triton included) shows signs of having either formed very late in the solar systems lifespan or been captured at some point. The reasons for this might also lie in Triton, as its capture might have disrupted the system to the point that most of Neptune's original moons were destroyed and/or thrown out.

The Doomed Quartet: Naiad, Thalassa, Despina and Galatea

Four small irregular moons imbedded within the ring system of Neptune and which keeps it in check with their gravity. All most likely having been formed by debris created when Triton disrupted Neptune's original satellite system.

All four are doomed like their progenitor (and Larissa) due to orbiting below Neptune's synchronous orbit radius. And will eventually break up and also become ring material.

You Always Hurt The One You Love: Larissa

The existence of this moon was first noticed in 1981 by occultation charting. Like Nereid, little else is known beyond the fact that it is a "rubble pile" of accreted rocks, is the fourth largest moon, and the location of its near-perfect circular orbit may mean that one day it will hurtle into Neptune like the rest of the inner moons. It finally received its name after a decade of observation in 1991 after one of Poseidon's many lovers. Like the other innermost moons, it may have formed from the remains of earlier satellites that were destroyed due to Triton messing with their orbits after the former was captured by the planet.

The Sea Shell: Proteus

The second largest moon of Neptune after Triton, discovered during Voyager 2's flyby. It's closer in shape to a polyhedron rather than a sphere, measuring roughly 400 kilometers, is heavily cratered and like the other inner moons, most likely formed by debris created by Tritons capture. Though unlike the rest it's not in the immediate threat of breaking up due to being too close to Neptune.

Only one feature is named on Proteus so far. The range of names for its geographical features are water related spirits and deities in human mythologies—except for Greek and Roman.

The Big Guy: Triton

Profile

• Diameter: 2,706 km
• Mass: 0.00359 of Earth
• Density: 2.061 g/cm3
• Surface Gravity: 0.08 g
• Semi-major Axis: 354,759 km from Neptune
• Orbital Period: 5 Days (Retrograde)
• Rotational Period: 5 Days (Tidally Locked)
• Axial Tilt: 156.88° to Neptune's Equator (28.32°), 129.81° to Ecliptic Plane, 129.61° to Neptune's Orbit
• Average Surface Temperature: -235° C
• Notable Features: Uhlanga Regio
• Discovered: 1846 by William Lassell

The largest and most-well known of Neptune's moons. Discovered just seventeen days after the planet itself, this moon enjoyed the status of "Neptune's Moon" until the discovery of Nereid in 1949. Alongside our moon, the four Galilean moons of Jupiter, and the Saturnian moon Titan, it is one of the seven large moons in the Solar System. Unlike all of its fellows, it has a retrograde orbit around Neptune; this indicates that Triton must be a captured object, rather than forming from the protoplanetary disc like most moons. Like the Moon, Triton is tidally locked, presenting exactly one face to Neptune at all times. Its surface chemical composition mirrors Pluto, sitting at approximately 55% frozen nitrogen, 15–35% water ice, and 10–20% dry ice (frozen CO₂). Its mantle is actually water, and like Europa, the possibility of a subterranean ocean exists. The most spectacular aspect of Triton are its cryovolcanic eruptions.

Triton's compositional similarities to Pluto are the root of the debunked theory that Pluto was an escaped moon of Neptune. The reasoning was that their similar compositions implies that the two have similar origins, and they do, but in reverse of the original theory. From the 1990s onward, the prevailing theory is that Triton originated as an independent dwarf planet in the Kuiper Belt before it was captured. Interestingly, computer simulations with currently known parameters suggests that Triton's odds of being captured were extremely low; it had a much higher chance of being launched into the Scattered Disk like Eris was when it got too close to Neptune. Even more interesting, this conundrum was solved by making Triton part of a binary system similar to Pluto and Charon: Triton's orbit around the barycenter created by its hypothetical partner gave it an angular momentum that greatly increased the chances of being captured. IF this scenario is correct, the fate of Triton's hypothetical ex-partner is unknown.

Unfortunately, thanks to its slow, decaying orbit, Triton is doomed, and likely to be torn apart by Neptune around 3.6 billion years from now when it gets too close. On the bright side, thanks to its icy composition, there's a chance it will become a gaudy ring system to rival Saturn's.

Given the great detail at which Triton has been examined compared to other worlds observed by Voyager 2, there are multiple naming conventions used for its geographical features. They tend to lean toward rivers and other bodies of water described by human mythologies, as well as islands considered sacred in the same.

There was a proposed mission named Trident that was intended to target Triton, taking advantage of a gravity assist window (which occurs every 13 years) between Jupiter and Neptune. However NASA declined to proceed with the mission in 2021 (which would have allowed a 2025 launch for a 2038 arrival), and it is therefore likely that further spacecraft will not visit Triton (and the Neptunian system in general) until the 2050s.

Long Distance Call: Nereid

The only other moon to be confirmed before Voyager 2 flew by in 1989. The third-largest moon of Neptune, it was discovered in 1949. Surprisingly, the actual shape of Nereid is unknown. While water ice has been detected, the actual moon is neutral in color. Voyager 2's camera did not have the resolution to get clear enough images to determine physical characteristics but studies shows its most likely a captured Trojan.

Nereid's biggest claim to fame is its highly eccentric orbit with a periapsis and apoapsis difference of around eight million kilometers. This irregularity either suggests it being a recent capture by Neptune or had its original orbit disrupted somehow. Possibly by Triton which would make Nereid the only known original satellite of Neptune.

And the Rest!

The Other six moons are very small and in mostly retrograde orbits outside of Triton, making them most likely captured Trojans. Expect more to join them soon as we discover more of the Neptunian system.

At the moment, the most notable of these moons is Neso, which has the most distant known orbit of any moon from its parent planet (possible because of Neptune's massive Hill sphere). It orbits Neptune, on average, more distantly than Mercury orbits the Sun.

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