For Neptune, or rather knowledge thereof, the early 1970s CE were a simpler time. In fact any time between 1949 and 1989 was a simpler time. Back then, Kuiper having discovered Nereid, a smaller and peculiar moon, at the end of the ’40s, Neptune only seemed to have two moons: Triton and Nereid. This state of affairs continued until the end of the ’80s, which was approximately one Neptunian season. Four decades during which the planet only appeared to have two moons. I’ll start with that.
I’ve already mentioned Triton, the oddball moon of the Neptunian system two hundred times as massive as all its other moons put together, orbiting backwards and at an angle, in an almost perfectly circular trajectory. I haven’t mentioned the equally oddball second moon discovered, Nereid, and I say the early ’70s were a simpler time but in fact its own orbit is very peculiar. Nereid has the most eccentric known orbit of any moon. It sometimes feels like discussing the orbit of a celestial body is a bit tangential to the core of its nature, but orbits have important consequences for the nature of planets, moons and their neighbours, and in this case it’s so odd that it would be strange not to mention it, particularly back in 1971 when that was practically all that was known about it. It sometimes feels like the Solar System “frays at the edges” with all this stuff, because things out here are really quite outré compared to the relatively regular innards of this system we call solar. Nereid’s orbit is entirely outside Triton’s, approaching Neptune by 1 353 600 kilometres at its closest and moving out to a maximum of 9 623 700 kilometres distance from the planet. It takes five days less than a year to go all the way round, which is appealingly similar to Earth’s sidereal period. In fact of all Solar System objects its year seems closest to ours. No other moon is remotely as eccentric. At its closest, Neptune would be a little larger than the Sun is in our own sky, and at its furthest, six months later (so to speak), about the size of a lentil on one’s dinner plate. This is probably the result of Triton’s capture, which to me suggests there are other former moons wandering far beyond Pluto or even in interstellar space, or maybe in the “Gap“.
Nereid is small and grey. There is no good image. The best one is this:
Not very impressive, eh?
Unlike Triton, Nereid orbits in the usual direction, as do two other irregular moons Sao and Laomedea, further out. Another moon, Helimede, is a remarkably similar colour but orbits the other way. It’s considered to be a bit that chipped off of Nereid. Nereid itself is about 360 kilometres across on average and may be somewhat spherical but by no means perfectly so. It’s one of several bodies in the system which are right on the border of being round, and is almost as large as the definitely round (sans Herschel) Mimas, but also rather denser. Its shape is therefore hard to determine. Certainly its gravity would be sufficient to pull Mimas-like material into a spheroid, since it’s higher, but that very density may result in the moon being tougher and more able to support its own weight without collapsing. However, its variation in brightness probably means it’s quite irregular in shape and closer to Hyperion in form. Its colour is markèdly unlike that of most centaurs, and it’s therefore probably a “native” Neptunian moon. There’s water ice on its surface.
Proteus is the one which really surprised me. On the whole, the Voyager probes and others only discovered small moons, although Charles Kowal’s discovery of Leda skews that for the Jovian satellites because it’s unusually small for a telescopic discovery of that time. Proteus is actually the second largest Neptunian moon, being somewhat larger than Nereid, and is shown at the top of this post. It orbits the planet at 117 647 kilometres from the barycentre on average in a fairly round orbit, though nowhere near as round as Triton’s. It can be determined not to be perfectly spherical and is in fact not even particularly rounded, with dimensions of 424 x 390 x 396 kilometres. Its surface consists of a number of planes (or plains) with sharp angles between them at their edges and it’s uniform in colour, being somewhat reddish like many other outer system worlds. It was discovered by Voyager, but two months before the space probe got to Neptune.
Unlike Nereid, Proteus was close enough to Voyager 2 to be mapped. As can be seen above, it’s heavily cratered and its surface is therefore likely to be quite old, meaning that nothing much has happened to it in a long time. NASA also had a very steep “learning curve” with Proteus compared to Nereid as it went from being unknown to being mapped within a few weeks, whereas Nereid’s existence has been established for six dozen years now and still there is no map available except possibly the kind of vague albedo feature map which used to be done for Pluto before a spacecraft got there. It can also be seen through the Hubble Space Telescope. It’s fairly dark, probably because its surface consists of hydrocarbons and cyanides. The only named feature on its surface is the relatively large crater Pharos, 260 kilometres across, but due to its somewhat irregular shape this fails to give it the “Death Star” appearance Mimas has. Proteus is also receding from Neptune due to tidal forces and is now eight thousand kilometres further from it than when it first formed. Unsurprisingly, given that it was undiscovered for so long, it’s a lot darker than Nereid.
The inner moons generally are coated in the same material as Proteus. A couple of them are quite notable. For instance, Larissa, which is 194 kilometres in diameter, was accidentally observed passing in front of a star in 1981, leading to the correct but unwarranted conclusion that Neptune has rings. The chances of a moon of that size being seen to cover a star are very small just anyway, but in Neptune’s case it’s even less likely because it moves against the “fixed” stars so slowly, taking almost three months to cover a distance equivalent to the face of the Sun. Larissa’s period is about twelve hours and it orbits only 73 400 kilometres above the centre of Neptune, putting it close to the Roche Limit, where large bodies are torn apart by gravity. It was, however, given a provisional designation in ’81, namely S/1981 N1, so it was accepted as a moon back then. Like the other inner satellites, it’s likely to be a rubble pile, without enough gravity to pull itself together as a solid object. It may be a future ring.
Another somewhat interesting moon is Hippocamp, which is so dim Voyager failed to notice it and had to wait for the Hubble Space Telescope to discover it, which was done by the combination of a number of images as even then it was too faint to be spotted. It seems to reflect less than ten percent of the light falling on it. It’s only seventeen kilometres across.
The closest moon to Neptune, and in fact to any solar gas giant at all, is Naiad, taking only seven hours to travel round the planet. It’s quite elongated at eight by five dozen kilometres, and will either become a ring or fall into the atmosphere in the relatively near future. Thalassa, the next moon out, is coörbital with it. Their orbits are only eighteen hundred kilometres apart but they never approach that closely because they move north and south of each other as they orbit, putting them a minimum of 2 800 kilometres apart. It’s about the planet’s radius from the cloud tops, making Neptune occupy most of its sky. This would make the surface look deep purple if it has a reddish coating like the others.
Like some other moons, the naming scheme has the prograde moons end in A, the retrograde in E and the highly tilted in O. The two outermost moons, Psamathe and Neso, are relatively close to each other, and stand in contrast to Naiad by being the most distant moons of any known planet at forty-six and fifty million kilometres. Neptune’s lower mass also gives them exceedingly long years of around a quarter of a century.
That’s it for Neptune and its moons, and I’ve already done Pluto, so next stop Eris.
A Box Of Chocolates 