Neptune is the furthest planet from the sun, at a whopping 30 Astronomical Units (where 1AU = distance between earth and sun). Its distance and low albedo (how much light it reflects) mean that it’s not visible to the naked eye- so its discovery was somewhat more complex than the other planets, most of which have been known for millennia to people using the night sky to navigate and tell stories. However I think the story of Neptune’s discovery is beautiful in its own way- the science way!
Astronomers (in the early 1800s) had been studying the orbit of Uranus (which surprisingly is classed as visible to the naked eye, but this was back before the high levels of light pollution we experience today) and realised that it didn’t match up with the expected path as predicted by Newton’s Law of Gravitation. It was thought that the gravitational interaction with a planet beyond Uranus could be the explanation, and after much calculation and observation, Neptune was discovered in 1846. Although this was the first time it was spotted and verified as a planet, it’s possible that Galileo had actually spotted it in the background of one of his observations of Jupiter, however he noted it only as a background star.
Despite its discovery nearly 200 years ago, not much was known about the planet itself, as with most telescopes it simply appears as a small blue disk, with changing observations of its brightness being attributed to potential weather systems changing the cloud cover and thus its albedo. Observations have improved enough in resolution to be more scientifically interesting, mostly thanks to the Hubble Space Telescope and better optics in the HUGE ground based telescopes like those in Hawaii. This allows us to track storms and changes in its atmosphere, as well as measure its composition using spectroscopy.
However it was Voyager that made some big discoveries, including 6 new moons, 4 new rings and the Great Dark Spot, an anticyclonic storm (ie. winds moving around an area of high pressure) rivalling Jupiter’s Great Red Spot in size! It also measured the speeds of winds on Neptune, and crowned it as having the fastest winds in the solar system, reaching up to 600m/s or 1300mph! As you can see from the photos below, its atmosphere is incredibly active with storms changing frequently.
This is the first difference between the ice giants- Uranus’ atmosphere appears homogeneous and mostly cloudless, a stark contrast to Neptune’s changing appearance.
The next mystery is how hot Neptune is. Or more specifically, how hot its interior is. Neptune radiates 2.61x as much energy as it receives from the sun, whereas Uranus is roughly in equilibrium (energy radiated = energy received). This would suggest that they have vastly different compositions and structure, but from what we can tell they seem pretty similar in that respect! The large temperature gradient between Neptune’s interior and its surroundings causes upwelling to power storms and fuels the speedy winds, which explains our first mystery- but why are the temperatures different in the first place?
One theory is that both these planets suffered collisions with other large planetary bodies in the early solar system, similar to how earth may have been hit with the Mars-sized body ‘thea’ to make the moon. The collision with Uranus was at angle, causing it to rotate onto its side and throwing up a disk of debris orbiting in that new plane of rotation, which Uranus’ moons formed out of. Neptune however was subject to a head on collision with a planetoid ~10% of Neptune’s mass/ similar size to the earth. This merged with Neptune’s core, transferring lots of energy which we see today as heat!
Neptune’s atmosphere is a swirling mix of Hydrogen, Helium and Methane. The methane is supposedly what gives Neptune its blue colour. The mystery here is what causes Uranus to be pale blue and Neptune to be deep blue, when they seem to have the same compositions? It probably makes up about 15% of its radius, though it’s difficult to tell exactly how thick its atmosphere is, because unlike on earth where there is a clear boundary between rocky/liquid surface and air, the atmosphere of Neptune merges with its mantle. But Neptune is an ice giant, not a terrestrial planet, so instead of a rocky mantle, the mantle is made of an icy mix of water, ammonia and methane. The heat from the core probably means that much of this is a very high pressure fluid, which could be regarded as a subsurface ocean, although it would be highly unlikely for life to exist here!
Neptune’s moons are also a source of mystery. Out of the 14 moons, 7 of these are regular, and sit amongst the ring system, fairly close to the planet, in prograde orbits (ie. they orbit in the same direction as Neptune rotates).
The inner prograde moons orbit anticlockwise 
Neptune’s Rings (NASA)
It’s currently thought that these moons and the rings system are relatively young, an artefact of the arrival of Neptune’s largest and so-far-most-interesting moon, Triton.
In itself, Triton isn’t ~particularly~ special. It’s very similar to Pluto (it’s actually slightly bigger!) and other large Kuiper belt bodies, with an icy (mainly frozen nitrogen, water and carbon dioxide) surface, a watery mantle (again, this could act as a subsurface lake, but the pressure could be low enough for it to be more conducive to life, perhaps similar to that of the Galilean moons), and rocky/metal core. Residual heat from the core and tidal forces may be enough to keep the mantle at least semi-molten, powering cryovolcanism at the surface.
Recent outpourings of Cryolava form smooth flat plains 
Clouds above Triton
This cryovolcanism is what renews the surface, explaining why it has so few craters and is quite flat, and also releases nitrogen into a thin atmosphere, which goes through a cycle of pressure changes as the day goes by: as the temperature decreases, the nitrogen condenses and coats the surface in a nitrogen frost!
Sure, this is interesting, but lots of bodies in the solar system have cryovolcanoes- what makes Triton special is its orbit. It orbits with large inclination, so much so that its orbit is retrograde (in the opposite direction to Neptune’s rotation). This makes it unlikely to have formed around Neptune in a debris disk, but instead was captured from the Kuiper belt. If this is true, Triton would be the largest captured moon by far- Phoebe, a moon of Saturn, would be in second place, and is 3 thousand times less massive than Triton!
For an object to be captured, it has to lose enough energy to enter orbit. For a while, this was another great mystery, but recently it has been theorised that Triton was once part of a binary system, similar to that of Pluto and Charon, and upon reaching Neptune, the energy was transferred to Triton’s pair, which was then flung away from the system, and potentially out of the solar system entirely! Triton’s entrance would have caused all sorts of gravitational mayhem, probably causing the original inner moons to collide and break up, creating the ring system which brought about a new generation of moons, as well as messing with the orbit of Nereid, Neptune’s third largest moon and the most eccentric moon in the solar system!
Not only did Triton cause the formation of Neptune’s current ring system, but tidal interactions and drag from debris orbiting Neptune are degrading its current orbit, bringing it closer and closer until it reaches the Roche limit, where tidal forces will see it ripped apart, forming a brand new ring system to rival that of Saturn’s! but this probably won’t be for another 3.6 billion years, and who knows what new dynamics will be in play in the meantime!
This blog post might make it seem like a lot of the mysteries have been solved, but these are all theories- good theories, but there’s little evidence at the moment to back it up. What is really needed is another mission to this distant world- technology has improved vastly since the 80s, and it would be great to get some more detail on the structure of Neptune, and the ages of the moons. This system shows just how dynamic and ever-changing the solar system as a whole is, and could reveal secrets as to how our own moon formed, or why Mars cooled so quickly.
Is the potential for new discoveries worth the time and money to send a mission there? Let me know in the comments!
Sources
- https://solarsystem.nasa.gov/planets/neptune/in-depth/
- https://en.wikipedia.org/wiki/Moons_of_Neptune
- https://en.wikipedia.org/wiki/Triton_(moon)
- https://solarsystem.nasa.gov/missions/voyager-2/in-depth/
- https://www.universetoday.com/21596/what-is-neptune-made-of-1/
- https://www.universetoday.com/21584/atmosphere-of-neptune/
- https://earthsky.org/space/why-neptune-uranus-different-collisions
- https://www.britannica.com/place/Neptune-planet/Neptunes-discovery
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