Survivor of an Ancient "Moonpocalypse"

 Sole survivor of an ancient "Moonpocalypse", A strange moon orbiting Neptune

NASA’s James Webb Space Telescope captured Neptune and its rings and inner moons in 2022. These moons could be made of the pulverized remains of Neptune’s original moons. Neptune’s moon Nereid may be the sole survivor of an ancient moonpocalypse. Neptune is definitely the odd one out of the gas giants. It’s tilted at a strange angle, and its moons are completely different from any other gas giant we know of. A new paper, published in Science Advances from researchers at CalTech, posits that might be because Triton, by far Neptune’s largest moon, absolutely obliterated the regular moon system it previously had, except for one particular exception, Nereid. 

A new study suggests that the strange satellite was born in a steady, circular orbit around Neptune, then tossed into its current elongated orbit during a chaotic encounter with a Pluto-sized body which ejected or pulverized all its sibling moons. This idea counters the assumption that Nereid formed in the Kuiper Belt, the cold reservoir of space rocks in the outer solar system, and was pitched into its present orbit later, researchers argue. Let's start with a little background on Neptune's moon system. Triton is weird. It rotates the opposite direction to Neptune itself, which means it did not form naturally as part of the planetary system. More likely, it was part of a Kuiper Belt Object (KBO) binary, similar to Pluto and Charon, which was captured by Neptune's gravitational well. Nereid is in itself an outlier.

“Maybe it got perturbed outward, rather than kicked inward,” says planetary scientist Matthew Belyakov of Caltech. “Nereid is that last remaining signature of the original satellite system.” Neptune’s largest moon, Triton, orbits backward and makes up more than 99% of the mass of all the planet’s moons combined. Most of Neptune’s other moons orbit the planet from a shorter distance and are small and rubbly, suggesting they’ve been through a lot of collisions. Planetary scientists think Triton came from the Kuiper Belt and wreaked havoc on the rest of the moons when Neptune captured it billions of years ago. Originally discovered in 1949 by Gerard Kuiper (after whom the Kuiper Belt is named), over 100 years after Triton was discovered, it remained Neptune's only other known moon until the Voyager 2 flyby in 1989. But its orbit is eccentric to say the least. It's highly elliptical and lasts 360 days, making astronomers believe for years that it was another captured KBO. This new study pretty clearly shows that it is not.

Nereid stands alone. It orbits in a wide ellipse far from Neptune. That puts it in a family of moons from across the solar system called irregular satellites, many of which are also thought to be captured Kuiper Belt objects. But it’s brighter, larger, more eccentric and closer to its host planet than other irregular satellites in the solar system. “Nereid always is an outlier,” Belyakov says. Maybe its origin story was an outlier, too. To prove that, the authors turned JWST's high-resolution infrared camera toward Nereid for the first time. They found that it looks much more like an icy native moon of Uranus or Saturn than a dark, dusty captured KBO. Compared with Phoebe, a known captured KBO, Nereid's water-rich craters appear completely different in infrared light. As the authors note , "Nereid's unique spectrum among outer Solar System bodies is not consistent with a scenario where Nereid is captured during the early Solar System's dynamic instability." So that pretty much rules out the possibility of Nereid as a KBO, which leaves the only other option as a naturally occurring moon of Neptune.

Belyakov and colleagues compared James Webb Space Telescope observations of Nereid’s makeup with those of other Kuiper Belt objects. Nereid wasn’t a good match for any of them. That left the possibility that Nereid formed locally. Belyakov and colleagues ran computer simulations of Triton’s known chaotic arrival at hundreds of different masses and orbits for Neptune’s original moons, including the destroyed ones. A computer rendering of a blue planet surrounded by moons, with two more distant moons labeled Triton and Nereid. The moons' orbits are depicted in grey. Nereid’s out-there, elongated orbit sets it apart from the other moons. Its distance, ellipticality and other factors are different from most other moons in the Solar System. To answer this question, the authors turned to simulations. They used a dynamic simulator called REBOUND to map Neptune as a series of normal, circular moons. Then they hit that nice, neat model with Triton. As the captured KBO entered a highly eccentric, backward orbit, it wreaked absolute havoc with Neptune's existing moon systems.

Most of the original moons were smashed to pieces or ejected from the system altogether as part of this process. Their debris eventually settled down to create Neptune's current ring system, and some of the tiny "ring-moons" like Proteus. None reproduced Nereid’s exact present-day orbit. And some ended with Triton leaving the system or crashing into Neptune. But about 20% produced a moon on a Nereid-like orbit, without destroying Triton. That’s enough to make the story believable, Belyakov says. But the simulations also showed another feature. In about 20% of all simulation runs, Triton kicked one of the native inner moons that was there before its arrival into a stable, highly elongated, tilted orbit. Just like Nereid's. So, in simulation at least, Nereid appears to be an original moon of Neptune which was kicked to its current wacky orbit by the capture of the planetary system's biggest current resident.

 If that's the case, it could offer pristine insight into the formation of the Neptunian system, since its distant orbit would have kept it relatively well preserved compared to other gas giant moons. We likely won't be able to confirm that theory until we send another probe that way, though. Planetary scientists have been clamoring for one for over a decade, to no avail as of yet. But until we do, we can shift our thinking of this specific gas giant moon from that of a captured ice ball to a battle-scarred survivor of one of the most violent moonpocalypses the Solar System has ever witnessed. Nereid itself is still largely mysterious. The best picture we have of it is about five pixels across, from the Voyager 2 mission in 1989. Belyakov is holding out for a spacecraft flyby someday. “That’s the next frontier, missions to the ice giants,” he says.