Neptunian moon Nereid could be lone intact survivor from ancient satellite system

By Jacopo Prisco, CNN

(CNN) — Nereid, Neptune’s third-largest moon, could be the only intact survivor from an ancient set of moons destroyed early in the solar system’s history, according to a new analysis based on data from the James Webb Space Telescope.

Neptune, the eighth and most distant planet from the sun, stands out among the outer planets in our solar system for its odd group of moons. The other outer giants — Uranus, Saturn and Jupiter — all have a broadly similar, orderly set of satellites, with several larger moons orbiting in the same direction as the host planet’s rotation.

But Neptune has a far smaller and more chaotic collection of moons: Triton, Neptune’s largest satellite, dwarfs all the others and orbits in the opposite direction of its host’s rotation. It is the only large moon in the solar system to do so.

Astronomers suspect the reason for Triton’s odd behavior is that it didn’t originate from the remnants of Neptune’s formation, which would make it orbit in the same direction as that planet. They hypothesize instead that Triton might have originated from the Kuiper Belt, a ring-shaped region of icy bodies at the edge of the solar system, and entered the Neptunian environment over 4 billion years ago.

Previous studies have suggested that Triton may have been captured by Neptune’s gravity after a close pass and flung inward to smash into Neptune’s primordial satellite system.

If Neptune did have an original set of moons that more closely resembled those of its planetary neighbors, the arrival of Triton — which is just smaller than our own moon — would have wreaked havoc, crashing into the other satellites and annihilating some of them. The current features of Neptune’s system support this scenario, and its seven inner moons appear to be leftovers of this ancient clash.

But now, new research using data from the James Webb Space Telescope suggests that one object might have been entirely spared from the chaos.

“I think Nereid is the only intact survivor of this process,” said Matthew Belyakov, a graduate student in planetary science at the California Institute of Technology and first author of a study on the subject published Wednesday in the journal Science Advances.

“The other survivors are Neptune’s innermost moons, but they are not intact because we have images of them from Voyager, and they look like disrupted rubble piles. So they are surviving material from the initial system, but not fully intact moons.”

This hypothesis would upend previous assumptions that Nereid was, much like Triton and a few other Neptunian moons, a captured Kuiper Belt object, as the new James Webb data revealed that Nereid’s composition doesn’t match what scientists know about Kuiper Belt objects.

A pixilated image

Astronomers don’t know a lot about Nereid, because it is faint and distant from Earth and the sun. The only image scientists have of it is a blurry photo taken in 1989 by NASA’s Voyager 2 spacecraft during its brief fly-by of Neptune. Nereid is the outermost of Neptune’s known moons, and it has one of the most eccentric (meaning noncircular) orbits in the solar system. It takes 360 Earth days for the moon to complete one lap around Neptune.

Named after the sea nymphs of Greek mythology, Nereid is believed to be around 210 miles (338 kilometers) in diameter. Even if it’s part of an original set of moons that Neptune had shortly after its formation, about 4.5 billion years ago, it’s difficult to speculate what that system might have looked like, according to Belyakov. “It’s kind of anyone’s guess what was there before Triton,” he said.

Just like Triton, Nereid is an irregular satellite, a class of objects whose orbits are inclined, backward or distant from their host, suggesting they were captured by their host and previously orbited the sun independently.

But even among irregular satellites, Nereid is an outlier, Belyakov said. “It’s twice as big in diameter as the next largest one, which is Phoebe around Saturn, and it’s not all that distant from its host planet compared to a lot of the other irregular satellites.” Some of Nereid’s features, he added, have long made some astronomers doubt the moon’s Kuiper Belt origin.

A 10-minute and 40-second observation commissioned for the study and made using James Webb’s infrared capabilities, which can reveal the composition of a distant object, gives credence to those doubts.

“What we found was an object that was highly water-rich on the surface, brighter than a lot of Kuiper Belt objects, and with some presence of CO2,” Belyakov said. “The overall signature was more similar to that of regular satellites around Uranus rather than Kuiper Belt objects.” The results were compared with data from 54 Kuiper Belt bodies, also from James Webb observations.

Belyakov and his colleagues then performed computer simulations to test whether the hypothesis of Nereid as part of an original moon system would stand true. “What we found in our simulations is that in the cases where Triton survives, rather than get destroyed or kicked into Neptune,” Belyakov said, “around 25% of the time one or more moons can survive the Triton encounter on distant orbits.” That number compares favorably to the chances of Nereid being a captured object instead, he added.

In this scenario, in the first 100 million or 200 million years of solar system history, Triton would have slammed into the Neptunian system, colliding with a number of the original moons. But Nereid would have been spared and sent on an eccentric orbit instead. The event would have also slowed down Triton’s own eccentric orbit and put it in its current path, closer to Neptune.

“I think people have already wanted this to be true,” Belyakov said about Nereid’s possible new origin story. “Now we can start the actual science feedback loop. There is more data to be gathered for Nereid compositionally that can help us really talk through the formation of the Neptunian system, and if we treat Nereid as a regular satellite, maybe that can tell us a lot about how satellites form around ice giants.”

Further James Webb observations can help, but the final word on what Nereid really looks like would require a mission to Neptune. At the moment, none is planned, and Voyager 2, launched in 1977, is the only spacecraft to have studied the system.

‘A compelling idea’

The new study is a lovely and simple examination of how the Neptunian moon system became the way it looks today, according to Carolyn Porco, an American planetary scientist who worked on NASA’s Voyager and Cassini missions and was not involved with the new study.

“It is Triton that was the captured body and it subsequently caused havoc, gravitationally scattering Neptune’s original moons hither and thither but mostly out of Neptune orbit,” Porco wrote in an email. “The authors show it is plausible that Nereid lucked out by remaining in orbit around Neptune but at a much larger distance than Triton. This would explain why its composition observed by James Webb does not match that of the bodies in the Kuiper Belt.”

James Webb is once again showing its tremendous power as a solar system explorer, said Leigh Fletcher, a professor at the School of Physics and Astronomy of the University of Leicester in England who also did not participate in the work.

“We’ve long known there’s something special about Neptune’s collection of moons, having been severely disrupted by the arrival of Triton and other satellites captured by Neptune’s gravity over the years,” Fletcher wrote in an email. “Given those destructive processes, I don’t think we expected to see anything left of Neptune’s original satellite system, other than rubble and debris.”

But James Webb data shows otherwise, and more work with the telescope could reveal finer scale features and maybe strengthen the case for Nereid as an original satellite, he added. “It’s a compelling idea, and something that can certainly be tested with future JWST observations, and hopefully with an ambitious future mission to the Neptune system.”

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