Astronomers believe Neptunian moon is lone intact survivor of ancient collision - CNN

- New data from the James Webb Space Telescope suggests Neptune moon Nereid survived an ancient collision that destroyed the planet's original moon system.

- Researchers found Nereid's composition doesn't match Kuiper Belt objects, challenging long-held assumptions about the moon's origin.

- Computer simulations show that when Triton entered the Neptunian system over 4 billion years ago, Nereid likely escaped intact.

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.

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.