Neptune’s third-largest moon Triton may be the only intact survivor of a group of ancient moons that were destroyed early in the solar system’s history, according to a new analysis of data from the James Webb Space Telescope.
Neptune, the eighth and farthest planet from the sun, stands out among the solar system’s outer planets for its peculiar constellation of moons. The other outer giants—Uranus, Saturn, and Jupiter—all have roughly similar, ordered sets of moons, including several larger moons that rotate in the same direction as the parent planet.
But Neptune’s satellite constellation is much smaller and more chaotic: Neptune’s largest moon, Triton, dwarfs all the others and orbits in the opposite direction to its host’s rotation. It is the only large moon in the solar system that can do this.
Astronomers suspect that the reason for Triton’s strange behavior is that it does not originate from remnants of Neptune’s formation, which would cause it to orbit in the same direction as Neptune. Instead, they hypothesize that Triton may have originated in the Kuiper Belt, a ring of icy bodies at the edge of the solar system, and entered the Neptune environment 4 billion years ago.
Previous research has suggested that Triton may have been captured by Neptune’s gravity after a close pass and slammed inward, crashing into Neptune’s primitive satellite system.
Triton, a mosaic image taken by Voyager 2 in 1989, is the largest moon of Neptune. – NASA/JPL/USGS
If Neptune does have a primordial set of moons more similar to its planetary neighbor, then the arrival of Triton – which is only smaller than our own moons – could wreak havoc, crashing into other moons and wiping out some of them. Current characteristics of the Neptune system support this scenario, with its seven inner moons appearing to be remnants of this ancient conflict.
But now, new research using data from the James Webb Space Telescope shows that one object may be completely untouched by the chaos.
“I think Poseidon is the only intact survivor of this process,” said Matthew Belyakov, a graduate student in planetary science at Caltech and lead 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 complete because we have images from Voyager where they look like broken piles of rubble. So they are surviving material from the original system, but they are not complete moons.”
This hypothesis would upend previous assumptions that Triton, like Triton and some other Neptune moons, is a captured Kuiper Belt object, because new James Webb data show that Triton’s composition does not match scientists’ knowledge of Kuiper Belt objects.
pixelated image
Astronomers don’t know much about Neptune because it is so faint and so far away from both the Earth and the Sun. The only image scientists have of it is a blurry shot taken by NASA’s Voyager 2 spacecraft during a brief flyby of Neptune in 1989. Triton is the outermost of Neptune’s known moons, and its orbit is one of the most eccentric (i.e., non-circular) in the solar system. The moon takes 360 Earth days to orbit Neptune.
Voyager 2 photo of Neptune, taken during the space probe’s 1989 flyby of Neptune. – NASA
Nereid, named after the sea nymph in Greek mythology, is believed to be about 210 miles (338 kilometers) in diameter. Even if it were part of a primitive set of moons shortly after Neptune formed, about 4.5 billion years ago, it’s difficult to speculate on what the system might have looked like, Belyakov said. “Everyone is guessing what existed before Triton,” he said.
Like Triton, Triton is an irregular satellite, a class of objects whose orbits are tilted, backward, or away from their host, indicating that they were captured by their host and previously orbited the Sun independently.
But even among irregular moons, Poseidon is an outlier, Belyakov said. “It is twice the diameter of the next largest moon (Phoebe around Saturn), and compared to many other irregular moons, it is not that far away from its host planet.” He added that some features of Triton have long led some astronomers to doubt the moon’s Kuiper Belt origin.
The study, which commissioned 10 minutes and 40 seconds of observations from James Webb’s infrared capabilities, which can reveal the composition of distant objects, confirmed those suspicions.
“What we found is an object with a water-rich surface, is brighter than many Kuiper Belt objects, and has some carbon dioxide present,” Belyakov said. “The overall characteristics are more similar to conventional satellites around Uranus than to Kuiper Belt objects.” The results were compared with data from 54 Kuiper Belt objects, also derived from James Webb’s observations.
Belyakov and his colleagues then conducted computer simulations to test the hypothesis that Triton was part of a primitive lunar system. “We found in our simulations that in the cases where Triton survived, rather than being destroyed or kicked into Neptune,” Belyakov said, “about 25 percent of the time, one or more satellites were able to survive encounters with Triton in distant orbits.” He added that this number compared favorably with the likelihood of Poseidon being captured.
In this scenario, Triton would have slammed into the Neptune system, colliding with many of the primitive moons during the first 100 million or 200 million years of the solar system’s history. But Poseidon might survive and be sent into an eccentric orbit. The event would also slow Triton’s own eccentric orbit, bringing it closer to Neptune along its current path.
“I think people already want this to be true,” Belyakov said of a possible new origin story for the sea god Nerid. “Now we can start the actual science feedback loop. Compositionally, there’s a lot more data to be collected about Trina, which can help us really understand the formation of the Neptune system, and if we treat Trina as a normal moon, maybe that can tell us a lot about how moons form around ice giants.”
Further observations by James Webb may be helpful, but a mission to Neptune will be needed to finally understand what the star is really like. There are currently no plans, and Voyager 2, launched in 1977, is the only spacecraft to study the system.
Voyager 2 is the only spacecraft to visit Uranus and Neptune. – NASA
“A compelling idea”
Carolyn Porco, an American planetary scientist who worked on NASA’s Voyager and Cassini missions but was not involved in the new study, said the new study is a lovely, simple look at how the Neptune satellite system came to be what it is today.
“The captured object was Triton, which subsequently wreaked havoc, with Neptune’s original moons being gravitationally dispersed everywhere but mostly out of Neptune’s orbit,” Porco wrote in an email. “The authors show that Triton was lucky to stay in orbit around Neptune, but at a much greater distance than Triton. This could explain why its composition as observed by James Webb does not match the composition of objects in the Kuiper Belt.”
Leigh Fletcher, a professor at the School of Physics and Astronomy at the University of Leicester in the United Kingdom, who was also not involved in this work, said that James Webb once again demonstrated his great power as a solar system explorer.
“We have long known that there is something special about Neptune’s moon collection, which has been severely disrupted over the years by the arrival of Triton and other moons captured by Neptune’s gravity,” Fletcher wrote in an email. “Given these destructive processes, I don’t think we would expect to see anything left of Neptune’s original satellite system other than rubble and debris.”
But James Webb’s data suggests otherwise, he adds, and more studies with telescopes could reveal finer-scale features and potentially strengthen Triton’s case as a primitive moon. “This is a compelling idea that can certainly be tested with future JWST observations and, hopefully, ambitious future Neptune system missions.”
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