Neptune's Inner Moons May Be Shattered Remains of Ancient Icy Worlds

In 1989, the Voyager 2 mission discovered six new moons orbiting Neptune, including five tiny ones that orbit just outside the planet's main rings. Because of their small sizes and locations, these satellites have been difficult to study from Earth. Now, using NASA's James Webb Space Telescope (JWST), a team of Caltech researchers has observed Neptune's rings and three of those moons—Larissa, Galatea, and Proteus—and found that their composition is unique among outer solar system bodies.

These findings point to the existence of an original moon system that was demolished when Triton, Neptune's biggest moon, was likely captured by the planet's gravity after it formed elsewhere in the solar system. The researchers believe remnants left behind from this smashup then came together to form the inner moons seen today.

"If Neptune once had a system of moons that looked something like what we see at Uranus today, we expect it would've been completely destroyed by the process of Triton getting captured," says former Caltech graduate student Ryleigh Davis (PhD '26), lead author of a paper about the team's findings publishing July 29 in Science Advances. "This is exciting new evidence that something catastrophic happened at Neptune that completely destroyed its original satellites, and we're getting to see the fingerprints left behind by that process."

Neptune is a curious planet, as it is the only one in the solar system that lacks a "typical" moon system of large, ordered satellites, meaning its evolutionary pathway might also be unique. In fact, another recent study from the same JWST research program, and led by Caltech postdoctoral scholar Matthew Belyakov (PhD '26), provides independent evidence for the existence of the original satellite system, suggesting that Neptune's moon Nereid may be the sole surviving intact member.

To look for more clues that could help reconstruct Neptune's history, Davis, who did her graduate studies in the lab of Mike Brown, Caltech's Richard and Barbara Rosenberg Professor of Planetary Astronomy, and the team turned to the planet's inner moons.

Until recently, no spectroscopy data existed for the moons. Spectrographs such as the near-infrared instrument on JWST split light into its many wavelengths to obtain information about the chemical makeup of astronomical targets, enabling scientists to identify the molecules present.

In collaboration with Belyakov, Davis designed and co-led a research program using JWST data to determine the composition of the satellites to see if the team could learn more about how the objects might have formed.

"Phyllosilicates had never been detected anywhere in the outer solar system beyond Jupiter, so that was not on our list of things to look for," says Davis, who is now a postdoctoral researcher at UC San Diego. "We were shocked to find the observed clays, which had to come from objects that were much, much bigger than Neptune's small inner ring moons."

One interesting result, Davis says, is that signatures of magnesium-rich phyllosilicates were found in the spectra of Larissa, Galatea, and the rings—minerals that only form in the presence of liquid water. And yet, no water ice is indicated by the spectra of any of the three moons studied or their rings.

"That's really surprising because everything out in this part of the solar system is really icy," she says. "So, we're fairly confident that they had to come from deep inside something that was big enough to generate enough heat that it melted its water ice. We think the most likely place would be an original system of icy moons, although it's a bit of a mystery where the ice may have gone."

This diagram illustrates the proposed origin for Neptune's present-day inner moon and ring system. Neptune once had a system of large moons that were destroyed when Triton—a captured Kuiper Belt object—arrived. The destruction exposed clay minerals from deep inside these ancient worlds, and a small fraction of the debris reaccreted into the small inner moons and rings we observe today. Credit: R. Davis

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This diagram illustrates the proposed origin for Neptune's present-day inner moon and ring system. Neptune once had a system of large moons that were destroyed when Triton—a captured Kuiper Belt object—arrived. The destruction exposed clay minerals from deep inside these ancient worlds, and a small fraction of the debris reaccreted into the small inner moons and rings we observe today.

Credit: R. Davis

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Brown, who is also the Terence D. Barr Leadership Chair and director of the Caltech Center for Comparative Planetary Evolution, remembers everyone on the team having the same big question when they saw the spectra of the satellites: "What is that?"

"It took diligent detective work from Ryleigh before we understood what we were seeing," he says. "Sometimes in science you are trying to find evidence to evaluate a specific hypothesis, and, sometimes, something that you had not been thinking about just hits you in the face."

Furthermore, the phyllosilicates were not present on Proteus, the largest of the small moons investigated, suggesting it may have reaccreted from a different region of the debris disk or undergone subsequent heating that destroyed any clay minerals present. In addition, the team found that each of the three moons has the same hydrated mineral that the team has not been able to identify.

"We see something that doesn't really look like anything else we've identified in the solar system; it doesn't match anything we have in our spectral libraries." Davis says. "We assume it's some form of hydrated rock from the moons as well, but there's still a lot of mystery."

While the team favors the primordial satellite destruction scenario, they note that an alternative explanation cannot be ruled out: the tidal shredding of a large, differentiated Kuiper Belt object, similar in size to Pluto, that passed too close to Neptune and was torn apart by the planet's gravity.

"Either way, what we're seeing on these moons had to come from deep inside something much larger," says Davis, noting that Neptune's inner moons are the only place in the solar system where we can look directly at the deep interior composition of a large icy world. "That material is normally permanently buried—we can only infer what's there. Here, a catastrophic event essentially turned these ancient moons inside out, and we get to see what was hidden inside."

She notes that the team's findings leave more open questions than answers; future projects, she says, might look at the dynamics of the evolution of the moon-destruction and recreation process.

"If you bring Triton in, and you smash up your large moons, we think only 1 percent or so of that material stayed around in the system," Davis says. "But the actual behavior of that material might be really different if Triton is still there shaking things up for a long time. So, looking forward, understanding how that process actually proceeds would be interesting. From there, the question is: 'Can we learn anything about how big the initial moons had to be to have formed and provided this material?'"

The paper is titled "Neptune's Inner Moons and Rings Are Exposed Icy Body Interiors." In addition to Belyakov, Brown, and Davis, postdoctoral scholar Zachariah Milby (PhD '26) and former Caltech graduate student Ian Wong (PhD '18), now with the Space Telescope Science Institute in Baltimore, Maryland, are co-authors. Funding was provided by NASA through a grant to the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy.