NASA’s James Webb Space Telescope Reveals Unprecedented Insights into Neptune’s Inner Moons

In a groundbreaking study, researchers using NASA’s James Webb Space Telescope (JWST) have identified a unique chemical composition in Neptune’s inner moons—Larissa, Galatea, and Proteus—that challenges existing notions about the planet’s satellite system. This research, led by a team from Caltech, builds on findings from the 1989 Voyager 2 mission, which discovered six moons around Neptune, including five tiny satellites just outside the planet’s rings.

The team’s observations suggest that Neptune may have once hosted a significantly different array of moons, which were likely destroyed when Triton, Neptune’s largest moon, was captured by the planet’s gravitational pull after forming elsewhere in the solar system. Lead author and former Caltech graduate student Ryleigh Davis (PhD ’26) notes, “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.” This phenomenon may provide vital clues to the planet’s tumultuous history.

Unexpected Discoveries in Composition

The study utilized JWST’s near-infrared spectrograph to analyze the moons’ surfaces, a feat previously unachievable due to the moons’ small size and distant position. The analysis revealed unexpected signatures of magnesium-rich phyllosilicates—clays that require liquid water to form. Remarkably, however, the spectra of Larissa, Galatea, and Proteus showed no evidence of water ice, raising questions about the origins of these minerals.

“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,” said Davis, currently 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.” The discovery indicates that these clays likely originated from deeper inside a larger icy body that generated heat sufficient to melt water ice.

Proteus and Further Mysteries

Interestingly, Proteus did not exhibit the same phyllosilicate signature as Larissa and Galatea, suggesting that it may have formed from different material or underwent later heating that destroyed any clay minerals that were initially present. All three moons displayed a recurring unidentified hydrated mineral, compounding the enigma of their formation.

“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 states. “We assume it’s some form of hydrated rock from the moons as well, but there’s still a lot of mystery.” While initial findings point towards the destruction of Neptune’s original moons, researchers have not ruled out the possibility that material could have originated from a large Kuiper Belt object that was disrupted by Neptune’s gravity.

Towards a New Understanding of Neptune’s Moons

The findings lead to numerous questions regarding the destruction of Neptune’s original satellites and the evolution of their debris. Davis added, “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.” Understanding the dynamics of this destruction and subsequent moon formation will be crucial for reconstructing the history of Neptune’s moons.

The research paper, titled “Neptune’s Inner Moons and Rings Are Exposed Icy Body Interiors,” features contributions from postdoctoral scholar Zachariah Milby (PhD ’26) and former Caltech graduate student Ian Wong (PhD ’18), now at the Space Telescope Science Institute. Funding for the project was provided by NASA through a grant to the Space Telescope Science Institute.