Researchers using the James Webb Space Telescope have detected magnesium-rich phyllosilicates — clay minerals that require liquid water to form — on Neptune's inner moons Larissa and Galatea and in the planet's rings. The findings, published July 29 in Science Advances, mark the first detection of phyllosilicates in the outer solar system beyond Jupiter.
Lead author Ryleigh Davis, a former Caltech graduate student now at UC San Diego, said the team was shocked to find the clay signatures because no water ice appears in the spectra of the three moons studied or their rings. Everything in this region of the solar system is typically icy, so the minerals likely originated deep inside bodies large enough to generate heat and melt their internal ice.
The researchers propose that Neptune once hosted a system of large, ordered moons similar to those at Uranus. When Triton, a captured Kuiper Belt object, arrived, its gravitational disruption demolished that original system. Debris from the shattered moons reaccreted into the small inner moons and rings observed today, exposing material from the deep interiors of the ancient worlds.
An alternative scenario — the tidal shredding of a single large, differentiated Kuiper Belt object passing near Neptune — cannot be ruled out. In either case, the observed minerals had to come from deep inside something much larger than the current inner moons, which are only tens to hundreds of kilometers across.
Proteus, the largest of the small moons studied, lacks the phyllosilicate signatures, suggesting it may have reaccreted from a different region of the debris disk or experienced later heating that destroyed any clays. All three moons also share an unidentified hydrated mineral not matching any known spectral library.
Mike Brown, Caltech professor of planetary astronomy and co-author, described the discovery as something that "hits you in the face" rather than confirming a pre-existing hypothesis. Davis noted that Neptune's inner moons are now the only place in the solar system where the deep interior composition of a large icy world can be observed directly, because a catastrophic event effectively turned those ancient moons inside out.
The team estimates only about 1 percent of the original satellite material remained in the system after Triton's capture. Future work will explore the dynamics of the destruction and reaccretion process and the minimum size the original moons must have been to produce the observed minerals.
Neptune's Inner Moons May Be Shattered Remains of Ancient Icy Worlds
This is an independent summary. The complete reporting, supporting context and any primary documents remain with Caltech News.