In 1989, Voyager 2 discovered six previously unknown moons around Neptune — five of them tiny satellites orbiting just beyond the planet's main rings. Their small size and remote location have made them almost impossible to study from Earth. Now, NASA's James Webb Space Telescope has given scientists their first real look, and what it revealed points to a catastrophe that shattered an entire family of ancient worlds.
A Caltech-led team used Webb's near-infrared spectrograph to observe Neptune's rings and three of its inner moons — Larissa, Galatea and Proteus. The spectra of Larissa, Galatea and the rings showed signs of magnesium-rich phyllosilicates: clay minerals that only form in the presence of liquid water. Yet the observations found no water ice on any of the three moons or in the rings — a combination never seen before in the outer solar system. "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 Ryleigh Davis, lead author of the study published in Science Advances.
Because clays of this kind require a large, heated interior to form, the team argues the material must have come from deep inside much bigger bodies — likely an original system of icy moons that was destroyed when Triton, Neptune's largest moon, was captured by the planet's gravity after forming elsewhere in the solar system. "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," said Davis, who conducted her graduate work in the lab of Mike Brown at Caltech.
Proteus, the largest of the small moons studied, showed no clay signature — possibly because it re-accreted from a different part of the debris disk or was heated later. The team also detected an unidentified hydrated mineral on all three moons that "doesn't match anything we have in our spectral libraries," Davis said. An alternative explanation — the tidal shredding of a Pluto-sized Kuiper Belt object that passed too close to Neptune — cannot yet be ruled out, but either way the material had to originate far below the surface of a much larger body.
That makes Neptune's inner moons uniquely valuable: they are the only place in the solar system where scientists can directly examine the deep interior composition of a large icy world. "A catastrophic event essentially turned these ancient moons inside out, and we get to see what was hidden inside," Davis said. A companion study from the same JWST research program, led by Caltech's Matthew Belyakov, independently suggests Neptune's moon Nereid may be the sole surviving member of the original satellite system.
