Neptune's inner moons and rings are exposed icy body interiors.

Neptune's single large moon, Triton, is accompanied by a set of dusty rings and small moons whose composition and origin are uncertain. Using the James Webb Space Telescope, we observed Neptune's rings and three moons: Larissa, Galatea, and Proteus. These moons and rings have spectra that are distinct from other outer Solar System objects and show no evidence of water ice despite hosting deep 3-micrometer OH absorption bands. In addition, Larissa, Galatea, and the rings show a compositional signature unique among outer Solar System bodies: a 2.72-micrometer absorption band diagnostic of magnesium-rich phyllosilicates-a signpost of extensive aqueous alteration. These minerals likely formed in the interior of primordial satellites destroyed during Triton's violent capture or a tidally shredded dwarf planet. Our findings suggest that Neptune's present-day inner moons and rings reaccreted from this deep interior material and therefore uniquely access the interior composition of icy outer Solar System differentiated bodies.