Neptune's Inner Moons and Rings Are Exposed Icy Body Interiors

Abstract

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. Remarkably, 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 micron OH absorption bands. Additionally, Larissa, Galatea, and the rings show a compositional signature unique among outer solar system bodies: a 2.72 micron 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.