Neptune's tiny moons tell the story of Triton's destructive capture

Neptune's moon system, including its one large moon, Triton, and many smaller moons, exhibits a puzzling imbalance. Triton, the largest moon, orbits in the opposite direction of Neptune's rotation and comprises over 99% of the planet's satellite mass. A new study in Science Advances suggests that Triton's capture from the Kuiper Belt caused significant disruption in Neptune's early moon system.

Spectroscopic evidence from the James Webb Space Telescope's NIRSpec detected clay-like minerals on two small moons, Larissa and Galatea, along with Neptune's rings. The presence of magnesium-rich phyllosilicates, which require liquid water for formation, is unusual as these moons and rings are too cold for liquid water near their surfaces.

The study authors propose that the moons and rings could be debris from the cores of larger, primordial moons that were shattered during Triton's capture. This would indicate that Neptune's early moon system may have contained differentiated, heated cores, similar to icy satellites or even dwarf planets. The findings suggest that outer solar system materials can undergo mineralogical evolution akin to that observed in main-belt asteroids when heated sufficiently.