Nereid was a Regular Satellite of Neptune

Neptune is the only giant planet that lacks a regular satellite system. Triton constitutes over 99% of the Neptunian satellite system's mass and follows a retrograde and synchronous orbit, suggesting an early capture from the protoplanetary disk followed by tidal circularization. Discovered in 1949 by G. P. Kuiper, Nereid is the third largest Neptunian satellite. While typically classified as an irregular satellite due to its eccentric and inclined orbit, Nereid is unique among this population. Of the irregular satellites, Nereid has the lowest pericenter (0.012 Hill Radii), the highest mean eccentricity (e = 0.75), the lowest prograde inclination relative to the ecliptic of 6 degrees, and the largest radius at 175km -- twice as large as the next largest irregular satellite, Saturn's Phoebe. Its compositional properties also stand out: it is bluer and higher albedo (0.25) than similarly-sized Centaurs and Kuiper belt objects. We present JWST near-infrared spectroscopy of Nereid, finding it has a distinct composition among the giant planet satellites and small bodies observed with JWST NIRSpec thus far, with abundant water ice and a notable blue slope in the near-infrared that is unlike that of any Kuiper belt object. The uniqueness of Nereid's spectrum suggests the moon was not captured during the giant planet instability from the same population that formed the Kuiper belt. Instead, we propose that Nereid was once a regular satellite. By simulating the capture of Triton alongside an initially regular Nereid, we show the existence of a plausible dynamical pathway in which Nereid was initially a regular satellite that Triton perturbed onto its current orbit during a phase of rapid tidal circularization.