JWST Reveals the Kuiper Belt-like Compositions of the Uranian Rings and Small Moons

JWST NIRSpec collected 1–5 micron spectra of the Uranian rings and four ringmoons: Puck, Perdita, Portia, and Juliet. The observations show that the compositions of the rings and moons strongly resemble water-type Kuiper belt objects, with deep 3.0 micron O–H and water ice features as well as the CO2 band at 4.27 micron.

The volatiles show a radial trend: volatile abundance decreases at closer radial distances to the planet, a result consistent with recent estimates of the densities of the innermost moons.

Given these compositional signatures, the authors suggest two ways to reconcile the history of the small inner moons with that of the regular moons. The first is that the Uranus-tilting collision produced a disk from an impactor formed out of material similar to water-rich Kuiper belt objects, and the ringmoons are small undifferentiated remnants of that process. Alternatively, the rings and ringmoons postdate the formation of the regular moons and formed from the later tidal disruption of a several-hundred-kilometer-sized undifferentiated water-type Kuiper belt object.