Saturnian Irregular Satellites as a Probe of Kuiper Belt Surface Evolution

Abstract We present the JWST NIRSpec PRISM 0.7–5.3 μ m spectra of Albiorix and Siarnaq and the NIRSpec G235H/G395M 1.7–5.3 μ m spectra of Phoebe, the three largest Saturnian irregular satellites. The irregular satellites of the giant planets are thought to be captured planetesimals from the same population as Kuiper Belt objects (KBOs). They are emplaced inside Saturn’s Hill sphere during the giant-planet instability described by the Nice model and are thus valuable tracers of Kuiper Belt surface evolution. Phoebe’s JWST spectrum matches the global average from Cassini VIMS, and by comparing the spectrum to the library of KBO spectra from JWST, we demonstrate Phoebe’s compositional similarity to water-rich KBOs. On the smaller Albiorix and Siarnaq, we observe a broad 3 μ m O–H band but do not see a Fresnel peak or the 1.5/2.0 μ m features characteristic of H 2 O ice. We posit that after capture, the frequent high-velocity collisions between smaller irregular satellites sublimate the water ice, while the much larger Phoebe is resistant to disruption and retains its water ice. We suggest that the presence of CO 2 on the smaller satellites, despite the lack of water ice, indicates later formation of CO 2 on these surfaces through irradiation of organic compounds.