The State of CO and CO₂ Ices in the Kuiper Belt as Seen by JWST

JWST has shown that CO₂ and CO are common on the surfaces of objects in the Kuiper Belt and have apparent surface coverages even higher than that of water ice, though water ice is expected to be significantly more abundant in the bulk composition. Using full Mie scattering theory, we show that the high abundance and the unusual spectral behavior around the 4.26 μm ν₁ band of CO₂ can be explained by a surface covered in a few micron thick layer of ∼1–2 μm CO₂ particles. CO is unstable at the temperatures in the Kuiper Belt, so the CO must be trapped in some more stable species. While hydrate clathrates or amorphous water ice are often invoked as a trapping mechanism for outer solar system ices, the expected spectral shift of the absorption line for a CO hydrate clathrates or trapping in amorphous ice is not seen, nor does the H₂O abundance appear to be high enough to explain the depth of the CO absorption line. Instead, we suggest that the CO is created via irradiation of CO₂ and trapped in the CO₂ grains during this process. The presence of a thin surface layer of CO₂ with embedded CO suggests volatile differentiation driving CO₂ from the interior as a major process driving the surface appearance of these mid-sized Kuiper Belt objects, but the mechanisms that control the small grain size and depth of the surface layer remain unclear.