Fig. 3. Possible CO₂ ice Fresnel peak at 4.265 μm.

a, b Comparison of the JWST/NIRSpec grand-average spectrum of Charon (black points with 1-σ errors) with a spectral model (red line), which consists of an areal mixture of two surface units (refer to text and “Spectral modeling” section in the Methods for detailed explanation). c, d The reflectance of each component within the model is shown. The model effectively captures the primary spectral features of Charon’s spectrum, though the match is not perfect. The goal of this model is to support the potential interpretation of the 4.265-μm peak as a CO₂ ice Fresnel peak originating from the interface between two layered media, with crystalline CO₂ ice and amorphous H₂O ice on top of a crystalline H₂O ice and tholins layer. Source data are provided as a Source Data file.