Fig. 4. The nature of the 3.5-μm feature.

a JWST observations of Charon (black points) and Europa (normalized to Charon’s I/F level at 2.52–2.54 μm, cyan points) are shown alongside infrared absorption studies detailing the effects of 5 keV electron irradiation on CO₂ diluted in H₂O ice films. Two specific irradiation fluences are shown: 9.00 ×  10¹⁵ e⁻ cm⁻² (dotted magenta) and 9.30 × 10¹⁷ e⁻ cm⁻² (dashed magenta line). The red lines show the baseline continuum fit. b Spectra in panel a, with continuum removed, are compared to the spectrum of H₂O₂ resulting from 100 keV H⁺ irradiation of pure H₂O ice³⁰ (yellow solid line) and the spectrum of Europa from Galileo/NIMS³¹ (red crosses). All spectra are normalized to Charon’s band depth. c The continuum-removed spectra of Charon (gray) and Europa (cyan), each maintaining their native band depths. Source data are provided as a Source Data file.