Extended Data Fig. 4: Observed absorption profile of the ¹³CO₂ asymmetric stretching, around 4.39 μm, in NIR38 (left panel) and J110621 (right panel).

To demonstrate the ice chemical environment that best reproduces the observed feature peak, the coloured curves show the scaled profiles of ¹³CO₂ in laboratory spectra of the following ice mixtures at 10 K: pure CO₂ (blue), H₂O:CO₂ (orange), CO₂:CO (green), and CO₂:CH₃OH (red). In all the ice mixtures, CO₂ is diluted in a ratio of ~ 1:10, with ¹²CO₂/¹³CO₂ ~ 90.