TNO2024

54 TNOs have been observed with NIRSpec during the large JWST Cycle 1 General Observers Program “Discovering the Surface Composition of trans-Neptunian Objects” (DiSCo-TNOs). Both $\mathrm{CO}{}_2$ and $\mathrm{CO}$ have been firmly detected, respectively, on 53 and 28 TNOs (De Prá et al. 2024). TNO surfaces fall into 3 spectral groups (Pinilla-Alonso et al. 2024) and $\mathrm{CO}{}_2$ is seen across all of them while $\mathrm{CO}$ is only detected in the “Double-dip” spectral type, rich in $\mathrm{CO}{}_2$, and in the “Cliff” spectral type, more chemically complex. Thermodynamic considerations (e.g. Lisse et al. 2021) predicted TNOs of the sizes investigated here (140 to 800 km) to be depleted in hyper volatiles like $\mathrm{CO}$, unless it is stored in a more refractory ice or steadily produced.

We investigated the origin of the observed $\mathrm{CO}$ by studying its relationship with $\mathrm{CO}{}_2$ in objects from the “Double-dip” and “Cliff” spectral types. We determined band parameters of the 2.7 μm $\mathrm{CO}{}_2$ combination band and the 4.68 μm $\mathrm{CO}$ fundamental band. We found that in the “Cliff“ group, $\mathrm{CO}$ band depth is linearly related to that of $\mathrm{CO}{}_2$, while in “Double-dip” spectra, $\mathrm{CO}$ band depth doesn't correlate with that of $\mathrm{CO}{}_2$. This behavior hints at different chemical pathways in each spectral group. We explored the possible irradiation origin of $\mathrm{CO}$ by performing ion irradiation experiments on $\mathrm{CO}{}_2$ and $\mathrm{CH}{}_3\mathrm{OH}$ ices with the INGMAR experimental setup (IAS/IJCLab, France). We found that the relative abundance of $\mathrm{CO}$ and $\mathrm{CO}{}_2$ in “Double-dip” spectra matches that of irradiated $\mathrm{CO}{}_2$. Whereas, in the “Cliff” group, the relative abundance of $\mathrm{CO}$ and $\mathrm{CO}{}_2$ is closer to that of irradiated $\mathrm{CH}{}_3\mathrm{OH}$. We will discuss the implications of this scenario with the goal of understanding the primordial composition of TNOs.