Irradiation-induced formation of CO at the surface of TNOs revealed by JWST/DiSCo-TNOs
Session 1.11 Spectroscopy
Monday 06-24 | 14:30 - 14:50

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 CO2 and 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 CO2 is seen across all of them while CO is only detected in the “Double-dip” spectral type, rich in CO2, 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 CO, unless it is stored in a more refractory ice or steadily produced.

We investigated the origin of the observed CO by studying its relationship with CO2 in objects from the “Double-dip” and “Cliff” spectral types. We determined band parameters of the 2.7 μm CO2 combination band and the 4.68 μm CO fundamental band. We found that in the “Cliff“ group, CO band depth is linearly related to that of CO2, while in “Double-dip” spectra, CO band depth doesn't correlate with that of CO2. This behavior hints at different chemical pathways in each spectral group. We explored the possible irradiation origin of CO by performing ion irradiation experiments on CO2 and CH3OH ices with the INGMAR experimental setup (IAS/IJCLab, France). We found that the relative abundance of CO and CO2 in “Double-dip” spectra matches that of irradiated CO2. Whereas, in the “Cliff” group, the relative abundance of CO and CO2 is closer to that of irradiated CH3OH. We will discuss the implications of this scenario with the goal of understanding the primordial composition of TNOs.

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