Irradiation origin and stability of CO on trans-Neptunian objects

Abstract

Context. The James Webb Space Telescope large program DiSCo-TNOs has recently shown that CO_{2 ice is ubiquitous on 54 mediumsize trans-Neptunian objects (TNOs). TNO surfaces are found to define three main spectral and thus compositional groups that are likely linked to their position before planetary migration. CO ice is observed on the spectral type that is richest in CO2 and on the type that is richer in CH3OH and organics. Considerations on the thermal evolution of TNOs predicted the depletion of hypervolatiles such as CO from their surface layers, however.Aims. We investigate a potential irradiation origin of CO as well as its stability by studying the distribution of CO in two TNO compositional types and compared it with irradiation experiments.Methods. We studied the 4.68 µm band of CO and the 2.70 µm band of CO2 to probe the relation between the two molecules in 33 TNOs. We performed ion irradiation experiments on CO2 and CH3OH ices at 45 and 60 K with 30 keV H^{+ . We compared the laboratory spectra to TNO observations by focusing on the band areas and positions.Results. We find that the two types of surfaces in which CO is detected are very distinct in terms of their relative abundances and chemical environment. CO that is observed on surfaces that are rich in CO2 are consistent with being produced by CO2 irradiation, specifically, at 45 K. On objects that are rich in CH3OH and complex organics, CO is more likely formed by irradiation of CH3OH. As the CO band areas are only partly related with temperature, the chemical environment plays a major role in the CO retention.Conclusions. We find that the CO that is observed on TNO surfaces is compatible with being a secondary molecule that is entirely formed by late irradiation processes. Its abundance and stability is mostly controlled by the matrix from which it formed.}}