Carbon dioxide (CO2) is one of the most abundant ices in the solar system. It has been detected in giant planet atmospheres and on their moons, on and around comets, and even in regions of Mercury, the Moon, and Mars. However, despite their formation in the coldest regions of the protoplanetary disk, CO2 was not previously detected on trans-Neptunian objects (TNOs). Carbon monoxide (CO) was only expected to be present in the largest TNOs. Out of 45 total, we report the detection of both CO2 and CO in 42 and 24 TNOs, respectively, observed with the NIRSpec instrument on the JWST by the DiSCo-TNOs project (PID 2418). Our analysis shows that \diox is widespread in the trans-Neptunian region, independent of dynamical class, while CO is a common constituent of the reddest objects in our sample. The abundance and characteristics of \diox suggest that most of it could be primordially accreted. While, CO is probably being produced by ion irradiation of CO2 and other C-bearing species. Our results suggest the prevalence of two compositional groups among TNOs based on the abundance and properties of CO2 and CO. We discuss the implication of these results in the context of solar system formation.
Context. Cosmic rays and solar energetic particles induce changes in the composition of compounds frozen onto dust grains in the interstellar medium (ISM), in comets, and on the surfaces of atmosphere-less small bodies in the outer Solar System. This induces the destruction of pristine compounds and triggers the formation of various species, including the precursors of complex organics. Aims. We investigate the role of energetic ions in the formation of formaldehyde (H 2 CO) and acetaldehyde (CH 3 CHO), which are observed in the ISM and in comets, and which are thought to be the precursors of more complex compounds such as hexamethylenetetramine (HMT), which is found in carbonaceous chondrites and in laboratory samples produced after the irradiation and warm-up of astrophysical ices. Methods. We performed ion irradiation of water, methanol, and ammonia mixtures at 14-18 K. We bombarded frozen films with 40-200 keV H + that simulate solar energetic particles and low-energy cosmic rays. Samples were analysed by infrared transmission spectroscopy.Results. Among other molecules, we observe the formation of H 2 CO and CH 3 CHO, and we find that their abundance depends on the dose and on the stoichiometry of the mixtures. We find that the H 2 CO abundance reaches the highest value after a dose of 10 eV/16u and then it decreases as the dose increases. Conclusions. The data suggest that surfaces exposed to high doses are depleted in H 2 CO. This explains why the amount of HMT in organic residues and that formed after irradiation of ices depends on the dose deposited in the ice. Because the H 2 CO abundance decreases at doses higher than 10 eV/16u, a lower quantity of H 2 CO is available to form HMT during the subsequent warm-up. The H 2 CO abundances caused by ion bombardment are insufficient to explain the ISM abundances, but ion bombardment can account for the abundance of CH 3 CHO towards the ISM and comets.
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