Organic macromolecular matter is the dominant carrier of volatile elements such as carbon, nitrogen and noble gases in chondrites—the rocky building blocks from which Earth formed. How this macromolecular substance formed in space is unclear. Here we show that its formation could be associated with the presence of dust traps, which are prominent mechanisms for forming planetesimals in planet-forming disks. We demonstrate the existence of heavily irradiated zones in dust traps, where small frozen molecules that coat large quantities of microscopic dust grains could be rapidly converted into macromolecular matter by receiving radiation doses of up to several tens of electronvolts per molecule per year. This allows for the transformation of simple molecules into complex macromolecular matter within several decades. Up to roughly 4% of the total disk ice reservoir can be processed this way and subsequently incorporated into the protoplanetary disk midplane where planetesimals form. This finding shows that planetesimal formation and the production of organic macromolecular matter, which provides the essential elemental building blocks for life, might be linked.

Context. Recent observations of protoplanetary disks suggest that they are depleted in gas-phase CO up to a factor of 100 with respect to predictions from physical-chemical (or thermo-chemical) models. It has been posed that gas-phase CO is chemically consumed and converted into less volatile species through gas-grain processes. Observations of interstellar ices reveal a CO2 component in a polar (H2O) ice matrix, suggesting potential co-formation or co-evolution. Aims. The aim of this work is to experimentally verify the interaction of gas-phase CO with solid-state OH radicals on the surface of water ice above the sublimation temperature of CO. Methods. Amorphous solid water (ASW) is deposited in an ultra-high vacuum (UHV) setup at 15 K and irradiated with vacuum-UV (VUV) photons (140-170 nm, produced with a microwave-discharge hydrogen-flow lamp) to dissociate H2O and create OH radicals. Gas-phase CO is simultaneously admitted and only adsorbs with a short residence time on the ASW. Formed products in the solid state are studied in the infrared through Fourier transform infrared spectroscopy and once released into the gas phase with quadrupole mass spectrometry. Results. Our experiments show that gas-phase CO is converted into CO2 when interacting with ASW that is VUV irradiated with a conversion efficiency of 7-27%. Between 40 and 90 K, CO2 production is constant, above 90 K, CO2 production is reduced in favor of O2 production. In the temperature range of 40-60 K, the CO2 remains in the solid state, while at temperatures 70 K the majority of the formed CO2 is immediately released into the gas phase. Conclusions. We conclude that gas-phase CO reacts with OH radicals, created on the surface of ASW with VUV irradiation, above its canonical sublimation temperature. The diffusion during the short, but nonzero, residence times of CO on the surface of ASW suggests that a Langmuir-Hinshelwood type reaction is involved. This gas-phase CO and solid-state OH radical interaction could explain (part of) the observed presence of CO2 embedded in water-rich ices when it occurs during the build up of the H2O ice mantle. It may also contribute to the observed lack of gas-phase CO in planet-forming disks, as previously suggested. It should be noted though that our experiments indicate a lower water ice dissociation efficiency than originally adopted in model descriptions of planet-forming disks and molecular clouds. Incorporation of the reduced water ice dissociation and increased binding energy of CO on a water ice surfaces in physical-chemical models would allow investigation of this gas-grain interaction to its full extend.

The emergence of life on Earth may have its origin in organic molecules formed in the interstellar medium. Molecules with amide and isocyanate groups resemble structures found in peptides and nucleobases and are necessary for their formation. Their formation is expected to take place in the solid state, on icy dust grains, and is studied here by far-UV irradiating a CH4:HNCO mixture at 20 K in the laboratory. Reaction products are detected by means of infrared spectroscopy and temperature programmed desorption-mass spectrometry. Various simple amides and isocyanates are formed, showing the importance of ice chemistry for their interstellar formation. Constrained by experimental conditions, a reaction network is derived, showing possible formation pathways of these species under interstellar conditions.

The identification of complex organic molecules, COMs, in inter- A nd circumstellar gas phase environments is steadily increasing. The formation of such COMs takes largely place on the icy dust grains, as has been shown in recent laboratory studies. Until now solid state features of smaller molecular species have been directly identified in these environments. The presented work on acetaldehyde (CH3CHO), ethanol (CH3CH2OH), and dimethyl ether (CH3OCH3) in different astronomically relevant ice environments and for temperatures in the range 15 to 160 Kelvin, provides the necessary tools to guide or interpret astronomical observations, specifically for upcoming James Webb Space Telescope observations.

Molecules with an amide functional group resemble peptide bonds, the molecular bridges that connect amino acids, and may thus be relevant in processes that lead to the formation of life. In this study, the solid state formation of some of the smallest amides is investigated in the laboratory. To this end, CH4:HNCO ice mixtures at 20 K are irradiated with far-UV photons, where the radiation is used as a tool to produce the radicals required for the formation of the amides. Products are identified and investigated with infrared spectroscopy and temperature-programmed desorption mass spectrometry. The laboratory data show that NH₂CHO, CH₃NCO, NH₂C(O)NH₂, CH₃ C(O)NH₂, and CH₃NH₂ can simultaneously be formed. The NH₂CO radical is found to be key in the formation of larger amides. In parallel, ALMA observations towards the low-mass protostar IRAS 16293-2422B are analysed in search of CH₃NHCHO (N-methylformamide) and CH₃C(O)NH₂ (acetamide). CH₃C(O)NH₂ is tentatively detected towards IRAS 16293-2422B at an abundance comparable with those found towards high-mass sources. The combined laboratory and observational data indicate that NH₂CHO and CH₃C(O)NH₂ are chemically linked and form in the ice mantles of interstellar dust grains. A solid-state reaction network for the formation of these amides is proposed.