Ion bombardment induces the formation of macromolecular organics on TNO ices

Introduction
Macromolecular organic refractory materials are widespread in primitive bodies in the Solar System, including meteorites, comets, and trans-neptunian objects (TNOs) [1].
Information on the composition of TNOs mainly comes from observations in the visible (Vis) and near-infrared (NIR) that allow us to reveal the ices at the object surfaces. Data also show positive slopes that are attributed to reddish complex organics. James Webb Space Telescope observations revealed three main compositional classes of mid-sized TNOs, namely water-rich (or bowl), CO₂-rich (double-dip), and organics-rich TNOs [2,3]. Both CO₂-rich and organics-rich types are the most likely to contain macromolecular matter. 
Constraining the properties of organics on the body surfaces is of primary importance to shed light on their origin and on alteration processes at work. On the one hand, various evidence suggests that TNOs inherited matter from the protoplanetary disk, where ices and organics formed in the earlier stages of the Solar System formation are exposed to heating and mixing. On the other hand, post-accretion processes, including the exposure to solar particles and galactic cosmic rays (GCRs) of TNO surfaces contributed to enriching their inventory of organics and macromolecular materials [4]. 
The faint signatures of macromolecular organics in the Vis-NIR hinder a detailed characterization of their properties. Experiments allow us to reproduce in the laboratory the formation of macromolecular organics possibly at the surface of TNOs, following the exposure of surficial ices to ion bombardment by solar particles and GCRs. We provide insights on the chemical composition and physical properties of the organics, as well as on the irradiation timescales necessary for their formation in the outer Solar System.

Methods
We perform laboratory experiments focused on the production and spectroscopic analysis of organic refractory residues (ORRs), samples left over after irradiation and warm-up to room temperature of simple C-bearing ices [5, 6, 7, 8]. 
ORRs are produced in an ultra-high vacuum chamber (P~10^-9 mbar) by depositing volatile C-bearing species at low temperature (18 K) on substrates placed in thermal contact with a He-cryocooler. The deposited ices contain compounds revealed on TNO surfaces, including CO, CO₂, CH₃OH, CH₄, also in presence of H₂O and N-bearing compounds, such as N₂ and NH₃. Ices are then exposed to a 200 keV ion beam (H⁺ or He⁺) to simulate the effect induced by solar particles and GCRs. After ion bombardment, processed ices are gently warmed-up to room temperature, and ORRs are formed. Some ORRs are also exposed to further ion bombardment or thermal processing to high temperatures.
During the experiments, we acquire mid-IR spectra that allow us the characterization of the chemical changes induced by the ion bombardment and the subsequent heating. ORRs are further analyzed by Raman spectroscopy, which is sensitive to the presence of amorphous carbon.

Results
The IR spectra acquired during ion bombardment show that all infrared features attributed to the deposited species decay following an exponential trend with increasing the irradiation dose. At the same time, new absorption bands attributed to irradiation by-products appear. The irradiation by-products, the produced quantities, as well as their formation rate are strongly related to the initial mixture. With increasing temperature, we observe IR features typically associated with the growth of carbon-rich networks, compatible with the formation of macromolecular organic matter (Urso et al., in prep.). We also reveal the formation of OCN^- and nitriles, whose IR features were recently revealed on organics-rich TNOs and are proposed as key compounds to clarify the origins of materials in these bodies [9].
The exposure to further ion bombardment and thermal processing of ORRs causes strong changes in the IR spectra of the samples. In particular, during warm-up we observe shifts and decrease in the intensity of all features. No IR feature is detected in the spectra acquired after warm-up to about 700 K. Interestingly, the Raman spectra acquired after ion bombardment or thermal processing of ORRs show the presence of the D and G bands of amorphous carbon [10].  The properties of the amorphous carbon detected in ORRs exhibit dependence on initial ice composition, irradiation and thermal history, in some cases showing a close match with carbonaceous extraterrestrial particles (Germanà et al., submitted MAPs, Urso et al., in prep).

Conclusion
Our data show that exposure of simple, pristine ices observed on the surface of TNOs to ionizing radiation can lead to the formation of macromolecular organic materials. The properties of organic matter primarily depend on the composition of ices and on the dose. Ion bombardment is thus of primary relevance in the formation of complex organic materials in the outer Solar System, where amorphous carbon could also be present.

References:
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