Spectral changes induced by thermal processing of organic refractory materials

Introduction
The study on the composition of meteorites, asteroids, comets, and trans-neptunian objects suggest that they contain materials originated during the early stages of the solar system formation. A relevant fraction of organic materials trapped in small bodies are thought to form from chemical reactions that involve carbon-bearing frozen volatiles species that are revealed in various star-forming regions and protoplanetary disks [1, 2]. Complex reactions are triggered by the interaction of solid-phase materials with energetic charged particles, such as galactic cosmic-rays (GCR), and energetic photons (x-rays, UV) that induce both the destruction of the pristine species and the formation of new compounds [3]. In addition, solar wind (SW) and solar energetic particles (SEP) also induce chemical reactions at the surface of outer icy small bodies [4]. As a result, pristine volatile species are destroyed and more complex molecules can form. The chemical complexity is further increased by the thermal processing that matter can suffer both during the star-formation process and in the event of migrations in the inner solar system. Processing at high temperature also affects the properties of meteorites during their entry into the atmosphere. Understanding how this event affects the physical, chemical, and spectral properties of meteorites is of particular relevance because these samples are often used to interpret the spectra of asteroids and other small bodies. 
Information on the chemistry triggered by energetic charged particles and heating in the formation of organic matter comes from laboratory experiments where simple carbon-bearing molecules are deposited at very low temperature (≤ 20 K), exposed to ion beams (keV-MeV), and further heated to room temperature (~ 300 K). These experiments lead to the formation of the so-called organic refractory residues, samples whose characterization revealed the presence of thousands of organic species, including compounds of astrobiological relevance, providing insights into the composition of extraterrestrial organics [5, 6, 7]. However, no information is available on the heating of organic refractory residues at temperatures higher than 300 K. 
Methods
We present new experiments of ion bombardment and further thermal heating of simple frozen volatile compounds representative of the pristine composition of icy materials in the presolar cloud. Ion bombardment experiments were performed with the facilities available at the Laboratory for Experimental Astrophysics (LASp) at INAF-Osservatorio Astrofisico di Catania (Italy). Ice mixtures containing H₂O, CH₃OH, NH₃ and CO, CH₄, and N₂ were deposited in a ultra-high vacuum chamber (P ≤10^-8 mbar) at low temperature (18 K) and exposed to 200 keV H⁺ and He⁺, respectively. After the bombardment, processed ices were warmed-up to 300 K with a constant heating rate in order to produce organic refractory residues. These samples were then extracted from the UHV chamber, stored in vacuum sample holders and transferred to the Planetary Spectroscopy Laboratory (PSL) at the DLR Berlin. At the PSL, organic refractory residues were placed in a oven and heated in vacuum (P≤10^-2 mbar) up to 970 K. The heating was performed in various step and with a constant heating rate.
During the ion bombardment, samples were analysed by means of Fourier-transform infrared (FT-IR) spectroscopy in the near- and mid-IR (1.25-10.5 µm, 8000-950 cm^-1) in transmittance mode. Several spectra were acquired in-situ during the processing, allowing us to follow the changes induced by energetic charged particles in the composition of ices. Spectra were also acquired during the warm-up to 300 K to follow the formation of organic refractory residues. At the PSL, FT-IR spectroscopy was also used to characterize samples after each step of warm-up.
Results
The spectra collected during ion bombardment at 18 K show the destruction of the pristine frozen compounds and the formation of new species, including the precursors of complex organic species, such as aldehydes (5.81 µm, 1720 cm^-1) and CN-bearing compounds (4.42 µm, 2260 cm^-1). The characterization performed after ion bombardment and during warm-up to 300 K show the sublimation of the most volatile species and spectral changes that point to the formation of organic refractory materials, as reported in previous similar experiments [8]. 
The further heating performed at the DLR shows relevant changes in the spectral properties of samples. In all cases, we observed a decrease in the intensity of the signal that is associated to a loss of material during warm-up. Shifts in the band position and changes in the relative intensity of absorption features also testify the alteration of the chemical properties of samples. In particular, we reveal strong changes in the 5.8-6.8 µm (1720-1470 cm^-1) region that includes the absorption features of C=C, C=O, and C=N bearing compounds as well as in the region around 4.5 µm (2200 cm^-1) that contains the absorption features of nitriles.
The data obtained are used to interpret the IR spectra of organic-rich meteorites and will support the understanding of the alteration induced by thermal processing of organic-rich surfaces in the inner solar system. 
Acknowledgements
This work is supported by  the Istituto Nazionale di Astrofisica (INAF) through the grant Organic Refractories Sustaining microOrganisms - ORSO, CUP C63C23001250005. RGU acknowledges the support from the Società Italiana di Scienze Planetarie - Angioletta Coradini (SISP-AC).

References
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