The role of thermal processing in the alteration of complex organic matter

Introduction
Complex organic matter revealed in primitive small bodies shows evidences of early origins from physico-chemical processes taking place in the interstellar medium (ISM) and in the protosolar cloud [1, 2]. In this regard, a relevant contribution to the formation of organic molecules is expected from the exposure of simple H-, C-, N-, and O-bearing ISM ices to low-energy Galactic cosmic rays (GCR) [3, 4]. In the further stages of the Solar System formation, the diffusion and desorption of most volatile species from processed ices, possibly supported by their heating, lead to the production of complex refractory organics [5]. Later on, significant changes are expected to take place in the protoplanetary disk, where thermal processing to high temperatures can determine the formation of amorphous carbon [6] and the loss of astrobiologically-relevant compounds possibly present in pristine organic matter.

We simulate in the laboratory the formation of complex organic matter from the exposure of laboratory analogues of ISM ices to energetic charged particles, their subsequent heating to room temperature to form organic refractory residues (ORRs), and their further thermal processing to higher temperatures, with the aim to shed light on the alteration of pristine complex organic matter.

Methods
Ice mixtures containing (i) H₂O, CH₃OH, NH₃, and (ii) CO, CH₄, and N₂ are exposed to 200 keV H⁺ and He⁺ ions, respectively, at the Laboratory for Experimental Astrophysics (LASp) at INAF-Osservatorio Astrofisico di Catania (Italy). Ices are deposited in an ultra-high vacuum chamber (P ≤10^-9 mbar) at low temperature (18 K) on MgF₂ inert substrates. After the ion bombardment, ices are warmed up to 300 K, leading to the formation of complex refractory organic matter [4]. These materials, named organic refractory residues (ORRs), are considered analogues of the pristine complex organic matter that can form in space. Thus, their characterization allows us to shed light on the properties of pristine organics, including the presence of compounds relevant in astrobiology [7].

ORRs then undergo thermal annealing by means of a vacuum (P≤10^-2 mbar) oven available at the Planetary Spectroscopy Laboratory (PSL) at the DLR Berlin. The annealing is performed with a constant heating rate of 3 K min^-1 and up to about 970 K.

During the experiments, we use Fourier-transform Infrared Spectroscopy (FT-IR) in the mid-IR and in transmittance mode to monitor the changes induced by ion bombardment and thermal annealing.

We also analyze ORRs by means of Raman spectrometers equipped with 532 and 488 nm excitation lasers available at LASp and at the Max-Planck-Institut für Extraterrestrische Physik, respectively, to shed light on the presence and properties of amorphous carbon.

Results & Discussion
The mid-IR spectra of the icy mixtures deposited at 18 K show the typical absorption of the deposited species. During the ion bombardment, we observe a decrease in the band area of the features attributed to the deposited species and the appearance of new IR bands due to the formation of new molecular species in the processed ices. The warm-up to room temperature determines further spectral changes induced by the desorption of volatile species and the formation of new compounds. At room temperature, we observe the IR spectrum of ORRs, with the most intense absorption feature centered at about 6 µm that includes the contributions from C=C, C=O, and C=N bearing compounds. The Raman spectra of these samples only show a strong fluorescence that is due to the presence of H bonded in the C-rich matrix. During thermal annealing, IR spectra acquired at increasing temperature show  the severe variation in the profile of all the IR features, with their intensity decreasing following a linear trend. IR absorption bands are observed up to about 700 K. ORRs annealed up to 970 K do not show any IR absorption feature, although their Raman spectra show the presence of the typical features associated with amorphous carbon, that is, the D and G bands at about 1360 and 1590 cm-1, respectively.

 Conclusions
Our experiments allow us to simulate the origin of pristine organic materials and to characterize their evolution in protoplanetary disks. The analysis of annealed ORRs provides information on the spectral changes induced by heating. In the IR spectra, we observe the ORR’s 6 µm feature up to about 700 K, while at higher temperature (973 K) we observe the presence of amorphous carbon, an IR inactive material that in our samples is revealed only by means of Raman spectroscopy. We use our data to shed light on the evolution of organic matter in protoplanetary disks and on how thermal annealing can determine the present physico-chemical properties of organics trapped in meteorites.

Acknowledgements
This work is supported by  the Istituto Nazionale di Astrofisica (INAF) through the grant Organic Refractories Sustaining microOrganisms - ORSO, CUP C63C23001250005 and through the grant “Misure in riflettanza di campioni extraterrestri e analoghi di superfici planetarie”, CUP C63C24001150005.

 

References

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