RAMAN ANALYSIS OF ORGANIC REFRACTORY MATERIALS AFTER ENERGETIC PROCESSING. IMPLICATIONS FOR COMETS AND TNOs.

Introduction

     Organic refractory materials are present in several Solar System small bodies, including comets and Trans Neptunian Objects (TNOs) [1, 2]. Recent observations by the JWST of TNOs reveal the presence of various compounds on their surfaces, including N₂, CO and CH₄ [3-5]. The icy surfaces of TNOs and comets in the Oort cloud are affected by energetic charged particles, that together with heating that bodies can experience in space determine the formation of organic refractory materials [6]. Experiments show that the irradiation of C-rich icy mixtures, followed by heating, determines the formation of organic refractory materials [7-9].

     High doses of irradiation to organic refractories lead to the formation of amorphous carbon (αC) [10-13]. Experiments also show that αC can form after ion bombardment of C-bearing ices [14]. Hence, the surface of outer Solar System bodies might contain αC. However, αC cannot be detected by IR spectroscopy. We thus use laboratory analogues of the refractory organics that could be present at the surface of TNOs and comets, namely organic refractory residues (ORRs), to study the formation of αC after their exposure to energetic processing.

     Samples are characterized by Raman spectroscopy, providing insights into the structural properties and degree of disorder of carbonaceous materials [15-17].

     Our aim is to shed light on the origins and properties of refractory carbon in Solar System primitive bodies. We also inform on the origin of refractory carbon in the cold and dense regions of the interstellar medium, e.g., the central regions of pre-stellar cores, where volatiles are frozen onto dust grains, just before forming protoplanetary disks. Their exposure to energetic processing and heating can thus occur.

 

Methods

     ORRs were produced at the Laboratory for Experimental Astrophysics at INAF-OACT (Catania, Italy), after the irradiation with 200 keV ions at 18 K and subsequent warm-up to room temperature of icy mixtures in a ultra-high vacuum chamber(P ≤ 10^-9 mbar). ORRs differ in the initial icy mixture and/or irradiation conditions. The icy mixtures (consisting of N₂, CH₄ and CO) mimic the surface composition of TNOs and comets in the Oort cloud [18]. Once at room temperature, some samples were exposed to further bombardment with 200 keV ions.

     Raman spectra were acquired at the Center for Astrochemical Studies of the Max-Planck Institute for Extraterrestrial Physics (Garching, Germany) by means of a spectrometer equipped with a 488 nm laser. Several spots on each sample were probed with low laser power (P=0.1 mW), and further spectra were collected at increasing laser power.

 

Results 

      We searched for amorphous carbon (αC) by looking for its bands, the D (disorder) line at ~1360 cm^-1 and the G (graphitic) line at ~1580 cm^-1 [10] in our samples.  We reveal that icy mixtures exposed to low dose (~120 eV/16u) lead to the formation of ORRs that do not show any features attributed to αC  (Fig. 1, red curve). Contrariwise, ORRs which were further irradiated (up to relatively high doses) exhibit a clear αC band signature (hereafter we refer to these ORRs as αC-ORRs) as seen on the blue curve of Fig. 1.

     Since the profile of the αC band reflects the structural properties of the carbonaceous material, we have used a Lorentz + Breit-Wigner-Fano fit to obtain the parameters of D and G lines [19], which in turn inform on the dimension of the αC sp² clusters (L_a). Samples analysed at a laser power of 0.1 mW show values of L_a ranging between 5-11 Å, meaning that sp² clusters are relatively small and the degree of disorder is high. The distribution of fitting parameters suggests a dependence on the initial icy mixture of the αC-ORRs.

     Raman spectra of αC-ORRs acquired at increasing laser power (0.2-0.5 mW) exhibit significant annealing effects attributed to the graphitization of the αC structure: conversion from sp³ to sp² bonds takes place, L_a increases.      

Figure 1. Raman spectra of ORRs produced from N₂ : CH₄ : CO + 200 keV He⁺ up to 120 eV/16u. Red curve: as formed ORR showing only the high fluorescence continuum; blue curve αC-ORR further exposed to +200 keV He⁺ reveals the αC band.

Conclusions 

      We shed light on the properties of carbonaceous refractory materials possibly present at the surface of outer solar system bodies. Due to their remote location, the characterization of their composition is mainly achieved in the IR. However, our data shows that IR-inactive αC can form from the exposure of complex organics to the high extent of energetic processing suffered by icy surfaces.

 

 

Acknowledgements

     MG acknowledges the Società Italiana di Scienze Planetarie - Angioletta Coradini (SISP-AC) through the call “Bando per la mobilità di giovani ricercatori e dottorandi 2024” and the support of Max Planck Society and the CAS group at Max-Planck Institute for Extraterrestrial Physics. This work is supported by the Istituto Nazionale di Astrofisica (INAF) through the funds provided for the PhD in Physics, XXXIX cycle, Università degli Studi di Catania.

 

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