Abstract
Context. Near- and mid-infrared observations have revealed the presence of organic refractory materials in the Solar System, in cometary nuclei and on the surface of centaurs, Kuiper-belt and trans-neptunian objects. In these astrophysical environments, organic materials can be formed because of the interaction of frozen volatile compounds with cosmic rays and solar particles, and favoured by thermal processing. The analysis of laboratory analogues of such materials gives information on their properties, complementary to observations. Aims. We present new experiments to contribute to the understanding of the chemical composition of organic refractory materials in space. Methods. We bombard frozen water, methanol and ammonia mixtures with 40 keV H+ and we warmed the by-products up to 300 K. The experiments enabled the production of organic residues that we analysed by means of infrared spectroscopy and by very high resolution mass spectrometry to study their chemical composition and their high molecular diversity, including the presence of hexamethylenetetramine and its derivatives. Results. We find that the accumulated irradiation dose plays a role in determining the composition of the residue. Conclusions. Based on the laboratory doses, we estimate the astrophysical timescales to be short enough to induce an efficient formation of organic refractory materials at the surface of icy bodies in the outer Solar System.
Highlights
The frozen surfaces of outer objects in the Solar System show the presence of various volatile molecules as well as refractory red materials, whose presence may be due to the effect of bombardment by cosmic rays, solar energetic particles, and solar wind
Only limited information is available on the composition of organic refractory materials on the surfaces of frozen bodies in the outer Solar System, as astronomical observations only show the presence of red slopes in the visible and near-infrared
Very high resolution mass spectrometry data show the presence of CHO, CHN, and CHNO molecular groups
Summary
Astronomical observations have enabled the detection of various frozen compounds on the surface of dust grains (icy grain mantles) in the interstellar medium (e.g. van de Hulst 1949; Tielens & Hagen 1982; Whittet et al 1996; Caselli & Ceccarelli 2012; Boogert et al 2015) as well as on the surface of small bodies in the Solar System, such as comets, centaurs, and Kuiper-belt objects (e.g. Cruikshank et al 1998; Barucci et al 2006; Biver et al 2006; Altwegg et al 2017; Stern et al 2019). Biver et al 2006; Willacy et al 2015; Altwegg et al 2017; Nesvorný 2018; McKinnon et al 2020) During their lifetime, frozen volatiles experience both irradiation by UV photons, cosmic-rays (CR) and stellar or solar particles, as well as heating. Johnson 1990; Cooper et al 2003; Strazzulla et al 2003; Urso et al 2020) Such bodies exhibit red slopes in the visible and near-infrared (IR) spectra that are related to the presence of a refractory C-rich material, whose formation is attributed to the irradiation of volatiles on their surfaces Such bodies exhibit red slopes in the visible and near-infrared (IR) spectra that are related to the presence of a refractory C-rich material, whose formation is attributed to the irradiation of volatiles on their surfaces (e.g. Cruikshank et al 1998; Barucci et al 2006; Brown et al 2011; Grundy et al 2020) or to the incorporation of red materials present in the presolar cloud (e.g. Dalle Ore et al 2011)
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