Nature and evolution of the dominant carbonaceous matter in interplanetary dust particles: effects of irradiation and identification with a type of amorphous carbon

Abstract

Aims. Interplanetary dust particle (IDP) matter probably evolved under irradiation in the interstellar medium (ISM) and the solar nebula. Currently IDPs are exposed to irradiation in the Solar System. Here the effects of UV and proton processing on IDP matter are studied experimentally. The structure and chemical composition of the bulk of carbon matter in IDPs is characterized. Methods. Several IDPs were further irradiated in the laboratory using ultraviolet (UV) photons and protons in order to study the effects of such processing. By means of infrared and Raman spectroscopy, IDPs were also compared to different materials that serve as analogs of carbon grains in the dense and diffuse ISM. Results. The carbonaceous fraction of IDPs is dehydrogenated by exposure to hard UV photons or 1 MeV protons. On the other hand, proton irradiation at lower energies (20 keV) leads to an efficient hydrogenation of the carbonaceous IDP matter. The dominant type of carbon in IDPs, observed with Raman and infrared spectroscopy, is found to be either a form of amorphous carbon (a-C) or hydrogenated amorphous carbon (a-C:H), depending on the IDP, consisting of aromatic units with an average domain size of 1.35 nm (5−6 rings in diameter), linked by aliphatic chains. Conclusions. The D-and15 N-enrichments associated to an aliphatic component in some IDPs are probably the result of chemical reactions at cold temperatures. It is proposed that the amorphous carbon in IDPs was formed by energetic processing (UV photons and cosmic rays) of icy grains, maybe during the dense cloud stage, and more likely on the surface of the disk during the T Tauri phase of our Sun. This would explain the isotopic anomalies and morphology of IDPs. Partial annealing, 300−400• C, is required to convert an organic residue from ice photoprocessing into the amorphous carbon with low heteroatom content found in IDPs. Such annealing might have occurred as the particles approached the Sun and/or during atmospheric entry heating.

Mid-IR, Far-IR, Raman micro-spectroscopy, and FESEM–EDX study of IDP L2021C5: Clues to its origin

“…The measured CH 2 /CH 3 ratio is well above the value of $2 observed for interstellar hydrocarbons (Sandford et al, 1991) and for some chondritic IOM (e.g. Pendleton and Allamandola, 2002), but is in perfect agreement with previous determinations (in the 3-5.5 range) of this ratio for IDPs (Flynn et al, 2008b;Matrajt et al, 2005;Muñoz-Caro et al, 2006;Raynal et al, 2000). The CH 2 /CH 3 ratio in Wild 2 grains (Stardust mission) presents a higher value: Rotundi et al (2008) estimated values in the range 5.7-6.1 for particles #35,17; 35,21; and 35,26; and 9.6 for particle #35,18.…”

“…For example amorphous carbon in IDPs with different degrees of order could be indicative of different irradiation doses by solar wind particles and fast solar protons in the interplanetary medium before collection . Muñoz-Caro et al (2006) proposed that the amorphous carbon in IDPs could be formed by energetic processing of icy grains, maybe during the dense cloud stage, and more likely on the surface of the disk during the T Tauri phase of our Sun, before the accretion of the individual carbon grains into porous aggregates, and that a partial annealing (300-400°C) would be required to convert an organic residue from irradiated ice (rich in heteroatoms, such as O and N) into an amorphous carbon similar to that found in IDPs. This might have occurred as the particles approached the Sun and/or during atmospheric entry heating.…”

“…(d): G-FWHM as a function of G band, for five representative points of L2021C5 (locations in (a); the C point comes from another matrix region, close to point 1 of Fig. 1) compared with literature data for IDPs, carbonaceous meteorites, and grains from comet Wild 2 (Wopenka, 1988;Raynal et al, 2001;Quirico et al, 2003Quirico et al, , 2005Raynal, 2003;Bonal et al, 2006;Muñoz-Caro et al, 2006;Sandford et al, 2006;Rotundi et al, 2007;Busemann et al, 2007;Rotundi et al, 2008). Two arrows mark the evolutionary paths due to ion irradiation and thermal metamorphism.…”

Comparison of the Raman spectra of ion irradiated soot and collected extraterrestrial carbon

“…Mennella et al, 2003). Irradiation processes are also important in the Solar System, as they are related to the production of complex organics in the pre-solar nebula (Muñoz-Caro et al, 2006) and on minor bodies surfaces (Brunetto et al, 2006), as well as to the alteration of pre-existing carbon (Moroz et al, 2004;Baratta et al, 2008). Moreover, many studies point out that the early Sun was much more active than today, passing through an active T-Tauri phase before entering the main sequence (e.g.…”

Organic matter in space - An overview

“…Examples include gaseous polycyclic aromatic hydrocarbons (PAH) (e.g., Tan & Salama 2005, Rouille et al 2007 or carbon chains (e.g., Dzhonson et al 2007, Linnartz et al 2000, in addition to matrix-isolation studies of large samples of PAHs (e.g., Hudgins & Allamandola 1999). Spectroscopy data bases of solids, including silicates (e.g., Jaeger et al 1998Jaeger et al , 2003, carbonates (e.g., Posch et al 2007), ices (e.g., Bisschop et al 2007a, Bernstein et al 2005 and carbonaceous material (e.g., Mennella et al 1997, Jaeger et al 2006, Muñoz-Caro et al 2006 continue to grow.…”