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Astron. Astrophys. 330, 1175-1179 (1998)

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2. Experimental

2.1. Sample preparation and chemical composition

Samples were obtained during the course of other studies. Experiments are described in details by Kaiser and Roessler 1992; Kaiser et al. 1992a,b; Patnaik et al. 1990; Roessler 1992; Roessler et al. 1992a,b. Samples were prepared as follows: CH4 and CH4 /Ar (1:12) gas mixtures were condensed on Si wafers at 15 and 77 K at a rate of [FORMULA] mole [FORMULA] h-1 at a pressure of 2 10-5 mbar to give typically a [FORMULA] m thickness. The methane impurity volume concentration was [FORMULA] ppm (Messer Griesheim - Germany). Samples were irradiated with 18 MeV protons and 17 MeV 3 He [FORMULA] ions respectively, during 5 to 90 min. This irradiation corresponds to doses of 0.007 to 16.8 eV per C atom. The fluence at the cometary surfaces being [FORMULA] eV [FORMULA] y-1, these doses correspond to exposure ages of 7 104 and 1.7 108 years respectively.

After warming up, a heterogeneous solid residue with a typical thickness of [FORMULA] m remained on the silicon wafer. The chemical composition of these solid residues was obtained in another study (Kaiser & Roessler 1992) by visual microscopic inspection and chromatography. The volatile species outgassed during methane irradiation at 15-30 K were determined: H, H2, C, CH, CH2, CH3, CH4, C2 H2, C2 H [FORMULA], up to linear alkanes in C8. After heating the left over solid residues at room temperature, several mono- and di-subsituted benzene molecules were outgassed from the residues as well as cyclic compounds such as C3 H6, C5 H10 etc... The remaining solid residues consisted of linear alkanes, alkadienes, aromatic species and polycyclic aromatic hydrocarbons.

2.2. Composition and isotopic analysis

Isotopic composition of carbon in the solid residues was analysed as follows: Organic matter was oxidised with an excess of pure oxygen in a vacuum apparatus. CO2 was collected in a U-trap for which the temperature can be adjusted precisely and then analysed by mass spectrometry. The carbon isotopic composition of methane before irradiation was determined by mass spectrometry after its oxidation, by oxygen released from CuO at 1000 [FORMULA] C.

The main part of the vacuum line was described previously by Lécluse and Robert (1994). Moreover a glass volume was added for the oxygen supply (see Fig. 1). It consisted in a 500 cm3 reservoir in which industrial O2 gas was liquefied at 77 K. From this reservoir, a saturated vapour pressure corresponding to room temperature (i.e. pressure equal to 18 cm Hg) could be expended in the vacuum line. Such an apparatus allows the expansion of almost infinite quantities of pure O2. Samples were sealed in a platinum foil and placed under vacuum in 250 cm3 quartz reaction tubes. Then a minimum quantity of [FORMULA] moles of O2 was expanded into the reaction vessel to ensure a total oxidation of the organic matter. The tubes were maintained in solenoid coils and heated by induction at temperatures above 1000 K during 10 min. Compounds formed by oxidation (CO2 and H2 O) were condensed in the U-trap at liquid nitrogen temperature and CO2 was released at temperatures between 113 and 123 K. CO2 purity was verified by mass spectrometry and its carbon isotopic composition was measured using a [FORMULA] mass spectrometer from Finnigan Mat.

[FIGURE] Fig. 1. Oxidation apparatus for organic solid residues. Oxygen supply is ensured by the left part of the line. The oxidation occurred in the quartz reaction tube under oxygen atmosphere at temperature above 1000 K. U-trap allowed the separation of CO2 from H2 O and sample tube received pure CO2 for analysis on the mass spectrometer (not represented there). See text for comments.

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© European Southern Observatory (ESO) 1998

Online publication: January 27, 1998