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

4. Discussion

In Fig. 2, the isotopic compositions (in units) of the residues are reported as a function of (Q standing for the carbon content expressed in mole). The isotopic composition of the initial methane is also shown (-39.9). All organic residues exhibit an enrichment in ¹³ C relatively to the initial methane and their ¹³ C values vary linearly as a function of . According to this correlation, residues appear to reach a fractionation limit of -24 for sample size greater than moles deposited.

Fig. 2. (in ) versus (Q in moles) in organic residues resulting from irradiation of methane ices. Error bars are due to the blank contribution. The initial isotopic composition of methane is also shown. The decrease in with sample size is interpreted as a progressive maturation by sputtering of initial polymers having values close to -24 (i.e. for ).

The correlation between the ¹³ C and cannot result from a two component mixing process with the low ¹³ C end-member standing for the blank contribution. Indeed, several samples exhibit clearly lower than those found for blanks ().

The correlation between the and the carbon content can be interpreted as a two step process for the formation of the organic polymers. In the first step, methane is polymerised (probably under the form of aliphatic compounds) and the resulting organic polymers (see flow chart in Fig. 3) are isotopically fractionated relative to methane by 16 with a mean value of (i.e. ). In the second step, aliphatic compounds are sputtered by the incoming ion beam (H or He ). As a consequence, an additional isotopic fractionation occurs and the isotopically heavier species are lost preferentially. Evaporation by sputtering is not supposed to cause any isotopic fractionation between the gas and the remaining solids if no chemical reaction takes place between the two phases. Therefore, the observed isotopic evolution must be linked to the loss of carbonaceous fragments, different in isotopic composition from the sputtered solid, implying in turn, that organic matter is re-arranged during this second step. Thus one can suppose that this type of isotopic fractionation is caused by the progressive polymerisation of aliphatics into aromatic refractory carbon phases. Simple mass balance equations illustrate this second step:

with

and

with:

The subscripts `aliph.', `lost' and `arom.' stand for the aliphatic carbon produced by the polymerisation of methane, for the carbon lost during irradiation and for the aromatic carbon produced during irradiation, respectively. The subscript `i ' designates the initial carbon phases formed by the polymerisation of methane (that is according to Fig. 2, mole and ). The subscript `mes.' stands for the measured values reported in Fig. 2. Since the isotopic fractionation occurs in a solid phase, aliphatic compounds which are not sputtered by the incoming beam are not fractionated relative to their initial values; hence in Eq. 2. The conversion yield for aromatic compounds can be defined as:

k in Eq. (6) represents the number of carbon atoms combined into an aromatic structure for 1 carbon atom lost by sputtering.

Fig. 3. Flow chart depicting the irradiation model described by Eqs. (1) to (7) (see text). The carbon isotopic composition of each species is indicated in . The species cij are outgassed form the solid during irradiation. Methane and the other organic compounds remain as solid phases during irradiation.

Eqs. (1) to (6) give:

In Eq. (7) the measured isotopic composition of the residues (i.e. ) is a linear function of Numerical simulations of Eq. (7) that fit the results reported in Fig. 2, show that: 1) Assuming that the more ¹³ C depleted samples (; mole; see Fig. 2) represent almost pure aliphatic free residues, the carbon isotopic fractionation between aliphatic and aromatic is 8 (i.e. ) and the conversion yield k cannot be higher than 9%; 2) If the isotopic fractionation is somewhat higher than 8, the conversion yield k must be lower than 9%. For example, an isotopic fractionation of 15 (i.e. an aromatic polymer with a ) would correspond to a conversion yield k of 5%.

It has been shown by hydrogen nuclear magnetic resonance, that the formation of complex polycyclic aromatic hydrocarbons in solid CH₄ such as coronene (C₂₄ H₁₂) already takes place at low radiation doses within one collision cascade (Kaiser 1991; Kaiser 1993; Kaiser et al. 1992a, b; Kaiser and Roessler 1992; Patnaik et al. 1990). It is rather a function of linear energy transfer than of the dose. The mechanism discussed here is a multicentre reaction of hot carbon and hydrogen atoms, their intermediate reaction products and free thermal radicals lost in the gas phase being located within a zone of 10 Å radius from the surface (Roessler 1992; Kaiser 1993). As microscopically observed on the wafers (Kaiser et al. 1992; Kaiser and Roessler 1992), the successive transformations of CH₄ into longer and longer aliphatic chains, polycyclic structures and finally amorphous carbon are likely related to the irradiation dose. But even here, multicentre processes will minimise the number of reaction steps. The small isotopic enrichments of ¹³ C in the residues can be considered as an additional evidence for the co-ordinated and concomitant multicentre mechanism.

These observations bear also interesting consequences as far as the origin and evolution of organic material in carbonaceous meteorites is concerned. Gilmour et al. (1991) and Gilmour & Pillinger (1992) detected organic molecule under the form of Poly-Aromatic Hydrocarbons (PAHs) in Murchison and Orgueil meteorites. These authors found that the carbon isotopic composition of PAHs increased with the molecular weight and with the degree of aromatisation. If the present interpretation is correct, isotopic fractionation of carbon linked to irradiation results in a decrease in the values associated with an increase in the degree of aromatisation. This conclusion is opposite to observations reported for PAHs in carbonaceous chondrites, suggesting they were formed by a different process.

© European Southern Observatory (ESO) 1998

Online publication: January 27, 1998
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