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

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5. Conclusion

Organic residues formed by irradiation of CH4 ice were enriched in 13 C by a maximum of 15 relatively to the CH4. An evolution of the isotopic composition with the sample size was interpreted as a maturation of organic matter during its irradiation.

In carbonaceous meteorites, [FORMULA] variations up to 75 are observed between different organic compounds (e.g. amino-acids, macromolecules etc...). These variations are attributed to the preservation, of a small fraction of interstellar organic matter. Since isotopic fractionation between the gas and the organic residues is markedly smaller (i.e. 15 compared with 75), the isotopic composition of interstellar matter (if present...) preserved in comets should not be blanked out by an organic maturation taking place at the cometary surface.

In addition, irradiation of comets by the solar wind or cosmic rays does not contribute significantly to the carbon isotopic fractionation between surface ices, surface polymers derived from these ices and their corresponding sputtered gaseous organic species lost in space. Accordingly, the mean 12 C/13 C ratio of the gas phase should be regarded as a face value for the bulk cometary nucleus.

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© European Southern Observatory (ESO) 1998

Online publication: January 27, 1998