Carbon isotopes of irradiated methane ices: implications for cometary ^12C/^13C ratio

Frozen CH tief{4} and CH tief{4} / Ar were irradiated by protons and hoch{3}He hoch{2+} ions at 15 and 77K in order to simulate the interaction between the solar wind and the surface of comets. After irradiation, polymerised organic matter is left over as solid residues, when warming up the remaining CH tief{4}. The carbon isotopic composition of these residues was determined. Isotopic fractionation between organic residues and methane reveals a two step process in the maturation of organic matter: 1) a polymerisation of methane associated with an isotopic fractionation of 16{{psfig{figure = 1175004A.EPS}}; 2) a sputtering of the polymerised species associated with an isotopic fractionation lying between 8 and 15{{psfig{figure = 1175004A.EPS}} and with a corresponding conversion yield of the polymers (i.e. the number of carbon atoms converted into refractory polymers relative to those lost by sputtering) between 9 and 5%. From the present data and assuming that these experiments represent cometary surface conditions, it can be concluded that irradiation of comets by the solar wind 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 hoch{12}C / hoch{13}C ratio of the gas phase should be regarded as a face value for the bulk cometary nucleus.