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

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

C. Lécluse * , 1, 2, F. Robert 1, 3, R.-I. Kaiser 4, K. Roessler 4, C.T. Pillinger 2 and M. Javoy 1

1 Laboratoire de Géochimie des Isotopes Stables, Université Paris VII, 4, Place Jussieu, F-75251 Paris Cedex 05, France
2 Planetary Science Unit, Department Earth Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
3 Laboratoire de Minéralogie (URA no 736 CNRS), Muséum National d'Histoire Naturelle, 61, rue Buffon, F-75005 Paris, France
4 Institut für Nuklearchemie, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany

Received 17 May 1996 / Accepted 17 October 1997


Frozen CH4 and CH4 /Ar were irradiated by protons and 3 He [FORMULA] ions at 15 and 77 K 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 CH4. 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; 2) a sputtering of the polymerised species associated with an isotopic fractionation lying between 8 and 15 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 12 C/13 C ratio of the gas phase should be regarded as a face value for the bulk cometary nucleus.

Key words: methods: laboratory – comets: general – Sun: solar wind – ISM: cosmic rays

*Present address: CRPG-CNRS, 15 Rue Notre Dame des Pauvres, BP 20, 54501 Vandoeuvre-lès Nancy Cedex, France

Send offprint requests to: C. Lécluse, CRPG-CNRS, 15 Rue Notre Dame des Pauves, BP 20, 54501 Vandoeuvre-lès Nancy Cedex, France

© European Southern Observatory (ESO) 1998

Online publication: January 27, 1998