Over the two years of the Rosetta mission accompanying the comet, the COmetary Secondary Ion Mass Analyzer (COSIMA), on board the orbiter, had collected in situ more than 35,000 dust particles and particle fragments in the inner coma of the comet 67P/CG (67P/Churyumov-Gerasimenko). These particle agglomerates with size ranging from ~50 to ~1000 µm, were captured at a low impact velocity (< 10 m/s) on metal targets and imaged and identified in situ with the COSIMA optical microscope COSISCOPE. Among all the dust particles collected, around 250 were chemically analyzed by the secondary ion mass spectrometry (SIMS) technique.

We have compared the global composition measured for 67P/CG's dust to previous results obtained from the Giotto and Vega missions for comet 1P/Halley and the Stardust mission for comet 81P/Wild 2, to the composition of Chondritic Porous Interplanetary Dust Particles (CP-IDPs) and to the CI chondrite composition. We have demonstrated that the organic matter of 67P/CG was different from all the semi-volatile compounds analyzed during the calibration step. However, similarities with Insoluble Organic Matter (IOM) extracted from carbonaceous chondrites are notable. Moreover, we have demonstrated that cometary particles from 67P are among the most carbon-rich objects in the Solar System, containing about 50% in mass of organic matter. Estimated H/C elemental ratio of the 67P/CG cometary organic matter is also reported.

Acknowledgements

COSIMA was built by a consortium led by the Max-Planck-Institut für Extraterrestrische Physik, Garching, Germany in collaboration with Laboratoire de Physique et Chimie de l'Environnement et de l'Espace, Orléans, France, Institut d'Astrophysique Spatiale, CNRS/ Université Paris Sud, Orsay, France, Finnish Meteorological Institute, Helsinki, Finland, Universität Wuppertal, Wuppertal, Germany, von Hoerner und Sulger GmbH, Schwetzingen, Germany, Universität der Bundeswehr, Neubiberg, Germany, Institut für Physik, Forschungszentrum Seibersdorf, Seibersdorf, Austria, Institut für Weltraumforschung, Österreichische Akademie der Wissenschaften, Graz, Austria (FWF P26871-N20) and is led by the Max-Planck- Institut für Sonnensystemforschung, Göttingen, Germany with the support of the national funding agencies of Germany (DLR, grant 50 QP 1801), France (CNES), Austria and Finland. Rosetta is an ESA mission with contributions from its Member States and NASA.