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FROM COMETS TO METEORS J. MAYO GREENBERG Raymond and Beverly Sackler Laboratory & 8230; of intimately related silicate and carbonaceous materials (core-mantle structure & 8230; FROM COMETS TO METEORS J. MAYO GREENBERG Raymond and Beverly Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, Niels Bohrweg 2, Leiden 2300 RA, The Netherlands E-mail: [email protected](Received 8 May 2000; Accepted 15 July 2000) Abstract. A comet nucleus and dust properties & 8230;

2. Chemical composition of comets The chemical composition of a nucleus can be very strictly constrained by combining the latest results on: the core-mantle interstellar dust model, the solar system abundances of the elements, the space-observed composition of the dust of comet Halley, and the latest data on the volatile molecules of comet comae. A discussion of how interstellar dust comes to be incorporated into comets during the formation of solar system is beyond the scope of this paper. However, it is certainly recognized that both the comet coma volatiles and the comet dust composition are very closely related to what we infer to be the composition of the primitive solar nebula dust as it existed 4.6 billion years ago. There are striking similarities between the volatile composition of comets and hot cores of regions star formation. In the very beginning there are the particles blown out of cool evolved stars. These accrete mantles in the denser clouds, which are photoprocessed. The mantles contain molecules created by surface reactions, by gas phase reactions and by photoprocessing. What is followed here are the & 8220;large& 8221; tenth micron (mean radius) grains which contain much of the mass of the dust - all the volatiles and all the silicates. Other populations of interstellar dust consist of very small carbonaceous particles and even smaller particles, which are presumed to resemble large polycyclic aromatic hydrocarbons. The large grains cycle between low-density (diffuse) clouds y molecular clouds and star forming clouds.

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