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D. Bockelée-Morvan, D. Gautier, F. Hersant (Obs. Paris), J.-M. Huré (Obs. Paris, Univ. Paris 7), F. Robert (Mus. Hist. Nat., Paris)
Cometary nuclei contains Mg-rich silicates in both amorphous and crystalline form. Since silicates in the protosolar cloud were presumably amorphous, as observed in the ISM, crystalline silicates must have been formed in the inner hot regions of the Solar Nebula and transported to the region of formation of comets by some mechanism.
We have developed a time-dependent model which investigates radial mixing of microscopic grains in the Solar Nebula by turbulence. The model uses pseudo-time-dependent temperature and surface density profiles of the Solar Nebula describing its evolution subsequent to the formation of the Sun. These profiles are generated with the 2-D accretion disk model of Huré (2000, A&A 358, 378) based on the \alpha-prescription for turbulent viscosity. Silicatic grains are assumed to be initially amorphous. The thermally induced amorphous-to-crystalline transition occurring in the inner nebula is modelled using available experimental data.
The computed time-dependent radial profiles of the crystalline-to-amorphous mass ratio (Cc/Ca) show that Cc/Ca progressively increases in the outer regions due to turbulent diffusion to reach a plateau, while the silicates within the formation region of meteorites remain essentially crystalline. The diffusion time scale and the outer Cc/Ca plateau depend on the Solar Nebula model, defined by its initial mass accretion rate and radius and the \alpha viscosity parameter. Some of the solar nebulae which explain the D/H ratios measured in meteorites, comets, Uranus and Neptune (Hersant et al. 2000, in preparation and this conference), namely the warmest, provide outer Cc/Ca ratios in agreement with that measured in comet Hale-Bopp. The time scale for radial mixing ~ 5 \times 104 yr is well below the time needed to form kilometer-sized comets from a population of microscopic grains.