Theoretical Study of the Thermal Histories of Comets and Parent Bodies of Meterorites

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The study of the origins and thermal histories of comets and parent bodies of meterorites leads to the following results :1. A new model of the thermal history of cometary nuclei during residence in the Oort cloud has been obtained, in which particular attention has been paid to the extremely low thermal conductivity of amorphous ice is taken into account.2. Conditions for condensation and preservation of amorphous ice have been theoretically derived. Crystallinity of ices in space has been discussed on the basis of the results.3. Formation of organic materials has been studied by irradiating proton beams on simulated cosmic ices of various composition.4. A tensile strength of grain aggregates has been derived, and the results have been applied to the splitting of comet Shoemaker-Levy 9.5. The following study has also been done in relation to the results 1 to 4 : the origin of the Kuiper-belt objects, mesurements of infrared absorption coefficients of corundum grains, and infrared astronomical study of H_2O and CO ices in interstellar dark clouds.Some of the results have been presented as a series of lectures at the International Summer School on Cosmic Dust Connection held at Erice in June 1994.