Thermal conductivity of granular materials relevant to the thermal evolution of cometary nuclei

A formula is presented for the thermal conductivity of porous granular materials on the basis of the effective medium theory. The theory is also used to derive thermal conductivity of a mixture of amorphous and crystalline ices. The results are applied to the study of thermal evolution of a cometary nucleus, in which earch of the grains has a silicate core and an amorphous H2O icy mantle initially. It is shown that for such pure-H2O comets, H2O ice crystallizes almost completely independent of porosity and contact area between adjacent grains.