Basic physical properties of cometary nuclei relevant to their thermal evolution are discussed including 1) thermal conductivities of amorphous ice and an aggregate of grains consisting of silicate cores and icy mantles, 2) the tensile strength of the grain aggregate, and 3) degassing and recondensation of volatile molecules such as CO and CO₂ in the icy mantles of the grains.

A unified model for evolution of the temperature, chemical composition, and mechanical strength of cometary nuclei is presented with taking account of relevant elementary processes such as release of net latent heat associated with the phase transitions of amorphous ice containing the volatile molecules and sintering of the icy grains in the nucleus. It is shown that the evolution of cometary nuclei forks into three paths depending on the initial chemical composition of the volatile molecules in the ice:

1. Endothermic case, where both CO and CO₂ are contained in the ice in significant amount (~10%): The crystallization degree of amorphous H₂O ice is 40%, and CO trapped in the fraction of the ice is evolved at the final stage; this CO escapes eventually from the nucleus. On the other hand, CO₂ condenses on the surfaces of the dust grains immediately after crystallization of amorphous H₂O ice and is preserved. As the crystallization proceeds, sintering of CO₂ and H₂O proceeds, and as a result the tensile strength is enhanced by three orders of magnitude.
2. Exothermic case, where the contents of both CO and CO₂ are smaller (~1%) than that of 1): Complete crystallization of the amorphous H₂O ice takes place and runaway temperature increase occurs up to about 140 K with increasing the pressure gradient of CO and CO₂ gas released from the ice. Sintering of CO₂ and H₂O leads to the tensile strength increase by three orders of magnitude as in the case 1).
3. No CO₂ case, where CO is contained in considerable amount but the CO₂ abundance is small. The evolution in this case is essentially the same as in the case 2) but disruption of the cometary nuclei occurs depending on the magnitude of the activation energy of surface diffusion of H₂O. If the activation energy is large, sintering of H₂O proceeds slowly, and consequently the gas pressure gradient due to CO overwhelms the tensile strength at some point, leading to disruption of the nucleus.

Discussion is given on the implications of the results. It is pointed out that the diversity of evolution of cometary nuclei is related to the volatile composition of grains in the parent molecular clouds from which planetary systems form. The implications of the evolution of the tensile strength of cometary nuclei are discussed in connection with collisional accretion of icy planetesimals to the Jovian planets and outer solar system bodies.