THERMAL HISTORY OF COMETS DURING RESIDENCE IN THE OORT CLOUD - EFFECTOF RADIOGENIC HEATING IN COMBINATION WITH THE VERY-LOW THERMAL-CONDUCTIVITY OF AMORPHOUS ICE

The thermal history of cometary nuclei during residence in the Oort cl oud is studied with the use of the very low thermal conductivity of am orphous ice recently obtained by Kouchi et al. [1992a]. The heat sourc es included are (1) radioactive nuclides K-40, Th-232, U-235, and U-23 8 with their chondritic abundances, and (2) latent heat released in tr ansition from amorphous ice to crystalline ice. We model the cometary nucleus as a porous aggregate of grains with each individual grain bei ng composed of a refractory core and an icy mantle. It is assumed that the ice is initially amorphous. The bulk thermal conductivity of a co metary nucleus is assumed to be expressed by the product of the therma l conductivity of individual grains and a reduction factor resulting f rom the porous structure of the nucleus. Numerical results of the ther mal history are presented for various conditions including one case wh ich includes heating by Al-26 decay. It is shown that the thermal hist ories are clearly classified into two distinct types depending mainly on the nucleus thermal conductivity kappa. (1) Comets with small K exp erience a runaway increase in the internal temperature to higher than 120 K during residence in the Oort cloud, in which case most of the ic e in the nucleus crystallizes. (2) Comets with a sufficiently large ka ppa, on the other hand, do not exhibit a runaway heating and the tempe rature is limited to < 100 K so that the initial amorphous ice is almo st completely preserved. A criterion of nuclear ice crystallization is presented in an analytic expression derived from the analysis of the physical processes of the crystallization. A brief discussion is given on the implications of the results for the sources of volatile molecu les observed in the coma.