THERMAL HISTORY OF COMETS DURING RESIDENCE IN THE OORT CLOUD - EFFECTOF RADIOGENIC HEATING IN COMBINATION WITH THE VERY-LOW THERMAL-CONDUCTIVITY OF AMORPHOUS ICE
J. Haruyama et al., THERMAL HISTORY OF COMETS DURING RESIDENCE IN THE OORT CLOUD - EFFECTOF RADIOGENIC HEATING IN COMBINATION WITH THE VERY-LOW THERMAL-CONDUCTIVITY OF AMORPHOUS ICE, J GEO R-PLA, 98(E8), 1993, pp. 15079-15090
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.