THERMOCHEMISTRY OF COMETARY NUCLEI .1. THE JUPITER FAMILY CASE

New experimental results related to the physical characteristics of th e material components of the cometary nuclei as well as new ideas abou t several aspects of the modelling of the thermochemical process in th e interior of this objects lead us to make a new attempt to analyse th e physical evolution of Jupier family comets over time scales comparab le to their lifetime. A new model is described in this paper where we present results concerning the evolution of Jupiter family comets and make comparisons with previous models. Our model of the cometary mater ial includes a porous solid matrix and vapour filling the pores. As ba sic constituents of the solid matrix we consider three omnipresent spe cies: dust, water ice (in two phases: amorphous and crystalline), and H2O vapour. In addition to the above, we include one substance more vo latile than H2O, CO, initially trapped in the amorphous matrix. We imp roved on the earlier models by accounting for the state of near satura tion attained by the vapour inside the nucleus, by including a separat e treatment of an unsaturated surface layer and by explicitly includin g the erosional velocity of the surface. As far as physical parameters are concerned, our basic improvements on earlier models were: 1) the representation of this matrix as an aggregate of micron-sized core-man tle grains; 2) the adoption of a very low thermal conductivity of the amorphous ice mantles; and 3) a correct account of the energetics of g as release and the allowance for condensation of CO ice. We defined a ''standard'' nuclear model with the best guesses of the many unknown o r poorly known parameters, and we ran it for 500 years in a typical Ju piter family orbit (q = 1.5 AU, Q = 6 AU), a time comparable to approx imately 10 % of the their lifetime. The CO bursts (associated with cry stallization spurts) are notorious in the first revolutions, but then, they gradually evolve from the sharp spikes to a much more subdued ap pearance. A set of variant models were run to explore the consequences of some of our assumptions. Variations of the following parameters we re considered: dust to ice ratio, porosity and amorphous ice conductiv ity. We note the broad similarity between-our standard and variant mod els. The ''standard'' model was also run in a capture scenario, where the comets first stay n a high-q orbits nd then into a low-q one. For high-q orbits , the rate of CO out-gassing exceeds the perihelion H2O outgassing rate by several orders of magnitude. Upon capture, the come t basically behaves in accordance with the burial depth of the crystal lization zone independent of which previous orbital evolution has led to this state. Concluding on the behaviour of Jupiter family comets, w e find that the complete crystallization of a sizeable nucleus with an initial radius of several km should take approximately 10(4) years. T his means that Jupiter family comets with our assumed properties shoul d still retain their CO, although in most cases buried deep below the nuclear surface.