Thermochemistry of cometary nuclei I. The Jupiter family case.
Abstract
New experimental results related to the physical characteristics of the material components of the cometary nuclei as well as new ideas about several aspects of the modelling of the thermochemical process in the interior of this objects lead us to make a new attempt to analyse the physical evolution of Jupier family comets over time scales comparable 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 material includes a porous solid matrix and vapour filling the pores. As basic constituents of the solid matrix we consider three omnipresent species: dust, water ice (in two phases: amorphous and crystalline), and H_2_O vapour. In addition to the above, we include one substance more volatile than H_2_O, CO, initially trapped in the amorphous matrix. We improved on the earlier models by accounting for the state of near saturation attained by the vapour inside the nucleus, by including a separate treatment of an unsaturated surface layer and by explicitly including 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-mantle grains; 2) the adoption of a very low thermal conductivity of the amorphous ice mantles; and 3) a correct account of the energetics of gas release and the allowance for condensation of CO ice. We defined a "standard" nuclear model with the best guesses of the many unknown or poorly known parameters, and we ran it for 500 years in a typical Jupiter family orbit (q=1.5 AU, Q=6 AU), a time comparable to ~ 10 spurts) are notorious in the first revolutions, but then, they gradually evolve from the sharp spikes to a much more subdued appearance. A set of variant models were run to explore the consequences of some of our assumptions. Variations of the following parameters were considered: dust to ice ratio, porosity and amorphous ice conductivity. We note the broad similarity between our standard and variant models. The "standard" model was also run in a capture scenario, where the comets first stay n a high-q orbits and then into a low-q one. For high-q orbits , the rate of CO outgassing exceeds the perihelion H_2_O outgassing rate by several orders of magnitude. Upon capture, the comet basically behaves in accordance with the burial depth of the crystallization zone independent of which previous orbital evolution has led to this state. Concluding on the behaviour of Jupiter family comets, we find that the complete crystallization of a sizeable nucleus with an initial radius of several km should take ~10^4^ years. This means that Jupiter family comets with our assumed properties should still retain their CO, although in most cases buried deep below the nuclear surface.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- June 1994
- Bibcode:
- 1994A&A...286..659T
- Keywords:
-
- Amorphous Materials;
- Carbon Monoxide;
- Comet Nuclei;
- Dust;
- Ice;
- Matrix Materials;
- Thermochemistry;
- Water Vapor;
- Astronomical Models;
- Crystallization;
- Granular Materials;
- Thermal Conductivity;
- Astrophysics;
- COMETS: GENERAL;
- DIFFUSION