Modeling of the thermal behavior and of the chemical differentiation of cometary nuclei

Abstract

Taking into account the low values of average density reported for Comet P/Halley, a model of a porous comet nucleus has been developed. The model includes heat conduction through the solid matrix and heat and mass transfers due to the vapor phase circulating in the pore system. Sublimation and recondensation of volatile species within the nucleus as well as water ice crystallization are represented according to experimental results on the evolution of ice mixtures with time and temperature. The model is based on the resolution of two symmetric diffusion equations through the whole nucleus, one describing the transport of matter and the other the transport of heat. These equations are linked by the source term. The sensible heat term due to gas advection is also integrated into the heat diffusion equation. The calculations are performed for comets on the orbits of P/Halley and P/Churyumov-Gerasimenko. Different nucleus compositions with ice mixtures containing H ₂O and CO or CO ₂ are investigated. The consequences of the amorphous to crystalline phase transition of water ice on the thermal profiles and on the evolution of the composition of the nucleus are studied. Results are presented on the evolution of the stratigraphy of the nucleus and of the production rates of CO, CO ₂, and H ₂O as a function of the heliocentric distance. Several phenomena are evidenced such as the depletion of volatiles in the subsurface layers and their enrichment in deeper layers. The important role of CO and CO ₂ in controlling the propagation of the phase transition is emphasized. The relation between the composition of the nucleus and gas production rates is discussed.