THERMAL EVOLUTION AND DIFFERENTIATION OF A SHORT-PERIOD COMET

The evolution of the subsurface layers of a short-period comet has been studied. The structure and composition of the surface layers due to sublimation-recondensation phenomena, to gas diffusion processes through the pore system and to the ejection of dust particles have been investigated in detail. The nucleus has been modelled as a mixture of water ice, CO2 ice and dust in specified proportions. The icy matrix is assumed to be porous and crystalline. The model is based on the solution 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 a source term which accounts for production or loss of gas. We assume that the water vapour present in the pore system acts as a perfect gas, and that sublimation and recondensation are instantaneous in order to maintain the local thermodynamic equilibrium between the solid phase and its vapour. Under these assumptions, the source term depends on the variation of the pressure due to vapour diffusion, and on the variation of the saturation pressure of the vapour due to the evolution of the temperature. The diffusion regime, Knudsen or viscous, depends on the mean free path of the molecules of gas through the pore network, considered as a system of cylindrical pipes. The dust particles may be removed from the surface of the nucleus depending on the force balance. The calculations are performed for a nucleus on the orbit of P/Du Toit-Hartley, that was one of the possible targets for the Rosetta mission. Different nucleus compositions with various CO2/H2O ice and dust/ice ratios are investigated. Results are presented on the evolution of the stratigraphy of the nucleus and on the production rates of CO2, H2O and dust particles as a function of the heliocentric distance. Several phenomena are observed, such as the depletion of CO2 ice in the subsurface layers and the possible formation of a dust layer at the nucleus surface.