THERMAL EVOLUTION AND DIFFERENTIATION OF A SHORT-PERIOD COMET

The evolution of the subsurface layers of a short-period comet has bee n studied. The structure and composition of the surface layers due to sublimation-recondensation phenomena, to gas diffusion processes throu gh the pore system and to the ejection of dust particles have been inv estigated in detail. The nucleus has been modelled as a mixture of wat er ice, CO2 ice and dust in specified proportions. The icy matrix is a ssumed to be porous and crystalline. The model is based on the solutio n 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 produ ction or loss of gas. We assume that the water vapour present in the p ore system acts as a perfect gas, and that sublimation and recondensat ion are instantaneous in order to maintain the local thermodynamic equ ilibrium between the solid phase and its vapour. Under these assumptio ns, the source term depends on the variation of the pressure due to va pour 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 p ipes. The dust particles may be removed from the surface of the nucleu s 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 possib le targets for the Rosetta mission. Different nucleus compositions wit h various CO2/H2O ice and dust/ice ratios are investigated. Results ar e 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 format ion of a dust layer at the nucleus surface.