Reference comet model for the ROSETTA mission

The successful accomplishment of the ROSETTA mission to a cometary nucleus requires some preliminary knowledge of comet status and activity, such as surface temperature, percentage of active surface, intensity of gas and dust fluxes and so on. It would be impossible to answer these questions only on the basis of ground-based observations, so it is necessary to use a predictive model of the thermal evolution and differentiation of a cometary nucleus. The nucleus of the model built by the authors' group is a sphere, initially homogeneous, composed of a highly porous predefined mixture of ices of water (the dominant constituent), CO/sub 2/, CO and silicatic dust grains embedded in it. In this work the authors applied the model, with different choices of parameters, to a comet on the orbit of P/Wirtanen, the target of ROSETTA mission, with the aim of obtaining information necessary to the planning of the mission. The results of this simulation seem to indicate that: The range of expected comet surface temperatures is: at the equator-aphelion 130 K; at the equator-perihelion 210 K; at latitude 80/sup deg/-aphelion 90 K; at latitude 80/sup deg/-perihelion 160: K; The circumpolar regions are probably inactive and covered by a dusty crust, while the equatorial regions are probably characterized by active areas. The depth of the volatile-depleted ice layers varies from 5 m at the equator to 10 m at the pole.