67P/Churyumov-Gerasimenko nucleus model: Portrayal of the Rosetta target

Rosetta is a cornerstone ESA mission dedicated to the detailed study of a comet nucleus before and after the onset of activity. Comet 67P/Churyumov-Gerasimenko is the new Rosetta target. At the moment, little is known about this comet, but the successful design of the ROSETTA mission requires some preliminary knowledge of the comet's physical parameters. In fact, surface and interior temperature, day-night variations, onset of activity, percentage of active surface, intensity of different gas and dust fluxes are needed to select safe orbits around the comet and to optimize the scientific return. These quantities cannot be determined only through ground-based observations, so predictive models of the thermal evolution and differentiation of a cometary nucleus are necessary. We have developed a comet nucleus thermal evolution model that helps in the estimations of the above quantities. The ideal nucleus is an initially homogeneous and porous sphere, composed by a mixture of different ices and dust particles. The results have been computed with a numerical code solving simultaneously the heat conduction and gas diffusion equations through an idealized spherical comet nucleus. We show the results obtained applying this model to a comet nucleus with the characteristics of Churyumov-Gerasimenko. The results indicate that the peculiar dynamical history has a strong effect on the comet with the formation of a dust mantle before the 1959 close encounter. After that event, the perihelion distance was reduced and the comet was probably "reactivated". In our model, we see a slight increase of the activity passage after passage. We found a good agreement between the model results and the few data available. (c) 2006 COSPAR. Published by Elsevier Ltd. All rights reserved.