Towards a model of cometary nuclei for engineering studies for future space missions to comets

A brief review is given of the present state of knowledge on comets. Existing comet models are critically analysed in view of these observational facts. This analysis leads to the conclusion that the most promising model approach is one where the comet nucleus is considered as a porous medium containing an intimate mixture of dust particles and different ices. Compact nuclei can be treated as limiting cases. An outline of a pseudo-tridimensional model based on this approach is given. The hypothesis is made that comets initially contain a mixture of amorphous water ice, solid carbon monoxide and silicate dust. As examples, local production rates of CO and integrated production rates of H2O and CO for comets P/Schwassmann-Wachmann 1 and P/Schwassmann-Wachmann 3 are calculated, making the extreme assumption that all the devolatilized dust remains on the surface. The depletion of CO in the near surface layers is determined after ten revolutions. Under the assumptions that all the dust remains on the surface or that all the dust is lifted off, thermal profiles at the equator of a nucleus on the orbit of comet P/Wirtanen with the rotation axis perpendicular to the orbital plane are determined when the surface temperature is maximum or minimum. The evolution of the maximum and minimum surface temperature is computed over ten orbital periods for two values of the bulk thermal conductivity of the dust coverage. For the same comet, the relative H2O and CO content as a function of depth are calculated for a dust covered and a non-dust covered nucleus. Improvements of the present day model are suggested and a strategy is proposed for adapting this model for complex and thus more realistic situations.