Modeling the comet nucleus interior

Numerical simulation of the structure and evolution of a comet nucleus is reviewed both from the mathematical and from the physical point of view. Various mathematical procedures and approximations are discussed, and different attempts to model the physical characteristics of cometary material, such as thermal conductivity, or permeability to gas flow, are described. The evolution and activity of comets is shown to depend on different classes of parameters: Defining parameters, such as size and orbit, structural parameters, such as porosity and composition, and initial parameters, such as temperature and live radioisotope content. The latter are related to the formation of comets. Despite the large number of parameters, general conclusions, or common features, appear to emerge from the numerous model calculations - for different comets - performed to date. Thus, the stratified structure of comet nuclei, volatile depletion, and the role of crystallization of ice in cometary outbursts are discussed. Finally, an evolution model applied to comet C/1995 01 Hale-Bopp - using different assumptions - is described and analysed in the light of observations.