Modeling the structure and activity of comet nuclei

Numerical simulation of the structure and evolution of a comet nucleus is reviewed from both the mathematical and 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, permeability to gas flow, drag of dust grains, and dust mantling, 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 param-eters, such as porosity and composition; and initial parameters, such as temperature and live radioisotope content. 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. 1