The friable sponge model of a cometary nucleus

The mantle/core model of cometary nuclei, first suggested by Whipple and subsequently developed by Mendis and Brin, is modified and extended. New terms are added to the heat conduction equation for the mantle, which is solved in order to obtain the temperature distribution in the mantle and the gas production rate as a function of mantle thickness and heliocentric distance. These results are then combined with some specific assumptions about the mantle structure (the friable sponge model) in order to make predictions for the variation of gas production rate and mantle thickness as functions of heliocentric distance for different comets. A solution of the time-dependent heat conduction equation is presented in order to check some of the assumptions.