Understanding the power balance at the surface of the nucleus is essential to study the chemical and physical evolution of a comet. Surface temperatures and available effective energy strongly influence the mass flux of H 2 O and minor volatiles from the surface. We perform computer simulations to model the gas flux from volatile, icy components in porous ice-dust surfaces, layers in order to better understand results from observations of comets. The body's porous structure is modeled as a bundle of tubes with a given tortuosity and an initially constant pore diameter.
Heat is conducted by the matrix and carried by the vapors.
The one-dimensional model includes radially inward and outward flowing vapor within the body, escape of outward flowing gas from the body, complete depletion of less volatile ices in outer layers, and recondensation of vapor in deeper, cooler layers. From the calculations we obtain temperature profiles and changes in relative chemical abundances, porosity and pore size distribution as a function of depth, and the gas flux into the interior and into the atmosphere for each of the volatiles at various positions of the body in its orbit. We investigate the relationship of the observed relative molecular abundances in the coma with those in the nucleus, calculated from our model. We look into results of comet Hale Bopp and comets of the Jupiter family class.