On the flux of water and minor volatiles from the surface of Comet Nuclei

Surface temperature and the available effective energy strongly influence t he mass flux of H2O and minor volatiles from the nucleus. We perform comput er simulations to model the gas flux from volatile, icy components in porou s ice-dust surfaces, in order to better understand results from observation s of comets. Our model assumes a porous body containing dust, one major ice component (H2O) and up to eight minor components of higher volatility (e.g . CO, CH4, CH3OH, HCN, C2H2, H2S), The body's porous structure is modeled a s 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 mo del includes radially inward and outward flowing vapor within the body, esc ape of outward Rowing gas from the body, complete depletion of less volatil e ices in outer layers, and recondensation of vapor in deeper, cooler layer s. From the calculations we obtain temperature profiles and changes in rela tive chemical abundances, porosity and pore size distribution as a function of depth, and the gas flux into the interior and into the atmosphere for e ach of the volatiles at various positions of the body in its orbit. In this paper we relate the observed relative molecular abundances in the c oma of Comet C/1995 O1 (Hale-Bopp) and of Comet 46P/Wirtanen to molecular f luxes at the surface calculated from our model.