The Effect of Varying Porosity and Inhomogeneities of the Surface Dust Layer on the Modelling of Comet Gas Production

ABSTRACT We study how uncertainties in knowledge of the microstructure of the porous dust surface layer affect our estimates of effective gas production of comet 67P/Churyumov-Gerasimenko. We analyse the transport properties (distribution of free paths, mean pore size, permeability) for random densely packed porous layers. Bimodal layers, as well as various cases of inhomogeneous layers having cavities and cracks, are considered. Modelling is performed for various values of effective porosity and various particle sizes and thicknesses of the dust layer. The results of computer simulations are embedded in thermal models of the surface region of the cometary nucleus, including explicitly a radiation thermal conductivity and a resistance of the dust layer for the gas flow is used. The simulations show that the porosity variations within the range of values expected for the 67P can change the effective gas production by tens of percent for thin layers, and can vary by several times with increasing layer thickness. These effects are manifested at all heliocentric distances where sublimation of water ice makes a significant contribution to the energy budget of the nucleus. Modelling also shows that in all considered cases the microscopic cavities and cracks have a minor effect on the effective gas production: changes in the presence and number of inhomogeneities in comparison with a uniform layer of the same effective porosity are negligible.