Sensitivity of modelled cometary gas production on the properties of the surface layer of the nucleus

Explanation of the observed gas activity based on the parameters of the comet's nucleus is not an obvious matter. Solutions based on certain thermal models can be obtained, but the use of arbitrary values for poorly known model parameters is always required. In this work, we carry out a study of the dependence of gas activity on these parameters and assess the importance of each of them. We consider model porous dust layers of diverse structures. Solid state and gas thermal conductivities are examined to estimate the possible range of effective thermal conductivity. The simulation results are embedded in a thermal model, explicitly including a radiative thermal conductivity and a resistance of the dust layer against the gas flow. Sublimation of water ice and supervolatiles (CO2 and CO) is tested at different heliocentric distances. It is shown that when sublimation is the main energy sink, the role of uncertainties in the structure of the layer is small. As the relative contribution of sublimation decreases, the scatter of solutions reaches tens of per cent. The expected large uncertainties in the effective thermal conductivity can also significantly change the gas production. The analysis performed shows that, in spite of the parameter range having been narrowed down by results from the Rosetta mission, the unavoidable uncertainty in the values of some model parameters (e.g. thermal conductivity) blurs the theoretical simulation estimates. Instead of presenting a narrow set of specific solutions, it is desirable to analyse the entire range of possible solutions.