Properties of the gas escaping from a non-isothermal porous dust surface layer of a comet

Estimation of the properties of the sublimation products leaving the cometary nucleus is one of the significant questions in the study of the dusty-gas flow following the Rosetta mission. It is widely assumed that the temperature of the water molecules emitted is the temperature of ice directly exposed to the surface. However, it is the simplest non-verified idealization if the refractory porous material lays on the surface and controls the energy driving the ice sublimation. This highly non-isothermal surface layer should change the vapour temperature as the molecules pass through it from the icy region to the vacuum. A key sustaining observation here comes from the MIRO experiment on Rosetta which measured the velocity of water vapour. The observed gas velocities are visibly higher than can be explained by emission at typical ice surface temperature. To investigate the issue, we simulate a gas flow through a non-isothermal porous dust layer and analyse the temperature of molecules emitted. Monodisperse and bimodal layers, as well as layers made of porous aggregates, are considered. Modelling is carried out for various porosity values, different particle sizes, and dust layer thicknesses. The simulation results are embedded in two-layer thermal models including the effective thermal conductivity, volumetric light absorption, and the resistance of the dust layer to the gas flow.