Activity of comets: Gas Transport in the Near-Surface Porous Layers of a Cometary Nucleus

The gas transport through non-volatile random porous media is investigated numerically. We extend our previous research of the transport of molecules inside the uppermost layer of a cometary surface (Skorov and Rickmann, 1995; Skorov et al. 2001). We assess the validity of the simplified capillary model and its assumptions to simulate the gas flux trough the porous dust mantle as it has been applied in cometary physics. A new microphysical computational model for molecular transport in random porous media formed by packed spheres is presented. The main transport characteristics such as the mean free path distribution and the permeability are calculated for a wide range of model parameters and compared with those obtained by more idealized models. The focus in this comparison is on limitations inherent in the capillary model. Finally a practical way is suggested to adjust the algebraic Clausing formula taking into consideration the nonlinear dependence of permeability on layer porosity. The retrieved dependence allows us to accurately calculate the permeability of layers whose thickness and porosity vary in the range of values expected for the near-surface regions of a cometary nucleus.Comment: 25 pages, 9 figure

Cometary surface dust layers built out of millimetre-scale aggregates: dependence of modelled cometary gas production on the layer transport properties

The standard approach to obtaining knowledge about the properties of the surface layer of a comet from observations of gas production consists of two stages. First, various thermophysical models are used to calculate gas production for a few sets of parameters. Second, a comparison of observations and theoretical predictions is performed. This approach is complicated because the values of many model characteristics are known only approximately. Therefore, it is necessary to investigate the sensitivity of the simulated outgassing to variations in the properties of the surface layer. This problem was recently considered by us for aggregates up to tens of microns in size. For millimetre-size aggregates, a qualitative extension of the method used to model the structural characteristics of the layer is required. It is also necessary to study the role of radiative thermal conductivity, which may play an important role for such large particles. We investigated layers constructed from large aggregates and having various thicknesses and porosity and evaluated the effective sublimation of water ice at different heliocentric distances. For radiati ve conducti vity, approximate commonly used models and the complicated model based on the dense-medium radiati ve transfer theory were compared. It was shown that for millimetre-size aggregates careful consideration of the radiative thermal conductivity is required since this mechanism of energy transfer may change the resulting gas productivity by several times. We demonstrate that our model is more realistic for an evolved comet than simple models parameterizing the properties of the cometary surface layer, yet maintains comparable computational complexity

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. Ho we ver, 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 ef fecti ve thermal conductivity, volumetric light absorption, and the resistance of the dust layer to the gas flow