Transport Characteristics of a Hierarchical Near-Surface Layer of the Nucleus of Comet 67P/Churyumov–Gerasimenko.

In this paper, we develop previous studies that considered a free molecular gas flow through a dust porous mantle of a cometary nucleus. Before, we considered various types of both homogeneous and heterogeneous layers built of nonintersecting spheres, including the layers containing microscopic cracks and inner cavities. At the same time, data from the Rosetta space mission provide convincing evidence that the near-surface layer is composed of porous aggregates rather than homogeneous solids. In this study, we propose models, in which the layer is constructed of porous aggregates formed by ballistic agglomeration. The effective porosity of the model layers is in a range of the values resulting from the analysis of observations of comet Churyumov–Gerasimenko. With the test-particle method, we quantitatively estimated the distribution functions for the free paths, the layer's permeability, and the other effective kinetic characteristics of sublimation products that passed through a nonisothermal porous layer. In addition, we estimated the volume adsorption of the visible solar light in the near-surface absorbing layer. For all of the considered transport characteristics, we present approximating expressions that may effectively be used in nonstationary thermophysical models of the physics of a cometary nucleus. [ABSTRACT FROM AUTHOR]