Schematic view of a portion of the mantle layer as modelled by GRAINOBLE. All pores are assumed to be square, to have the same size, and no walls (see text). The edges sites, that connect the non-porous surface and the pores are depicted in blue. In this example, each pore is constituted by 16 sites, of which the 12 blue sites are edge sites, and the fraction of area occupied by the pores is 0.1.