Sketch of the energy exchanges (heat sources and sinks) at the surface and sub-surface of a cometary nucleus: is the vector normal to the surface and θ_i is the solar incidence angle; T₀ is the temperature at the surface of the nucleus; and are the temperatures at the sublimation fronts for the respective type of ice; and T_N is the lower boundary temperature of the nucleus. The coupling mode is represented as a switch in between the H₂O and CO₂ sublimation fronts. The shadowed layers at and represent the selected effective active fractions (EAF) used at the sublimation front of each molecule.