The thermal and structural evolution of multicomponent ice/mineral samples due to surface radiative heating has been detail by Greenberg (1990), leads to the presumption that on investigated experimentally. The components of the samples a large scale cometary nuclei are rather homogeneous bodwere water ice, carbon dioxide ice, and dunite. Starting with origies, with similar chemical composition in their interior and inally homogeneous mixtures, we observed the sublimation of close to the surface, as long as they reside far from the Sun different volatiles under low pressure and low temperature conin the Oort cloud. At these distances the only mechanism ditions. This process resulted in the formation of stratified samthat may modify the composition and structure of the surface ples, containing clearly divided regimes with different chemical layers is the influence of cosmic hard radiation (Johnson et composition and cohesivity. A new computer model, developed al. 1987). This situation changes dramatically when the comet along the lines of our previous work, is applied to provide a quanencounters the inner Solar System. With the onset of comtitative description of the observed temperature evolution and stratification within the samples. These processes may play a key etary activity more violent processes commence, which work role in the formation of surface layers of cometary nuclei. Within on a much shorter time scale: volatiles evaporate dragging the ROSETTA mission, several already accepted experiments part of the dust away, but on the other hand a part of the aim at a direct (in situ) investigation of just these near-surface sublimated volatiles flows deeper into the porous nucleus layers on the nucleus of Comet P/Wirtanen. Therefore, both the and recondenses there. These processes should lead to a rather generally applicable thermal model and the particular restrong chemical differentiation of the outer layers of a comet sults of our laboratory experiments may provide useful support nucleus. They are mainly controlled by the evolution of temfor the design of these experiments.