Schematic diagram of our multiphase models in which pores exist in the ice mantle. The yellow area indicates the gas phase. The blue, green, red, and light blue balls represent species in the active layers, normal species, interstitial species, and edge layer species, respectively. Species in the active layers can diffuse into the edge layers. Species in the edge layers can also diffuse into the active layers. So, the active layers and edge layers are combined together to form bigger active layers. Normal species are frozen in the partially active ice mantle.