Distribution of major icy species in different grain size bins, indicated by their size (nm) at final time t = 1.55 Myr. The four ice layers are numbered: layer (1) is the surface, while layer (4) is adjacent to the refractory grain core. Species within a single layer are intermixed, they are shown separately here to illustrate their proportions within the layer. Although thickness is similar for all bulk-ice layers for a given grain size, the species in the outer layers are more abundant because the grain has grown. This effect is more pronounced for the smallest grains. Most of the ‘other’ molecules are NH₃ and also CH₄.