Schematic view of our scenario and the layered ice structure. Stage I) The main formation stage of H₂O ice. Water deuteration is not efficient, f_D2< f_D1 < 10^-3. The majority of oxygen is locked in O-bearing molecules in this stage. Stage II) CO/CH₃OH-rich outer ice layers are formed, while the formation of water ice continues with much reduced efficiency, compared to Stage I. Nevertheless, the formation of HDO and D₂O ices is more efficient than in Stage I, owing to the enhanced deuteration processes, 10^-3 ≪ f_D2<f_D1.