Main destruction rates of key species. We plot the important destruction rates of H₂O (ice), CO (gas+ice), H₂CO (ice), and CH₃OH (ice) as a funtion of time. The destruction of CO through surface accretion is drawn in black, ⊥CO through hydrogenation and photodesorption in dark blue, ⊥H₂O through photodissociation in light blue, ⊥H₂CO through hydrogenation in yellow, and ⊥CH₃OH through photodissociation and dehydrogenation in red. For photo-processes, the rates obtained from cosmic rays and UV photons are added up.