Chemical composition of the mantle layers as a function of the monolayers at a time of 2 × 10⁵ years, for a density n_H = 10⁵ cm^-3, as in Cuppen et al. (2009) (see text): CO (red), formaldehyde (blue) and methanol (green). The four panels refer to the four cases of Fig. 4 of Cuppen et al. (2009) (see text): a) grain temperature 12 K, n(H)/n(CO) = 1 and n_d = 2 × 10¹²n_h; b) as a) but n_d = 1 × 10¹²n_h; c) as a) but temperature 15 K; d) as b) but n(H)/n(CO) = 0.5.