Representative timescales relevant to the formation of s-H₂S and s-H₂O as well as their freeze-out depths. Upper panel: the continuous black curve is the timescale for a grain to be hit by an H atom. Once in the grain surface, the H atom diffuses and can react with an adsorbed S atom to form s-SH. The dashed magenta curves show the timescale for thermal desorption of an s-S atom (E_b ∕k (S) = 1100 K left curve, and 2600 K right curve) and of an s-O atom (blue curve; E_b∕k (O) = 1800 K). The gray dotted curve is the photodesorption timescale of s-S. At G₀ values where the continuous line is below the dashed and dotted lines, s-O and s-S atoms remain on grain surfaces sufficiently long to combine with an adsorbed H atom and form s-OH and s-SH (and then s-H₂ O and s-H₂S). These timescales are for n_H = 10⁵ cm⁻³ and n(H) = 100 cm⁻³. Bottom panel: freeze-out depth at which most O and S are incorporated as s-H₂ O and s-H₂S (assuming no chemical desorption and T_k = T_d).