As Fig. 7 but for surface diffusivities. The cross indicates the CS column density and the arrows indicate the upper limits of the observed column densities for other S-bearing molecules. Left to right panels: three scenarios for the surface diffusivity are considered: (1) slow diffusion, E_diff ∕E_bind = 0.77, (2) average diffusion, E_diff∕E_bind = 0.5, and (3) fast diffusion, E_diff∕E_bind = 0.3.