Experimental results. Time evolution of N_H2O(s), normalized to its initial value, during isothermal experiments at 160 K (left), 165 K (middle), and 170 K (right) shown in the top panel, for several values of L, the thickness of the top layer of a-C grains. Bottom panels show the Molecular flux J_H2O derived from proportional fits of the linear decays of N_H2O(s) reported in the top panels, plotted as a function of 1/L. Error bars reflect the uncertainty introduced by ϵ and L_C (see Appendix B).