Species abundances as function of time at 50 (top panels), 150 (middle panels), and 250 K (bottom panels). The abundance of unoccupied silicate surface (*) and core (!) sites, of atomic hydrogen in the gas and solid phases, and of H₂ are shown in the left panels. Gas-phase water and water ice abundances are shown on the right panels together with gas-phase abundance of atomic oxygen and OH.