Experimental study of the penetration of oxygen and deuterium atoms into porous water ice

Context. Many interstellar molecules are thought to form on dust grains. In particular, hydrogenation is one of the major mechanisms of the formation of mantle ice. To date it is not clear if H atoms can penetrate the bulk of the ice mantle or if it only has chemical activity on the accessible surface of grains. Aims. We wish to study the efficiency of atoms deposited on the outer surface of the amorphous solid water to penetrate into the ice bulk. Methods. NO molecules react with O and H atoms. They are easily detected by infrared (IR) spectroscopy. These two properties make this molecule an ideal chemical tracer for the penetration of O and H atoms through water ice. In our experiments we first deposited a NO undercoat and covered this layer (at 40 K) with a variable amount of water ice. Then, we exposed this undercoat to D (10 K) or O (40 K) atoms, and we followed the NO consumption and the products that appeared via IR signatures, and we finally analyzed the desorption of all species through a temperature-programmed desorption technique. We experimentally characterize the accessible surface of the ice and provide a model to interpret quantitatively our measurements. Results. Water ice limits the destruction of tracer NO molecules. The thicker the ice, the more NO remains unreacted. H and O atoms lead to the same amount of NO consumption, pointing out that access to reactants for these two different atoms is identical. We discuss different possible scenarios of NO localization (in and/or on the ice) and determine how this affects our observables (IR data and desorption profiles). Conclusions. In our experimental conditions, it is not possible to measure any atom penetration through the bulk of the ice. The surface diffusion followed by reaction with NO or by self-reaction (i.e., H + H → H2) is faster than bulk diffusion. We propose lower limit values for penetration barriers. Therefore the building of astrophysical ice mantles should be mostly driven by surface reactivity