  The surface processes on interstellar dust grains have an important role in
the chemical evolution in molecular clouds. Hydrogenation reactions on ice
surfaces have been extensively investigated and are known to proceed at low
temperatures mostly below 20 K. In contrast, information about the chemical
processes of molecules within an ice mantle is lacking. In this work, we
investigated diffusive hydrogenation reactions of carbon monoxide (CO) embedded
in amorphous solid water (ASW) as a model case and discovered that the
hydrogenation of CO efficiently proceeds to yield H2CO and CH3OH even above 20
K when CO is buried beneath ASW. The experimental results suggest that hydrogen
atoms diffuse through the cracks of ASW and have a sufficient residence time to
react with embedded CO. The hydrogenation reactions occurred even at
temperatures up to ~70 K. Cracks collapse at elevated temperatures but the
occurrence of hydrogenation reactions means that the cracks would not
completely disappear and remain large enough for penetration by hydrogen atoms.
Considering the hydrogen-atom fluence in the laboratory and molecular clouds,
we suggest that the penetration of hydrogen and its reactions within the ice
mantle occur in astrophysical environments. Unified Astronomy
