Cold hydrogen atoms at T ≥ 7K were shown experimentally to react with graphite grains at the same temperature to produce CH4 and smaller amounts of C2H6, C2H4 and C2H2. At T < 20K the hydrocarbon mantle could polymerize to form carbonaceous substances, similar to those found in carbonaceous chondrites. Further encounters with H-atoms would result in their recombination on the hydrocarbon mantle around the grains. At higher grain temperatures, the hydrocarbons formed could be ejected into the gas phase.
Cold iron atoms at T < 5K were shown experimentally to react with molecular hydrogen in a T < 5K matrix. Mössbauer studies with 57Fe demonstrated the formation of an Fe-H2 bond. FeH2 and FeH molecules could be formed on grains by encounters of iron atoms with either H-atoms or H2 molecules.