Molecular clouds, which have dense concentrations of dust and gas at temperatures around 10 K, often form icy grains, in which the gas condenses on the carbon/silicate dust grains. Some of the major constituents of gases in molecular clouds include H2, H2O, CO, CO2, CH4, NH3, and CH3OH.
It was once thought that these molecular clouds in the interstellar medium were too cold for any chemical reactions to occur, but radiation chemistry has shown us that even at extremely low temperatures, complex molecules can form.
When subjected to ionizing radiation, such as UV photons, these molecules can break apart and undergo radical-radical recombination reactions. For example, the C-H bond in methane can be cleaved by a UV photon (CH4 + hv -> CH3 + H), and the methyl radicals generated in this reaction can then recombine to form ethane (C2H6). These reaction pathways can become increasingly complex with the addition of more molecules in the ice layer and with the addition of catalytic surfaces (such as the carbon/silicate dust grains). This process can also form less volatile species that do not sublimate along with the rest of the ice, leaving behind a residue filled with complex organics, some of which may be prebiotically relevant.