  Observational studies reveal that complex organic molecules (COMs) can be
found in various objects associated with different star formation stages. The
identification of COMs in prestellar cores, i.e., cold environments in which
thermally induced chemistry can be excluded and radiolysis is limited by cosmic
rays and cosmic ray induced UV-photons, is particularly important as this stage
sets up the initial chemical composition from which ultimately stars and
planets evolve. Recent laboratory results demonstrate that molecules as complex
as glycolaldehyde and ethylene glycol are efficiently formed on icy dust grains
via non-energetic atom addition reactions between accreting H atoms and CO
molecules, a process that dominates surface chemistry during the 'CO-freeze out
stage' in dense cores. In the present study we demonstrate that a similar
mechanism results in the formation of the biologically relevant molecule
glycerol - HOCH2CH(OH)CH2OH - a three-carbon bearing sugar alcohol necessary
for the formation of membranes of modern living cells and organelles. Our
experimental results are fully consistent with a suggested reaction scheme in
which glycerol is formed along a chain of radical-radical and radical-molecule
interactions between various reactive intermediates produced upon hydrogenation
of CO ice or its hydrogenation products. The tentative identification of the
chemically related simple sugar glyceraldehyde - HOCH2CH(OH)CHO - is discussed
as well. These new laboratory findings indicate that the proposed reaction
mechanism holds much potential to form even more complex sugar alcohols and
simple sugars.
