| Abstract: | Some two decades ago, Hoyle and Wickramasinghe (1976) proposed that the physical conditions inside dense molecular clouds
favour the formation of amino acids and complex organic polymers. There now exists both astronomical and laboratory evidence
supporting this idea. Recent millimeter array observations have discovered the amino acid glycine (NH2CH2COOH) in the gas phase of the dense star-forming cloud Sagittarius B2. These observations would pose serious problems for
present-day theories of molecule formation in space because it is unlikely that glycline can form by the gas-phase reaction
schemes normally considered for dense cloud chemistry. Several laboratory experiments suggest a new paradigm in which amino
acids and other large organic molecules are chemically manufactured inside the bulk interior of icy grain mantles photoprocessed
by direct and scattered ultraviolet starlight. Frequent chemical explosions of the processed mantles would eject large fragments
of organic dust into the ambient cloud. Large dust fragments break up into smaller ones by sputtering and ultimately by photodissociation
of individual molecules. Hence, a sizeable column density (N≈ 1010−1015 cm-2) of amino acids would be present in the gaseous medium as a consequence of balancing the rate of supply from exploding mantles
with the rate of molecule destruction. Exploding mantles can therefore solve the longstanding molecule desorption problem
for interstellar dense cloud chemistry. A sizeable fraction of the organic dust population can survive destruction and seed
primitive planetary systems throughout our galaxy with prebiological organic molecules needed for proteins and nucleic acids
in living organisms. This possibility provides fresh grounds for a new version of the old panspermia hypothesis first introduced
by Anaxagoras. It is shown that panspermia is more important than asteroid and cometary organic depositions onto primitive
Earth. Furthermore, no appeal to Miller-Urey synthesis in a nonoxidizing atmosphere of primitive Earth is then needed to seed
terrestrial life.
This revised version was published online in July 2006 with corrections to the Cover Date. |
