Circular Polarisation in Star Forming Regions: The Origin of Homochirality?

The origin of homochirality is one of the longest-standing puzzles in understanding the origins of life. In the laboratory, illumination by circularly polarised UV radiation (asymmetric photolysis) is an effective means of producing an enantiomeric excess in an otherwise racemic mix of chiral molecules. In the natural world, however, it has proven difficult to identify a suitable source of Circularly Polarised Light (CPL). Recent observations of L-excesses of 2–9% for a number of α-methyl amino acids in the Murchison meteorite and our discovery of large degrees of CPL in some star forming regions has added weight to the suggestion that the origin of homochirality is extra-terrestrial. Here we report initial modelling of the production of that CPL.

Astronomical Sources of Circularly Polarized Light and Their Role in Determining Molecular Chirality on Earth

The major source of circularly polarized flux potentially capable of inducing asymmetric photolysis of a racemic mixture of chiral molecules on the primitive Earth is the daytime sky. The degree of circular polarization (q) depends on the slope and orientation of the local terrain which obscures part of the sky. After allowance for dilution by direct sunlight values of |q| range between about 10−4 and 10−5. The rates of (1) asymmetric photolysis, (2) chiral selection by the electro-weak process and (3) racemization are not yet well enough known for the relative importance of these processes to be definitely established.