Astronomical sources of circular polarization in visible light and the implications for the origin of chirality

High levels of circularly polarized light (CPL) in the Orion nebula (up to 17%), thought to arise from scattering of starlight by interstellar dust, have been observed at 2.2 mum by Bailey et al. (1998). They suggest that the level of CPL may also be strong enough in the UV for the mechanism of asymmetric photolysis, by UV CPL, to act on molecules on the surfaces of interstellar grains, and that this might ultimately be the source of biomolecular chirality on Earth. This cannot be tested in the UV, so we have searched for CPL in the visible (0.5 mum) in the same region of Orion, and find no values above 0.9%. However, because of the much greater dust extinction at 0.5 mum the scattering geometry is radically different at the two wavelengths and we have therefore not yet been able to carry out a definitive test of the theory. This will require a study of a large area that includes regions that are optically thin at a wide range of wavelengths and in several molecular cloud regions.