The unique macroscopic homochirality of biological material offers the prospect of being detectable remotely using circular polarization. To explore this concept, we present spectropolarimetry of primitive photosynthetic microbial organisms. The reflected-light spectra show clear circular polarization signatures associated with the strong electronic transitions of the photosynthetic absorption bands. The polarization is apparently due to the molecular circular dichroism of the chromophores which in the case of chlorophyll displays a characteristic sign change at the absorption maximum. We compare these polarization signals to those of a leaf and iron oxide. The biological samples produce a polarization signature of their chirality which, while small, can be orders of magnitude larger than the abiotic control. Hence with sufficient light levels, circular spectropolarimetry can be an excellent biomarker with the virtues: (i) it can be used remotely using spectropolarization observational methods (ii) it exploits a very generic characteristic of biochemical life, homochirality, and hence is sensitive to life potentially different to terrestrial (iii) it may be very pure such that a spectropolarimetric signal strongly implies the presence of macroscopically chiral and hence biological material, with low probability of false positives.
Pub Type: Journals
astrobiology, chirality, circular dichroism, diffuse scattering