Date: 11 May 2011

Astrobiological remote sensing with circular polarization

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Abstract

One of the main drivers of astrobiology is the search for life in the Universe. Important evidence relevant to extraterrestrial life is the existence in space of organic molecules of prebiological or biological significance. Such molecules are often characterized by a special type of asymmetry called “homochirality” (domination of molecules of a specific handedness). This results in optical activity of materials containing such molecules. Due to optical activity, the light scattered by such materials is characterized by non-zero circular polarization. We review existing observations of circular polarization in space, including observations of molecular clouds, comets, and the planet Mars. We also review laboratory measurements of light scattered by biological (e.g., bacteria and leaves) and non-biological (minerals) samples. These reveal distinctive features in the circular polarization spectra in absorption bands for the biological samples. We also consider theoretical simulations of light scattering by homochiral materials. Significant progress in this direction has been achieved after the development of the superposition T-matrix code for clusters of optically-active spheres. This allows us to simulate light scattering by biological objects, e.g., colonies of bacteria, and by materials of prebiological value, e.g., cometary dust. We explore how circular polarization depends on the porosity and size of aggregates as well as on the degree of their homochirality.