Polarimetric technique to study (pre)biological organics in cosmic dust and planetary aerosols

Noticeable circular polarization was measured in several dusty environments, including star forming regions and comets. These measurements have raised the possibility that circular polarization may relate to the formation (in molecular clouds) and observation (in comets) of prebiological organics, characterized by homochirality (dominance of molecules of specific handiness). We present computer simulations of light scattering by aggregates to model circular polarization from realistic particles containing homochiral molecules. Using optical constants of chlorophyll a, we study the spectral and phase angle dependencies of circular polarization. We begin our study by reproducing laboratory measurements (Sparks et al., 2009a), which found a fast change and reversal of sign for circular polarization within absorption bands. We further study how this spectral effect depends on the size and number of monomers in aggregates. We show that larger aggregates are characterized by larger values of circular polarization and that circular polarization peaks at medium (40–140°) phase angles. We discuss at which wavelengths such a spectral feature can be expected for (pre)biological molecules other than chlorophyll. These findings may help observers to optimize spectral and geometrical parameters in the search of cosmic dust that contains prebiological organics or aerosols of biological origin in atmospheres of planets.