Light scattering by agglomerates: Interconnecting size and absorption effects (PROGRA2 experiment)

Linear polarization of the scattered light by clouds of dust particles and by very large agglomerates deposited on a surface are studied with the PROGRA² experiment. A first series of measurements use bare silica spheres and black-coated spheres to compare the phase curves obtained by different sizes of agglomerates with varying albedos. The refractive indices are evaluated by comparison with numerical simulations. Then, the maximum polarization, P_max, on the phase curves for irregular particles is studied as a function of the size of the grains (equivalent diameters from submicron-sized to hundreds of micrometres) and of the agglomerates (from micrometres to centimetres). A minimum value of P_max is obtained for silica (about 5% for lifted agglomerates and 3% for layers of particles with a grain size of about 50 μm) and amorphous carbon (about 40% for lifted agglomerates and layers with a grain size of about 0.2 μm). For smaller grain sizes, P_max increases when the grain size decreases. For larger grain sizes, P_max increases when the grain size increases. Differences between transparent and absorbing materials are underlined. Such studies may be used to interpret remote observations of light scattering by dust particles in cometary comae and Titan's atmosphere.