A systematic study of light scattering by aggregate particles using the microwave analog technique:: Angular and wavelength dependence of intensity and polarization

Microwave analog experiments [e.g., Gustafson, 1996a, 1999] allow us to simulate light-scattering properties of particles of almost any combination of composition, size, and shape. We have taken advantage of this ability and the broadband nature of the University of Florida microwave facility in a systematic study of light scattering by aggregates. We present angular dependencies of intensity and polarization in the scattering angle range 0 degrees-165 degrees, and in the wavelength interval 2.7-4 mm, which simulates 0.430-0.660 mu m in the visual. The data are for a variety of sizes, shapes, and compositions of the constituent particles, as well as the number of particles in an aggregate and their packing. Wavelength dependencies are shown to be powerful diagnostics for the interpretation of comet dust observations and we urge observers to report color and polarimetric color. As an example, we estimate the size of constituent particles in cometary grains (0.1-1 mu m). We also show that the grains evolve with the distance from the nucleus and that the reason apparently is the loss of some dark matter. Techniques and results reported here are general enough to apply to a broad range of natural and artificial aggregates.