Light scattering by aggregate particles

We have investigated scattering properties of aggregates, emphasizing the size of constituent monomers comparable with the wavelength of visible light, in order to model the scattering properties of cometary dust. This has differentiated our study from previous investigations of aggregates in which the size of the monomers was much smaller than the wavelength. For aggregates, the absorption cross sections tend to have less steep slopes towards longer wavelength than a single sphere, typically, C-abs similar to lambda(-1). Consequently aggregates of absorbing material are cooler than the individual monomers, because the aggregates radiate more efficiently in the infrared. The polarization is sensitive to the shape and size of the constituent monomers as well as to the fine structure of the aggregate. Generally aggregates of highly absorbing material produce a strong positive polarization around theta(p) = 90 degrees, but no negative polarization near the backward direction. In contrast, silicate aggregates are the major source of strong negative polarization at larger scattering angles. A mixture of both carbonaceous and silicate aggregates results in a polarization curve which largely matches the observed negative polarization at theta(p) less than or equal to 20 degrees and the maximum peak around theta(p) = 90 degrees for cometary dust. The same mixture also gives a reasonable rise of the phase function toward the backward direction, which is similar to the phase function of cometary dust. Thus, we find that aggregates with constituent monomers a few tenths of a micron in size and with intermediate porosity (approximate to 0.6), similar to chondritic aggregate interplanetary dust particles, are a reasonable analog for cometary dust.