Experimental light scattering by fluffy aggregates of magnesiosilica, ferrosilica, and alumina cosmic dust analogs

Abstract

Context. Fluffy aggregates are generally assumed to be important constituents of circumstellar and interplanetary environments as well as to be present among the solid debris ejected from active comets. Aims. We experimentally study light scattering properties of several fluffy aggregate samples. These cosmic dust analog aggregates are composed of coagulated magnesiosilica grains, ferrosilica grains, and alumina grains. The samples contain aggregates with different porosities. The individual grains have diameters of the order of a few tens of nanometers; the aggregates have diameters up to several micrometers. Methods. The samples were produced in a Condensation Flow Apparatus. Their light scattering properties were measured with the Amsterdam Light Scattering Facility at a wavelength of 632.8 nm. Results. We measured two scattering matrix elements as functions of the scattering angle, namely F11(θ) (phase function) and −F12(θ)/F11(θ) (degree of linear polarization for incident unpolarized light) for seven different samples of aggregates in random orientations in an aerosol jet. The samples consisted of fluffy aggregates with cosmic dust analog compositions. We provide detailed information about their production and nature. In addition, for four of these samples we measured F22(θ)/F11(θ). We covered an angle range of 5 ◦ to 174 ◦ ,i n small steps of 1 ◦ in the range from 5 ◦ to 10 ◦ and 170 ◦ to 174 ◦ and in steps of 5 ◦ for the rest of the angle range. Conclusions. The results for the analog samples show an extremely high −F12(θ)/F11(θ), with maxima between about 60% to almost 100%. This Rayleigh-like behavior has been demonstrated before for fluffy aggregates and suggests that the small-sized grains in the aggregates are the main cause. Measured results for phase functions are more scarce. The phase functions we measured show shapes that are similar to those of compact micron-sized particles, suggesting that it is the overall size of the aggregates that determines their shape. The modest negative branch of −F12(θ)/F11(θ) found for all seven samples seems to be mainly governed by aggregate structure. Thus, the unique combination of accurately measured phase functions and polarization functions over a fine mesh of scattering angles for cosmic dust analog aggregates enables the exploitation of the data as powerful diagnostic tools to constrain the different physical properties of dust in e.g. circumstellar clouds and in comet ejecta.