Experimentelle und Analytische Planetologie
Numerical Simulations of Light-Scattering Properties of Cosmic Dust Analogues
Light-scattering theories are an important tool for comparing astronomical observations to model assumptions
of dust properties. The light-scattering properties depend on the size, material, and structure of the dust grains.
Interplanetary dust particles (IDPs) collected in the stratosphere of the earth indicate that a fraction of the
unmelted IDPs are aggregates consisting of submicron grains. There is also evidence that cometary dust
particles have a porous structure as well as dust grains around other stars are possibly porous or aggregates of
submicron monomers. With known
light-scattering theories it is possible to determine the light-scattering properties of aggregates consisting of
submicron grains. Mie theory is used to calculate the light-scattering properties of spherical grains by solving
the Maxwell equations including boundary conditions. In combination with Maxwell-Garnett mixing rule the
light-scattering properties of porous spherical grains can be determined. This formalism does not account for
interactions between the monomers, which causes large uncertainties for aggregates consisting of transparent
materials. The generalized multiparticle Mie theory (GMM) determines the light-scattering properties of
aggregates including the interactions between the monomers. With the discrete-dipole approximation (DDA)
the particles are described as an agglomerate of dipoles. This approach allows to simulate the light scattering of
arbitrarily shaped particles by approximating their shape. T-Matrix method and GMM, in contrast, give exact
results for the calculation of aggregates but only for certain specific shapes.
In a first step we compared
the different light-scattering theories and consider the advantages of every theory. We calculated the
light-scattering properties of compact, spherical grains, porous, spherical grains and aggregates with the
appropriate theory. In this way the influence of the radiation pressure force for aggregates with up to 8192
monomers could be calculated which correspond to volume-equivalent sizes of 2 micrometer.
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