Modeling of the light scattering properties of cometary dust using fractal aggregates

Abstract

Aims. The nature of cometary dust is still elusive. In order to constrain the physical properties of dust in cometary coma, we modeled the observed scattering parameters, investigating different morphological and compositional effects. Methods. We used fractal aggregates as dust particles. The scattering behavior of extremely elongated dust particles, similar to those formed in micro-gravity aggregation experiments and more compact aggregates, have been studied with the discrete-dipole approximation theory. Results. Results show that particles comparable in size to the incident wavelength provide the best qualitative fits. A silicate composition gives a good fit to the polarization and the phase function, but is characterized by large albedo. Organic and mixed compositions provide low albedos but do not fit the polarization and phase function curves well. Compact particles seem to reproduce the observed parameters better, indicating a possible morphological metamorphism of dust since the time of formation. The observed different polarization classes may be explained in terms of different monomer sizes, if a cluster of spheres models are used.

The influence of the monomer shape in the first stage of dust growth in the protoplanetary disk

“…In this way random chains with extremely elongated loose structures and high porosity are obtained, as described in Bertini et al (2007). This kind of elongated aggregates morphologically resemble the dust particles obtained in the micro-gravity aggregation experiment CODAG (D f ≈ 1.4 for aggregates made by monodisperse spherical monomers and having the number of monomers, N [10-100]), where the structure of primitive preplanetary dust was investigated (Krause & Blum 2004).…”

“…A widely-used parameter to describe the effect of volume-filling factor in fractal structures, and irregular particles in general, is the porosity, p, defined as 1 − V/V t , where V is the volume of the solid part of the particle and V t is the total volume, generally intended as the volume of the solid part plus the pores (e.g Greenberg & Hage 1990;Mukai et al 1992;Kozasa et al 1992;Davidsson & Skorov 2002;Petrova et al 2004;Kimura et al 2006;Bertini et al 2007;Das et al 2008;Voshchinnikov & Henning 2008). It should be emphasized at this point that the porosity and the fractal dimension need not to be uniquely related.…”

“…The fractal dimension and the porosity are therefore independent parameters, both characterizing the morphology of fractal aggregates. In order to calculate the porosity of the aggregates, we applied the method by Mukai et al (1992), already used by several other authors (e.g Kozasa et al 1992;Petrova et al 2004;Kimura et al 2006;Bertini et al 2007;Das et al 2008). Firstly, although fractals do not possess a constant internal density, the so-called characteristic radius, R c , is calculated.…”

Photometric observations of comet 81P/Wild 2 during the 2010 perihelion passage

“…We considered the average scattering behavior of the entire coma, without taking into account possible dustparameter variations as a function of the distance from the nucleus. We used compact irregular fractal structures comparable in size with the incident wavelength to simulate the emitted dust particles because they have shown to provide the most accurate fits to the observed dust scattering properties in the visible range (Bertini et al 2007). A power-law grain size distribution, n(r) ∼ r −a , with a = 2.6 was used in our modeling, because our results are not sensitive to the choice of the exponent in the range a = [2−3].…”

“…Following these findings, three possible dust compositions were taken into account in the A(α) f ρ reduction to phase angle zero: pure silicates, pure organics, and a 1:1 in mass mixture of silicates and organics. For a detailed description of the dust model used, including the bulk density and the complex refractive index of the dust particles, we refer the reader to Bertini et al (2007).…”