Modelling the polarization properties of Comet 1P/Halley using a mixture of compact and aggregate particles

Abstract

Both the in situ measurement of Comet 1P/Halley and the Stardust‐returned samples of Comet Wild 2 showed the presence of a mixture of compact and aggregate particles, with both silicates and organic refractory being in the composition of the cometary dust. Results obtained recently from the Stardust mission suggest that the overall ratio of compact to aggregate particles is 65:35 (or 13:7) for Comet 81P/Wild 2. In the present work, we propose a model that considers cometary dust as a mixture of compact and aggregate particles, with a composition of silicate and organic. We consider compact particles as spheroidal particles and aggregates as both ballistic cluster–cluster aggregate (BCCA) and ballistic agglomeration with two migrations (BAM2) aggregate with a certain size distribution. The mixing ratio of compact to aggregate particles is taken to be 13:7. For modelling Comet 1P/Halley, the power‐law size distribution n(a) ∼a−2.6, obtained from a re‐analysis of the Giotto spacecraft data, for both compact and aggregate particles, is used. We consider a mixture of BAM2 and BCCA aggregates with a lower cut‐off size of about 0.20 μ m and an upper cut‐off of about 1 μ m. We also consider a mixture of prolate, spherical and oblate compact particles with an axial ratio (E) of 0.8–1.2 where a lower cut‐off size of about 0.1 μ m and an upper cut‐off of about 10 μ m are considered. Using a T‐matrix code for polydisperse spheroids (0.1 μ m ≤a≤ 10 μ m) and superposition T‐matrix code for aggregates (0.2 μ m ≤av≤ 1 μ m), the average simulated polarization curves are generated, which can best fit the observed polarization data at the four wavelengths: λ= 0.365, 0.485, 0.670 and 0.684 μ m. The suitable mixing percentages of aggregates obtained from the present modelling are 50 per cent BAM2 and 50 per cent BCCA particles, and the silicate‐to‐organic mixing percentages are 78 per cent silicate and 22 per cent organic, in terms of volume. The present model successfully reproduces the observed polarization data, especially the negative branch, for Comet 1P/Halley at the above four wavelengths, more effectively as compared to other work done in the past. It is found that among the aggregates, the BAM2 aggregate plays a major role in deciding the cross‐over angle and depth of the negative polarization branch.

Linear polarization measurements of Comet C/2011 W3 (Lovejoy) from STEREO

“…Mie theory, the scattering of light by transparent spherical particles, qualitatively replicates many of the polarization features seen in cometary dust, but aspherical particles (either irregular polydisperse submicron particles or porous aggregates of submicron components) are needed to completely explain the polarization and other scattering properties (Kolokolova et al, 2004). Recent work suggests that cometary particles consist of a mixture of aggregates and compact solid particles (Kolokolova and Kimura, 2010;Das et al, 2011). Much work has been done using the T-matrix method (e.g.…”

Dependence of light scattering properties on porosity, size and composition of dust aggregates

“…We utilize this concept to explore the NPB and its dependencies on various physical parameters. This negative polarization, on the other hand proves the availability of silicate materials in the dust particles [9,10,11,12]. It has been suggested that near field effect and the constructive interference of multiply scattered waves give rise to the phenomenon of negative polarization in dust particles though there is no direct evidence or reason behind the actual cause of negative polarization [13].…”

The effect of porosity of dust particles on polarization and color with special reference to comets

“…We also checked on some examples that both (4) and ( 5) gave very similar values of the polarization. However, under a different situation, the linear polarization should be expressed by (5).…”