| Abstract |
Both the in situ measurement of Comet 1P/ Halley and theStardust-returned samples of Comet Wild 2 showed thepresence of a mixture of compact and aggregate particles,with both silicates and organic refractory being in thecomposition of the cometary dust. Results obtained recentlyfrom the Stardust mission suggest that the overall ratio ofcompact to aggregate particles is 65:35 (or 13:7) for Comet81P/Wild 2.A model has been proposed that considers cometary dust as amixture of compact and aggregate particles, with acomposition of silicate and organic. The compact particlesare assumed to be spheroidal and aggregates as bothballistic cluster-cluster aggregate (BCCA) and ballisticagglomeration with two migrations (BAM2). The mixing ratioof compact to aggregate particles is taken to be 13:7. Thepower-law size distribution n(a) ~ a-2.6, obtained from are-analysis of the Giotto spacecraft data, were used forboth compact and aggregate particles. A mixture of prolate,spherical and oblate compact particles with an axial ratio(E) of 0.8-1.2 was considered. Using a T-matrix code forpolydisperse spheroids (0.1 μm < a < 10 μm) andsuperposition T matrix code for aggregates (0.2 μm < av < 1μm), the average simulated polarization curves aregenerated, which can be bestfit the observed polarizationdata at the four wavelengths: 0.365, 0.485, 0.670 and 0.684μm. The mixing percentages of aggregates obtained from thepresent modeling are 50 per cent BAM2 and 50 per cent BCCAparticles. The silicate-to-organic mixing percentages are 78per cent silicate and 22 per cent organic, in terms ofvolume. The present model successfully reproduces theobserved polarization data, especially the negative branch,for Comet 1P/Halley, more effectively as compared to otherwork done in the past. It is found that among theaggregates, the BAM2 aggregate plays a major role indeciding the cross-over angle and depth of the negativepolarization branch. |
