Computer Science
Scientific paper
Mar 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999icar..138..107y&link_type=abstract
Icarus, Volume 138, Issue Icarus, pp. 107-128.
Computer Science
27
Scientific paper
Scattering calculations for nonspherical particles have been carried out in order to explain observed optical properties of cometary dust. We focused on two optical properties of cometary dust sensitive to particle shape: negative linear polarization at phase angles <=21 deg and the 11.2-μm silicate emission feature. The discrete dipole approximation (DDA) method was employed to compute the scattering matrix for nonspherical silicate and absorbing particles of size comparable to the wavelength. Silicate particles with a variety of shapes and size parameter X_eq~2.5, corresponding to a linear dimension of 0.5-1.0 mum, can produce negative linear polarization at small phase angles, whereas carbon particles produce a strong positive maximum of polarization near phase angles of 90 deg. Mixtures of silicate and carbonaceous material, on a scale small compared to the wavelength, eliminate the negative polarization in this size range; however, macroscopic mixtures of silicate and carbon could yield the observed negative linear polarization at low phase angles (<=21 deg) and a maximum positive polarization at phase angle of 90 deg. The position of the 11.2-μm thermal emission peak observed in comets, attributed to crystalline olivine, depends strongly on particle shape even for particles much smaller than the wavelength and can be matched with anisotropic Mg-rich olivine for our model tetrahedra or moderately elongated bricks. Spheres and extreme shapes, such as disks or needles, appear to be ruled out. Approximately 20% crystalline olivine and 80% disordered olivine reproduces the observed spectra of comets with comparable peaks at 10 and 11.2mum, e.g., P/Halley, Bradfield 1987 XXIX, Mueller, Levy 1990 XX, and C/1995 O1 (Hale-Bopp). This study is an essential first step toward realistic modeling of comet dust as aggregates composed of nonspherical monomers having dimensions comparable to the wavelength of incident radiation.
Hanner Martha S.
Yanamandra-Fisher Padmavati A.
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