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We explore experimentally possible explanations of the polarization curves of the sunlight reflected by the Barbarian asteroids. Their peculiar polarization curves are characterized by a large-inversion angle, around 30°, which could be related to the presence of FeO-bearing spinel embedded in Calcium–Aluminum inclusions. In order to test this hypothesis, we have measured the phase function and degree of linear polarization of six samples of Mg-rich olivine and spinel. For each material, we have analysed the light scattering properties of a millimeter-sized grain and of two powdered samples with size distributions in the micrometer size range. The three spinel samples show a well-defined negative polarization branch with an inversion phase angle located around 24°–30°. In contrast, in the case of the olivine samples, the inversion angle is highly dependent on particle size and tends to decrease for larger sizes. We identify the macroscopic geometries as a possible explanation for the evident differences in the polarization curves between olivine and spinel millimeter samples. Although the polarization behaviour in near backscattering of the Barbara asteroid is similar to that of our spinel mm-sized sample in random orientation, this similarity could result in part from crystal retro-reflection rather than composition. This is part of an ongoing experimental project devoted to test separately several components of CV3-like meteorites, representative of the Barbarians composition, to disentangle their contributions to the polarization behaviour of these objects. [ABSTRACT FROM AUTHOR]

We present measurements of the complete scattering matrix as a function of the scattering angle of a sample of calcite particles. The measurements are performed at 647nm in the scattering angle range from 3° to 177°. To facilitate the use of the experimental data we present a synthetic scattering matrix based on the measurements and defined in the full range from 0° to 180°. The scattering matrix of the calcite sample is modeled using the discrete-dipole approximation. Two sets of shapes, flake-like and rhomboid-like particles giving a total of 15 different targets are considered since both types of shapes have been found in our calcite sample. In our computations we use the measured size distribution of the calcite sample truncated at 1.2μm. We present a theoretical study of the impact of birefringence on the computed scattering matrix elements for both sets of shapes. Four different cases regarding the composition of the calcite particles are considered: two isotropic cases corresponding to the ordinary and extraordinary refractive index of calcite, respectively; one equivalent isotropic case analogous to internal mixing; and birefringence fully accounted for. Numerical simulations are compared with the experimental data. We find that birefringence has little impact on the calculated phase functions but it has a significant effect on the polarization-related elements of the scattering matrix. Moreover, we conclude that the shape of the targets (flakes or irregular rhomboids) has a much stronger effect on the computed scattering matrix elements than birefringence. [ABSTRACT FROM AUTHOR]

Abstract: The Amsterdam Light Scattering Database proved to be a very successful way of promoting the use of the data obtained with the Amsterdam Light Scattering apparatus at optical wavelengths. Many different research groups around the world made use of the experimental data. After the closing down of the Dutch scattering apparatus, a modernized and improved descendant, the IAA Cosmic Dust Laboratory (CoDuLab), has been constructed at the Instituto de Astrofísica de Andalucía (IAA) in Granada, Spain. The first results of this instrument for water droplets and for two samples of clay particles have been published. We would now like to make these data also available to the community in digital form by introducing a new light scattering database, the Amsterdam–Granada Light Scattering Database (www.iaa.es/scattering). By combining the data from the two instruments in one database we ensure the continued availability of the old data, and we prevent fragmentation of important data over different databases. In this paper we present the Amsterdam–Granada Light Scattering Database. [Copyright &y& Elsevier]

Abstract: In this paper we present an overview of light scattering experiments devoted to measure one or more elements of the scattering matrix as functions of the scattering angle of ensembles of randomly oriented small irregular particles in air. A summary of the most important findings in light scattering experiments on ensembles of randomly oriented particles in air is given. The particles of interest are relevant for studies of atmospheres of planets and satellites and also for other astronomical bodies and environments. Some applications of light scattering experiments are also presented. [Copyright &y& Elsevier]

Abstract: We present the first results of measurements on solid particles performed at the Instituto de Astrofı´sica de Andalucı´a (IAA) cosmic dust laboratory located in Granada, Spain. The laboratory apparatus measures the complete scattering matrix as a function of the scattering angle of aerosol particles. The measurements can be performed at a wavelength (λ) of 483, 488, 520, 568, or 647nm in the scattering angle range from 3° to 177°. Results of special test experiments are presented which show that our experimental results for scattering matrices are not significantly contaminated by multiple scattering and that the sizes/shapes of the particles do not change during the measurements. Moreover, the measured scattering matrix for a sample of green clay particles is compared with measurements previously performed in the Amsterdam light scattering setup for the same sample. New measurements on a white clay sample at 488 and 647nm are also presented. The apparatus is devoted to experimentally studying the angle dependence of scattering matrices of dust samples of astrophysical interest. Moreover, there is a great interest in similar studies of aerosols that can affect the radiative balance of the atmosphere of the Earth and other planets such as silicates, desert dust, volcanic ashes, and carbon soot particles. [ABSTRACT FROM AUTHOR]

Abstract: Seven symmetry relations, with a wide range of validity, are presented for the phase matrix of horizontally oriented particles. Three of these relations constitute a fundamental basis from which four others can be derived by making combinations. All seven relations can be used for many types of particles, including hexagonal plates and columns as well as spheroids, cylinders and cubes. Some applications are also pointed out. [Copyright &y& Elsevier]

Abstract: We present a new apparatus for measuring the complete scattering matrix as a function of the scattering angle of dust irregular particles. The design is based on the well-known apparatus located in Amsterdam, The Netherlands. In this improved version we have extended the scattering angle ranging from 3° to 177°. Moreover, the measurements are performed with a tunable argon–krypton laser that emit at a wavelength of 483, 488, 520, 568, or 647nm. The apparatus has been developed at the Instituto de Astrofísica de Andalucía (IAA), Granada, Spain. To measure the scattering matrix elements we use a number of different optical components such as polarizers, a quarter-wave plate, and an electro-optic modulator. These components are used to manipulate the polarization state of light. By using eight different combinations for the orientation angles of the optical components, all scattering matrix elements are obtained as functions of the scattering angle. The accuracy of the instrument is tested by comparing the measured scattering matrices of water droplets at 488, 520, and 647nm with Lorenz–Mie calculations for a distribution of homogeneous water droplets. [Copyright &y& Elsevier]

Abstract: We present a concise survey of light scattering by small particles in the atmospheres and surface layers of various Solar System bodies, comets, the interplanetary medium, and in planetary rings. Current issues and recent developments are emphasized. Special attention is given to experimental and numerical approaches for solving problems of light scattering by non-spherical particles. References to more detailed papers, websites and books are presented. Key areas for further research are pointed out. [Copyright &y& Elsevier]

Abstract: We present an update of the Amsterdam Light Scattering Database located at http://www.astro.uva.nl/scatter. We give a detailed explanation and clarification of the nature of the scattering matrices in the database. Measured scattering matrix elements are presented as functions of the scattering angle, for aerosol particles in random orientation at 632.8nm. They pertain to seven volcanic dust samples from three different volcanoes, two samples with extreme refractive indices (hematite and rutile), and three forsterite samples with identical compositions, but different size distributions. For 15 phytoplankton species and two types of silt suspended in water, the database now contains two matrix elements, and as functions of the scattering angle at 632.8nm. Lastly, we have included all scattering matrix elements as functions of the scattering angle for spherical micron-sized water droplets, which may be used for testing purposes. [Copyright &y& Elsevier]

We present a new extensive database containing experimental scattering matrix elements as functions of the scattering angle measured at 441.6 and 632.8 nm for a large collection of micron-sized mineral particles in random orientation. This unique database is accessible through the World-Wide Web. Size distribution tables of the particles are also provided, as well as other characteristics relevant to light scattering. The database provides the light scattering community with easily accessible information that is useful, for a variety of applications such as testing theoretical methods, and the interpretation of measurements of scattered radiation. To illustrate the use of the database, we consider cometary observations and compare them with (1) cometary analog data from the database, and (2) with results of Mie calculations for homogeneous spheres, having the same refractive index and size distribution as those of the analog data. [Copyright &y& Elsevier]

A number of issues relevant to laboratory studies of scattering matrices as functions of the scattering angle for randomly oriented particles in the visible part of the spectrum are discussed. The usefulness of experiments is compared with that of numerical computations, in particular, for ensembles of natural nonspherical particles with broad ranges of sizes and shapes. It is argued that measurements of the entire scattering matrix have considerable advantages over measurements of only the intensity and polarization of the scattered light for incident unpolarized light. Results of special test experiments are presented which show that our experimental results for scattering matrices are not significantly contaminated by multiple scattering and that the orientation of the particles can be adequately described as random. Some ways are pointed out to overcome the lack of measurements for very small and very large scattering angles. A possibility to reduce the amount of material needed in the experiments is indicated. Finally, characterizations of the particles in terms of sizes, shapes and refractive indices are discussed. [Copyright &y& Elsevier]