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The coherent backscattering effect (CBE), the constructive interference of light scattering in particulate surfaces (e.g., regolith), manifests as a non-linear increase in reflectance, or opposition surge, and a narrow negative polarization feature at small solar phase angles. Due to a strong dependence of the amplitude and angular width of this opposition surge on the absorptive characteristics of the surface material, CBE also produces phase-angle-dependent variations in the near-infrared spectra. In this paper we present a survey of such variations in the spectra of icy satellites of Saturn obtained by the Cassini spacecraft's Visual and Infrared Mapping Spectrometer (VIMS) and in the ground-based spectra of Oberon, a satellite of Uranus, obtained with TripleSpec, a cross-dispersed near-infrared spectrometer on the Astrophysical Research Consortium 3.5-m telescope located at the Apache Point Observatory near Sunspot, New Mexico. The paper also presents computer modeling of the saturnian satellite spectra and their phase-angle variations using the most recent version of the Multi-Sphere T-Matrix (MSTM) code developed to simulate light scattering by layers of randomly distributed spherical particles. The modeling allowed us not only to reproduce the observed effects but also to estimate characteristics of the icy particles that cover the surfaces of Rhea, Dione, and Tethys. [ABSTRACT FROM AUTHOR]

The paper presents an attempt to extract information about the comet 9P/Tempel 1 nucleus from the characteristics of the ejecta cloud produced by the impactor of the Deep Impact mission. For this purpose we use two techniques. We first study the shadow cast on the nucleus surface by the ejecta cloud and investigate how areas of different brightness are related to the varying optical thickness or albedo of the ejecta cloud. The shadow was seen during the first 2.0 s after the impact (afterward it became obscured by the ejecta cloud). We have found that all brightness variations in the shadow are the result of the surface inhomogeneities, indicating that during first 2.0 s the ejecta cloud was homogeneous within the MRI spatial resolution. Our second technique is to study the obscuration of the nucleus limb by the ejecta. This study covers the period 0.76–68.8 s after impact and is based on comparison of the ejecta cloud brightness on the limb and just beyond the limb. At this stage we do see inhomogeneities in the ejecta cloud that relate to the albedo and optical thickness variations in the ejected dust. Specifically, we have found two distinct bands of low optical thickness and one band of a high optical thickness. Based on crater formation ideas we estimate the depth of excavation of the ejected material for the found inhomogeneities and, thus, define a potential layering structure for the comet nucleus, Our estimates suggest that the low-optical thickness material was excavated from a depth of 15–18 and 30–32 m in the case the porous nucleus material and 37–46 and 87–93 m in the case of a non-porous nucleus material, and a layer of high optical thickness originated from the depth 9–11 m for porous material or 20–23 m for non-porous material. Based on the crater diameter estimates, we expect that the real depth of the layers is between these two cases. The rest of the ejecta do not show any signs of layering but have significant azimuthal inhomogeneity with clumps of high optical thickness and patterns of high albedo. [ABSTRACT FROM AUTHOR]

Cosmic dust is a polydisperse mixture of irregular, often aggregated, particles. Previous attempts have tried to simulate polarimetric properties of this dust using aggregate dust models, but it has not been possible to consider particle sizes larger than a couple of microns due to limitations of computer memory and processing power. Attempts have also been made to replace aggregates by polydisperse regular particles (spheres, spheroids, cylinders), but those models could not consistently reproduce the observed photopolarimetric characteristics. In this study, we introduce to the astronomical community the software package developed by Dubovik et al. (2006) for modeling light scattering by a polydisperse mixture of randomly oriented smooth and rough spheroids of a variety of aspect ratios. The roughness of spheroids is defined by a normal distribution of the surface slopes, and its degree depends on the standard deviation of the distribution (which is zero for smooth surface and greater than zero for rough surface). The pre-calculated kernels in the software package allow for fast, accurate, and flexible modeling of different size and shape distributions. We present our results of a systematic investigation of polarization obtained with the rough and smooth spheroid models; we study differences in their phase angle dependence and how those differences change with the particle size distribution. We found that the difference between smooth and rough particles increases with increasing effective size parameter and affects mainly the value and position of the maximum polarization. Negative polarization was found to be typical only for silicate-like refractive indexes and only when the particles have size parameters within 2.5–25. As an example of an application of the rough spheroid model, we made computations for rough spheroids that have a size distribution and composition typical for cometary dust. We found that a mixture of porous rough spheroids made of absorbing material compositionally similar to comet Halley’s dust and solid silicate spheroids, dominated by particles of size parameter 5< x <20, can reproduce angular and spectral characteristics of the brightness and polarization observed for cometary dust. [ABSTRACT FROM AUTHOR]

We analyzed Deep Impact High Resolution Instrument (HRI) images acquired within the first seconds after collision of the Deep Impact impactor with the nucleus of comet 9 P/Tempel 1. These images reveal an optically thick ejecta plume that casts a shadow on the surface of the nucleus. Using the 3D radiative transfer code HYPERION we simulated light scattering by the ejecta plume, taking into account multiple scattering of light from the ejecta, the surrounding nuclear surface and the actual observational geometry (including an updated plume orientation geometry that accounts for the latest 9 P/Tempel 1 shape model). Our primary dust model parameters were the number density of particles, their size distribution and composition. We defined the composition through the density of an individual particle and the ratio of its material constituents, which we considered to be refractories, ice and voids. The results of our modeling indicate a dust/ice mass ratio for the ejecta particles of at least 1. To further constrain the parameters of the model, we checked for consistency between the ejecta mass resulting from our modeling with the ejecta mass estimated by the crater formation modeling. Constraining the particle size distribution by results of other studies of the Deep Impact ejecta, we find the number density of ejecta particles equal to ∼104 particles/cm3 at the base of the plume. [ABSTRACT FROM AUTHOR]

We model light scattering by aligned particles and particles that contain homochiral organics, i.e. organics that possess optical activity (circular birefringence and circular dichroism), to check for a method to distinguish between these particles based on their linear and circular polarization. For aligned particles, we consider alignment in a magnetic field that aligns the particles with the longest particle dimension perpendicular to the magnetic field lines. We model those particles as polydisperse ensembles of prolate and oblate spheroids made of ice, silicate and cosmic organics. We model optically active particles as aggregates of submicron monomers made of chlorophyll, which has optical constants with values similar to those of silicates but possesses distinct optical activity in the visible. The results of the modeling show that alignment and optical activity produce a rather similar shape of phase angle dependences of linear and circular polarization, making it difficult to distinguish between them. However, a difference was found in the exact backscattering and forward scattering directions, where aligned particles have non-zero linear polarization and zero circular polarization whereas optically active particles have non-zero circular polarization and zero linear polarization. We also studied correlations between linear and circular polarization and found that at small phase angles both aligned and optically active particles show correlation between linear and circular polarization. However, at phase angles larger than 100°, linear and circular polarizations correlate for aligned particles and anticorrelate for optically active particles. This difference in correlations may be used to distinguish between two mechanisms of formation of circular polarization. Also, the spectral dependence of circular polarization for these two mechanisms is very different because it is defined by the refractive index for aligned particles and circular dichroism and birefringence for optically active particles. Our results can be helpful in determining the mechanism responsible for formation of circular polarization at scattering of light by dust particles in comets, star-forming regions and circumstellar disks, and by aerosols in atmospheres of exoplanets. [ABSTRACT FROM AUTHOR]

This special issue is primarily devoted to the 6th meeting on Cosmic Dust (Cosmic Dust VI), which was held at CPS (Center for Planetary Science) in Kobe, Japan, on August 5-9, 2013. This meeting was coordinated in an order where a friendly and welcoming atmosphere persuaded the participants of the meeting to develop human relations and interactions among themselves. This has been our interdisciplinary approach to answering the question of where dust comes from and where dust goes. We briefly review some of the exciting papers presented at the meeting and provide perspectives for the development of cosmic dust research. [ABSTRACT FROM AUTHOR]

Abstract: We describe circular polarization as a remote sensing diagnostic of chiral signatures which may be applied to Mars. The remarkable phenomenon of homochirality provides a unique biosignature which can be amenable to remote sensing through circular polarization spectroscopy. The natural tendency of microbes to congregate in close knit communities would be beneficial for such a survey. Observations of selected areas of the Mars surface could reveal chiral signatures and hence explore the possibility of extant or preserved biological material. We describe a new instrumental technique that may enable observations of this form. [Copyright &y& Elsevier]