Negative Polarization Branch Research Articles

Polarization of Light Scattered by Surfaces with Complex Microstructure at Phase Angles 0.1°–3.5°

We present laboratory measurements of the phase dependences of linear polarization for surfaces with a complex microstructure in the range of phase angles 0.1°–3.5° A sample of freshly fallen snow (with particle sizes of about 50 × 500 μm) exhibits a nearly zero polarization. Surfaces with submicron structure show a narrow branch of negative polarization at small phase angles, irrespective of whether the surface is powderlike or solid with microcrystalline structure. This polarization is similar to that exhibited by Jupiter's satellites. The negative polarization branch becomes deeper with decreasing porosity of light dielectric surfaces. At the phase angles between 0.5° and 3.0°, the polarization for quartz powder with 10-μm particles is almost constant. The polarization for light dielectric surfaces depends on the geometry of illumination and observation. An inclination of the surface in the scattering plane produces a parallel shift of the negative polarization branch toward large values of the polarization modulus. The same inclination in a perpendicular direction produces the same shift toward positive degrees of polarization.

A Negative Branch of Polarization for Comets and Atmosphereless Celestial Bodies and the Light Scattering by Aggregate Particles

Optical observations of comets and atmosphereless celestial bodies show that a change of sign of the linear polarization of scattered light from negative to positive at phase angles less than ≈ 20° is typical of the cometary coma, as well as of the regolith of Mercury, the Moon, planetary satellites, and asteroids. To explain a negative branch of polarization, this research suggests a unified approach to the treatment of cometary-dust particles and regolith grains as aggregate forms. A composite structure of aggregate particles resulting in the interaction of composing structural elements (monomers) in the light-scattering process is responsible for the negative polarization at small phase angles, if the monomer sizes are comparable to the wavelength. The characteristics of single scattering of light calculated for aggregates of this kind turned out to be close to the properties observed for cometary dust. Unlike the cometary coma, the regolith is an optically semi-infinite medium, where the interaction between particles is significant. To find the reflectance characteristics of regolith, the radiative-transfer equation should be solved for a regolith layer. In this case, the interaction between scatterers can be modeled to a certain extent by representing the regolith grains as aggregate structures consisting of several or many elements. Although real regolith grains are much larger than the particles considered here, laboratory measurements have shown that it is precisely the surface irregularities comparable to the wavelength that cause a negative branch of polarization. The main observed features of the phase and spectral dependence of the linear polarization of light scattered from comets and atmosphereless celestial bodies, which are due to the difference of the elementary scatterers in composition, size, and structure, can be successfully explained using the aggregate model of particles.

Light Scattering by Aggregates with Sizes Comparable to the Wavelength: An Application to Cometary Dust

Cometary dust is likely to have aggregate structure. Therefore the optical properties of aggregates must be understood if one wants to obtain information on cometary dust from optical observations. In this paper the scattering properties of polydisperse clusters with sizes comparable to the visible wavelengths are studied. It is shown that the sizes of the constituent monomers play a vital role in establishing the phase functions of intensity and linear polarization. Irregularly structured aggregates composed of a moderate number of touching spheres (<50) with size parameters ranging from 1.3 to 1.65 display properties typical of cometary dust particles, namely, a weak increase of the backscattering intensity, negative polarization at small phase angles, and a positive wavelength gradient of polarization. More compact particles have a more pronounced negative branch of polarization. With increasing wavelength the negative branch of polarization will become shallower and the inversion angle will approach the backscattering direction (if the refractive index is constant). This is in agreement with observations of Comet Hale–Bopp in the K band. The increase of polarization with wavelength is reduced if the imaginary part of the refractive index decreases with wavelength. The negative wavelength gradient of polarization observed in Comet Giacobini–Zinner can be explained by a particularly strong decrease of absorption with increasing wavelength or by smaller particle sizes. The spectral dependence of extinction efficiency for aggregates is less steep than that of equivalent spheres, and its maximum is shifted to larger size parameters. Therefore, in order to avoid an underestimation of the particle size derived from extinction measurements, one should take into account the shape of the scattering particles.

Polarization Properties of the Galilean Satellites of Jupiter: Observations and Preliminary Analysis

We present new, detailed polarimetric measurements of the Galilean satellites of Jupiter with U, B, V, and R filters at phase angles ranging from 12° to nearly 0°. The polarization phase curves of Io, Europa, and Ganymede in the B, V, and R filters clearly show the presence of the polarization opposition effect in the form of a sharp peak of negative polarization centered at a very small phase angle of 06-07 and superimposed on the regular negative polarization branch. This phase angle is comparable to the width of the spikelike photometric opposition effect observed for Europa, thus indicating that both opposition phenomena are likely to be produced by the coherent backscattering mechanism. The U filter values of |Pmin| for Io and Europa are close to 0.60% and 0.47%, respectively, and exceed the respective BVR values by a factor of almost 2. The BVR polarization for the trailing hemispheres of Io, Europa, and, especially, Ganymede is systematically stronger than for the respective leading hemispheres. For Callisto, the leading hemisphere polarization is significantly stronger than for the trailing hemisphere. The inversion angles for Io, Europa, and Ganymede are nearly wavelength independent and close to 100, 86, and 88, respectively. The inversion angle for the trailing hemisphere of Callisto is also wavelength independent and is in the range of 12°-13°.

Physical properties of M class asteroids

The available data on M class asteroids have been collected and analysed. The possible number of M-asteroids in the main asteroid belt larger than 30 km was estimated to be about 90 objects. A list of the 60 asteroids currently classified as M is presented. Such parameters as UBV and JHK colour indices, colours from the ECAS data, albedo, rotation rate, lightcurve amplitude, opposition effect value of magnitude-phase dependences, and the depth of the negative polarization branch of M-asteroids are considered in the comparison with the data for other asteroid classes and the laboratory data for meteorites. The reliability of metal content determination from spectral observations is discussed. On average the physical properties of the M class are close to those of the S class. However a diversity in composition inside the M class is evident.

Light Scattering by Solar System Dust: The Opposition Effect and the Reversal of Polarization

The opposition effect and the reversal of linear polarization, or negative polarization, at small phase angles have been almost universally observed in light scattered from atmosphereless solar system bodies (e.g., Seeliger 1887, Lyot 1929). Recent investigations have indicated that both phenomena can be qualitatively understood as resulting from a common physical mechanism: coherent multiple backscattering (Shkuratov 1989, Muinonen 1989). These findings have cast doubt on the hitherto accepted explanation that mutual shadowing alone is responsible for the opposition effect, and for the first time offer an acceptable interpretation of the polarization reversal near opposition. As for interplanetary dust, the coherent backscattering mechanism contributes both to the Gegenschein and to the almost certainly existing negative polarization branch (Roosen 1970, Lumme and Bowell 1985).In the following, theoretical results supporting the coherent backscattering explanation are briefly presented. As future work, we suggest modeling light scattering by a particulate medium to include the first, second and, if necessary, higher orders of scattering in the range below the typical particle size.

44 NYSA - an iron-depleted asteroid

The minor planet 44 Nysa has a unique combination of photopolarimetric parameters, with the most nearly neutral UBV colors, the shallowest negative polarization branch, and by far the highest polarimetric albedo yet obtained for any asteroid. Its surface apparently consists of a low-opacity, iron-free silicate strongly suggestive of enstatite achondrites.

Optical polarimetry of planet mercury

Polarimetric measurements were collected at different areas of the surface of Mercury, and for the whole disk in six wavelengths. The curves of polarization are compared with telescopic observations of the Moon and laboratory studies of minerals and returned lunar samples. The negative branch of polarization proves that Mercury's surface is almost everywhere covered by a regolith layer of fines of the lunar type, also made of dark and adsorbing material, and most probably of the same impact generated origin. The polarization maximum of Mercury is reproduced by lunar samples of fines of intermediate albedo corresponding to the lightest regolith found in the Apollo explored maria. The albedo of Mercury at phase angle 5° deduced from telescopic photometry is to be corrected by a factor of 1.20 and the best “polarimetric” values of albedos are 0.130 at λ = 0.585 μm, 0.119 at λ = 0.520 μm, 0.093 at λ = 0.379 μm and 0.087 at λ = 0.354 μm. The contrast between light and dark-lined regions at the surface of Mercury is most probably much fainter than between the maria and continents on the Moon. The molecular atmosphere of Mercury, if any, has a surface pressure probably smaller than 2 × 10 −4 bars.

Minor planets and related objects. XVI - Polarimetric diameters

Polarimetric observations of 43 asteroids are presented. All objects show a well-developed negative polarization branch as an indicator of unconsolidated surface regoliths. The empirical slope-albedo law for diffusely reflecting solid surfaces is reexamined and used to compute polarimetric albedos and diameters for 30 asteroids. In many cases the results are in good agreement with infrared-radiometric diameters; the older visual diameter measurements were systematically too small. Radiometric albedos below 5%, however, are not confirmed by the polarimetry. Bimodal frequency distributions are noted for asteroid color, albedo, and the depth of the negative polarization branch. Correlations between B - V color and polarimetric parameters suggest that most of the asteroid population can be divided into silicaceous and carbonaceous opacity classes.

On the Nature of Iapetus

For solar system bodies with little or no atmosphere, a linear relation exists between the depth of the negative polarization branch and the reciprocal of the geometric albedo. Considered in this fashion, the polarization data for Iapetus imply an albedo difference by about a factor of 6 between its leading and trailing hemispheres, and are consistent with the amplitude of the light curve for Iapetus. The brightness of the satellite at any particular maximum or minimum is unpredictable by a certain factor because of inadequately understood variations in solar phase angle and rotation. Although other measurements of the Iapetus albedos give results different from those in the presented paper, it is still clear that the two hemispheres have very different albedos.