Search

We present reflectance and polarization phase curve measurements of highly reflective planetary regolith analogues having physical characteristics expected on atmosphereless solar system bodies (ASSBs) such as a eucritic asteroids or icy satellites. We used a goniometric photopolarimeter (GPP) of novel design to study thirteen well-sorted particle size fractions of aluminum oxide (Al2O3). The sample suite included particle sizes larger than, approximately equal to, and smaller than the wavelength of the incident monochromatic radiation (λ = 635 nm). The observed phase angle, α, was 0.056 o ∼95%). The incident radiation has a very high probability of being multiply scattered before being backscattered toward the incident direction or ultimately absorbed. The five smallest particle sizes exhibited extremely high void space (> ∼95%). The reflectance phase curves for all particle size fractions show a pronounced non-linear reflectance increase with decreasing phase angle at α∼ The polarization phase curves for particle size fractions with size parameter (particle radius/wavelength) r/λ ∼1 we detect no polarization. This polarization behavior is distinct from that observed in low albedo solar system objects such as the Moon and asteroids and for absorbing materials in the laboratory. We suggest this behavior arises because photons that are backscattered have a high probability of having interacted with two or more particles, thus giving rise to the CB process. These results may explain the unusual negative polarization behavior observed near small phase angles reported for several decades on highly reflective ASSBs such as the asteroids 44 Nysa, 64 Angelina and the Galilean satellites Io, Europa and Ganymede. Our results suggest these ASSB regoliths scatter electromagnetic radiation as if they were extremely fine grained with void space > ∼95%, and grain sizes of the order The GPP used in this study was modified from our previous design so that the sample is presented with light that is alternatingly polarized perpendicular to and parallel to the scattering plane. There are no analyzers before the detector. This optical arrangement, following the Helmholtz Reciprocity Principle (HRP), produces a physically identical result to the traditional laboratory reflectance polarization measurements in which the incident light is unpolarized and the analyzers are placed before the detector. The results are identical in samples measured by both methods. We believe that ours is the first experimental demonstration of the HRP for polarized light, first proposed by Helmholtz in 1856.

We present results of goniometric photometry of different samples at extremely small phase angles using a new laboratory setup with a laser light source (λ=0.63 μm). No opposition effect of carbon soot is found in the range 0.008–1.5°; whereas, MgO deposits and very disperse porous SiO2 powder demonstrate a narrow brightness spike of significant amplitude. We find a strong dependence of phase curve slope in the range on albedo when studying mixtures of white chalk and carbon soot. These measurements as applied to interpretation of opposition effect observed for Kuiper belt objects suggest that the albedo of these objects cannot be small. Size particle separates of olivine reveal prominent opposition spikes independent of the particle size. Spherical particles with a diameter near 50 μm show significant differences in their scattering behavior depending on whether they form a monolayer or thick particulate surfaces. The latter show a sharp opposition surge unlike the monolayer. Except for the monolayer of spheres, no particulate surface in our measurements yet exhibits phase curve rounding as the phase angle approaches zero.

Optical measurements in the laboratory are an important source of information on light scattering by particulate surfaces. One example of such surfaces is planetary regolith. Laboratory photometric and polarimetric measurements allow direct verifications of theoretical models of scattering (e.g., Shkuratov et al., 2007a; Zhang and Voss, 2005); they potentially may suggest new regularities that can improve theoretical interpretation. Moreover the measurements can suggest new effects, which require innovative theoretical approaches. For example, photometric and polarimetric opposition phenomena, well-known to astronomers and experimenters (e.g., Lyot, 1929; Oetking, 1966), were long without a satisfactory explanation. The effect of coherent backscattering enhancement explanation of these phenomena in astronomy was suggested in Shkuratov (1985,1988) and (1989) almost 60 years after their discovery.

We present results of our photometric measurements of three samples of particulate surfaces consisting of dielectric and metallic spheres at extremely small phase angles. Monolayers of the spheres show satisfactory coincidence with the results of Mie-theory calculations. In particular, no opposition effect of the monolayer was found at the phase angle range 0.008 – 1.6° in accordance with Mie theory prediction. On the other hand, thick layers of the spheres reveal the opposition effect at phase angles less than 0.8°. In the case of dielectric spheres the opposition spike is due to the coherent-backscattering effect; whereas, for iron particulate surfaces the main contributor is perhaps the shadow-hiding effect. Measured dependencies do not allow us to separate these effects. 1 Introduction The motivation of this study is an astrophysical problem: Photometric observations of Kuiper belt objects reveal a prominent brightness opposition spike that is very narrow [1]. These objects are observed at very small phase angles (< 2°) and to reproduce these conditions in a laboratory requires the use of very small angular apertures of the light source and the receiver. Using an old laboratory of our Institute, which had been exploited as an analog processor for Fourier transformation of large images, we have constructed a laboratory laser photometer to study extremely small phase angles; this setup provides measurements in the range 0.008 – 1.6° [2]. This instrument allows us to measure the scattering properties of structural analogs of the surfaces of Kuiper belt objects. First of all it allows studies of the opposition spike effect in a wide physical context. For example, we here compare phase dependencies of intensity for dielectric and metallic small spherical particles that form a monolayer and thick layer in order to study the opposition effect related to the shadowing and coherent backscatter enhancement.