Laboratory Simulations of Planetary Surfaces: Understanding Regolith Physical Properties from Remote Photopolarimetric Observations

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 (lambda = 635 nm). The observed phase angle, alpha, was 0.056 < alpha < 15 degrees. These Al2O3 particulate samples have very high normal reflectance (> similar to 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 (> similar to 95%). The reflectance phase curves for all particle size fractions show a pronounced non-linear reflectance increase with decreasing phase angle at alpha similar to <3 degrees. Our earlier studies suggest that the cause of this nonlinear reflectance increase is constructive interference of counter-propagating waves in the medium by coherent backscattering (CB), a photonic analog of Anderson localization of electrons in solid state media. The polarization phase curves for particle size fractions with size parameter (particle radius/wavelength) r/lambda < similar to 1, show that the linear polarization rapidly decreases as a increases from 0; it reaches a minimum near alpha = similar to 2 degrees. Longward of similar to 2 degrees, the negative polarization decreases as phase angle increases, becoming positive between 12 degrees and at least 15 degrees, (probably similar to 20 degrees) depending on particle size. For size parameters r/lambda > similar to 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 lo, Europa and Ganymede. Our results suggest these ASSB regoliths scatter electromagnetic radiation as if they were extremely fine grained with void space > similar to 95%, and grain sizes of the order < = lambda. This portends consequences for efforts to deploy landers on high ASSBs such as Europa. These results are also germane to the field of terrestrial geo-engineering, particularly to suggestions that earth's radiation balance can be modified by injecting Al2O3 particulates into the stratosphere thereby offsetting the effect of anthropogenic greenhouse gas emissions. 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. (C) 2017 Elsevier Inc. All rights reserved.