Optical polarization studies of latex beads in aqueous solution: An analog for radar scattering in icy regolith

Abstract

This study presents low phase angle $0°\text{–}5°$ measurements of polarimetric properties of icy planetary regolith analog materials acquired using the custom-built Multi-Axis- Goniometer-Instrument (MAGI). We present same sense $\left(SC\right)$, and opposite sense $\left(OC\right)$ backscatter circular polarization coefficients, circular polarization ratio $\left(CPR\right)$, and degree linear polarization $\left(DLP\right)$ of spherical latex (non-dye) beads of varying sizes and volume concentrations (v/v) in aqueous solutions ($\lambda =0.8\mu m$) in water. We also present measurements of alumina powder in air at $\lambda =1.064\mu m$. Measurements showed that at a low incidence angle $(i=0°)$, backscatter is dominated by surface specular single-bounce scattering, that hides other scattering processes. At high $(i=15°)$ incidence angle, surface single-bounce surface scattering becomes negligible, allowing for the detection of diffuse, dihedral (multiple bounces) scattering. We find that classical Mie alumina particles $(2.1\mu m,4.0\mu m)$ enhance subsurface scattering due to a larger void space relative to larger Mie particles $\left(30\mu m\right)$, that cause the radar signal to scatter forward off small imperfections, maintaining the polarization properties of the signal and generating high $>1$ $CPR$. Latex beads, representing impurities, demonstrate the impact of isotropic and anisotropic scattering on radar signatures. This study also found that the scattering medium’s anisotropy correlates to the size of the beads, while the void space of the medium inversely correlates with the bead size and the volume concentration (v/v) of the beads. Rayleigh beads, due to isotropic scattering from the reduced scattering cross-section and higher transparency relative to larger impurities, generate subsurface single bounce scattering from the sample platform, producing $OC\gg SC$ and a low $(<0.5)$ $CPR$ across all v/v. Rayleigh impurities in transparent water-ice simulate single bounce scattering from underlying layers. Conversely, classical and large Mie beads generate anisotropic scattering that intensifies scattering in the forward direction with high $CPR$, inversely proportional to the volume concentration. This study aids in interpreting radar observations of icy bodies in the solar system, providing insights into the interplay between radar waves and icy regolith compositions.