Optical Polarization Studies Of Latex Beads In Aqueous Solution: An Analog For Radar Scattering In Water-Ice Medium

This study presents low phase angle 0◦−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 (SC), and opposite sense (OC) backscatter circular polarization coefficients, circular polarization ratio (CPR), and degree of linear polarization (DLP) of spherical latex (non-dye) beads of varying sizes and volume concentrations (v/v) in aqueous solutions (λ=0.8μm) in water.

We leverage Mie scattering calculations to accurately simulate the polarization behavior of light in an aqueous solution of latex beads. We also present measurements of alumina powder in air at λ=1.064μm. Measurements showed that at a low incidence angle (i=0◦), backscatter is dominated by surface specular single-bounce scattering, which 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 (2.1μm, 4.0μm) enhance subsurface scattering due to a larger void space relative to larger Mie particles (30μm), which 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. Experiment and model 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.

Model and measurements show that Rayleigh-sized beads (impurities), due to isotropic scattering from the reduced scattering cross-section and higher transparency relative to larger impurities, generate subsurface single bounce scattering, producing OC≫SC and a low (<0.5) CPR and across all v/v with SC, and CPR proportional to v/v, but with OC, and DLP inversely proportional to v/v.

Model of Rayleigh-sized beads (impurities) has increased modeled transparency that results in more simulated single-bounce scattering relative to experiment. Conversely, model and measurements of classical and large Mie beads show anisotropic scattering that intensifies scattering in the forward direction with high CPR, inversely proportional to the volume concentration with discrepancies between 4◦−5◦ relative to model.