A Goniometric System for Photometric and Polarization Measurements of Planetary Regolith Analogs

Abstract

The Planetary Surface Texture Laboratory at the Johns Hopkins University Applied Physics Laboratory is a facility for the study of the photometric and polarization behavior of regolith analogs using an ∼1.5-m-radius arc goniometer system. This system characterizes the photometric and polarization response of granular materials (planetary regolith analogs) at various illumination and viewing angles in the principal plane, enabling both forward and backward scattering observations at phase angles as low as 20 ± 5°. A polarimetric camera with linear polarizers at 0°, 45°, 90°, and 135° captures visible-wavelength images of samples illuminated by semi-collimated, unpolarized light. Imaging polarimetry and photometric studies are important for their ability to reveal sub-pixel information about the nature of planetary surfaces, in particular, texture-related properties such as porosity and roughness. We develop a calibration pipeline for the system and illustrate how the system can be utilized to understand the polarization and photometric response of planetary surface analog materials relevant to a range of airless Solar System bodies. We present an initial case study using a lunar regolith simulant, JSC-1A, illuminated with broadband white light and observed between phase angles of 20 ± 5° and 120 ± 5°. We find that the radiance reflected from JSC-1A decreases with increasing phase angle, while the degree of linear polarization increases, consistent with previous studies. This work illustrates the system's potential to enhance the interpretation of data sets such as those from the PolCam instrument on the Danuri lunar orbiter, and astronomical observations of asteroids.