Optical Scattering Characteristics of 3-D Lunar Regolith Particles Measured Using X-Ray Nano Computed Tomography

Abstract

Shape, in addition to size and material composition, has a strong effect on the optical scattering characteristics of a particle. In this work, we computationally study how the measured 3-D shapes of individual lunar regolith particles affect their optical scattering characteristics. The shapes of 25 lunar regolith particles collected during the Apollo 11 mission were measured using X-ray nano computed tomography (XCT). The scattering cross sections (CSs) of particles that ranged in size between 400 and 1000 nm were computed using the Discrete Dipole Scattering (DDSCAT) software in the visible to the infrared frequency range. The orientation-averaged scattering CS of the lunar regolith particles was then compared with that of spheres of the same volume. The comparison shows that the shape of lunar regolith particles decreases the resonance wavelength by 20%, on average, compared with the spheres of the same volume. On the other hand, the peak scattering CS is found to be nearly independent of shape for the suite of particles studied. Also, lunar regoliths are found to generate a slightly lower single scattering albedo and have a higher phase function asymmetry parameter in comparison to spherical particles of the same volume. These differences highlight the importance of accounting for 3-D shape variations in understanding the optical response of lunar regolith particles.