PLANETARY REGOLITH ANALOGS APPROPRIATE FOR LABORATORY MEASUREMENTS. R. M. Nelson 1 , J. L. Piatek 2 , M. D. Boryta 3 , K. Vandervoort 4 , B. W. Hapke 5 , K. S. Manatt 6 , A. Nebedum 3 , Yu. Shkuratov 7 , V. Psarev 7 , D. O. Kroner 8 , W. D. Smythe 6 . 1. Planetary Science Institute, Pasadena CA 2. Central Connecticut State University, New Briton CT, 3. Mount San Antonio College, Walnut CA, 4. California State Polytechnic University, Pomona CA, 5. University of Pittsburgh, Pittsburgh PA, 6. NASA JPL Pasadena CA, 7. Karazin University, Kharkiv, Ukraine, 8. University of California, Los Angeles, Los Angeles, CA rnelson@psi.edu Introduction: Understanding remote sensing data from airborne or space-based platforms requires com- parison to similar measurements on candidate regolith materials in a laboratory controlled environment. Such measurements have been made for centuries [1]. Four centuries have elapsed since Galileo first observed Saturn’s rings with the telescope. His acute observa- tional skills allowed him to pioneer fundamental re- mote sensing photometric techniques simply by watch- ing the Moon rising over a sun illuminated wall in his garden. He noted the fully illuminated Moon appeared darker than the sunlit wall. He correctly inferred that the intrinsic reflectivity of the Moon’s surface was lower than the intrinsic reflectivity of his wall. Having made this observation he is to be credited with the first reported albedo measurement of the surface of an extra terrestrial object [2]. In the modern laboratory angular reflectance measurements of candidate regolith materi- al are made using instruments such as goniometric photoporarimeters (GPP). An instrument of this type is generally classified as a ‘polarization-sensitive well- collimated radiometer’; particulate samples used to simulate planetary regoliths are classified as ‘discrete random media’[3]. Background: There are many reputable laborato- ries around the world that conduct such measurements. Slight differences in samples at various facilities can lead to uncertainty when comparing measurements between laboratories and when applying these meas- urements to spacecraft results. High albedo surfaces have often been simulated in the laboratory using MgO, BaSO 4 , or powdered, compressed Polytetrafluoroethylene (aka Teflon, PFTE, or HALON). These materials, while easily available, are not well sorted into particle sizes that are larger than, comparable to, or less than the size of the incident light used in most GPP devices. Therefore, in 2000, we in- troduced powdered aluminum oxide Al 2 O 3 as a materi- al that might appropriate for simulating high albedo regoliths in the laboratory [4]. Powdered Al 2 O 3 is widely used as an optical abrasive. It is available in particle sizes that are as small at 0.1 μm to several hundred microns from various commercial suppliers. We acquired these materials in a wide range of particle sizes from Micro Abrasives Corp of Westfield Mass, USA and the Stutz Company of Chicago IL, USA. Both the GB and WCA designations were previously offered by Microabrasives Corp. These materials have since been measured at reputable GPP laboratories around the world [4- 7]. The product identifications and approximate particle sizes are shown in Table 1. Table 1. Manufacturer’s Product Identification and approximate particle diameter Product ID Diam (µm) Product ID Diam (µm) WCA 40 30.09 GB 1200 1.5 WCA 30 22.75 GB1500 1.2 WCA 20 12.14 GB2000 1 WCA 12 7.1 GB 2500 0.5 WCA 9 5.75 GB 3000 0.1 WCA 5 4 WCA 3 3.2 WCA 1 2.1 The Al 2 O 3 particulates are supplied in two different particle shapes. The sizes larger than 2 μm (WCA) are described by the supplier as ‘platelet shaped’; those smaller than 1.5 μm (GB) are described as ‘equant’. These morphology differences should be apparent with analysis of particle packing density and in photomi- crographs. The Particle packing density: We used 13 separate particle sizes 0.1 <d <30 1.5 μm. Five of these were 1.5 μm or smaller. The samples were poured into sam- ple cups of known height and diameter. The cup was gently shaken to flatten the surface and permit settling so as the surface might best replicate a powdered sur- face of a planetary regolith as viewed by a remote ob- server. The mass of the material was measured. The void space was calculated based on the density of Al 2 O 3 . The results, shown in Fig. 1, are consistent with the manufacturer’s description. The particle sizes <~1.5 µm (shown as ‘Equant’ in Figure 1 and designated GB) pack together with much larger void space than the particles of size >~2.1 μm (Shown as ‘Platelet’ in 1695.pdf 47th Lunar and Planetary Science Conference (2016)