Syntheses and Reflectance Analyses of Lunar Red Glass Compositions: Information to Improve Understanding of Remotely Sensed Spectral Data

Introduction: Compositions of many of the >100 lunar pyroclastic deposits identified remotely are poorly constrained. In addition, the absorption properties of glasses, which are ubiquitous in all regolith samples, strongly influence spectra of lunar soil and hence spectral remote sensing data of the lunar surface [1,2]. Previous spectral studies of lunar synthetic glasses have not reproduced all lunar glass compositions (Fig 1). To this end, synthetic glasses are made with specific compositions (Table 1) and under controlled oxygen fugacity to facilitate study of the relationship between the optical properties of “lunar” glass and FeO, TiO2 concentrations. Although the UV absorption is controlled by the sum of Fe and Ti, their relative importance is unknown. Also, the effect of Fe and Ti is believed restricted to the UV on the basis of little evidence. Thus, optical constants derived from this work have direct application to radiative transfer modeling of lunar pyroclastic glasses [3] and agglutinates by providing the ability to evaluate the relative effects of Fe and Ti and determine whether Fe-Ti has any affect outside the UV. The goal is to eventually use our derived optical constants to match the remotely sensed spectral data with model data, and from this infer the composition of the lunar surface. Methods: Sample Synthesis. The synthetic red glass composition for this study was based on the average lunar red glass composition (Table 1), and was synthesized with reagent-grade carbonate and oxide powders. The powders are heated for 30 minutes at 850C in a controlled fO2 environment. Flowing H2-CO2 gas mixture was used to impart an intrinsic fO2 just above ironwüstite to simulate lunar conditions. The sample is then lowered further into the furnace, where it is quickly (~1 min) heated above the liquidus temperature (~1400 °C), and then rapidly quenched. Quench is achieved when the crucible is tipped, allowing the molten sample to fall into a reservoir of water below the furnace. Spectral analysis requires a relatively large quantity of homogenized glass. This obviates the use of the conventional Pt wire loop method. High purity alumina crucibles allow gram-quantities of material to be conditioned and fused at once. Additional advantages of this ceramic are its low cost, stability at low fO2, and highly refractory nature. The compositional contrast between our lunar basalt compositions, which are strongly undersaturated in alumina, and the alumina container materials pose a potential contamination problem. Our preliminary experiments, however, exhibited minimal reaction with the container (Table 1), and the melt formed a ball inside the container (i.e., did not wet the container). Measuring Reflectance and Deriving Optical Constants. Bidirectional reflectance measurements of synthetic lunar glass allow the calculation of optical constants of glasses using radiative transfer theory. The synthetic red glass samples have sufficient optical defects at the macroscopic level (Fig 2) to prohibit standard transmission methods for deriving absorption coefficients. Instead, our samples were ground and dry sieved to achieve a fine <53 μm powder. Reflectance measurements were made at the University of Hawaii using an Analytical Spectral Devices spectrometer, which has a spectral range from 0.35 to 2.5 μm and a spectral resolution of 1 μm. Reflectance measurements are compared against a Spectralon reflectance standard. A quartz-halogen lamp was used as the illumination source, with a 40° incidence angle and 0° emission angle. Additional reflectance measurements will be made at Brown University’s Reflectance Laboratory for comparison and reproducibility. Optical constants were computed from the reflectance values of these powers using the methods of [4,5], as implemented by [6] for deriving mineral optical conFig 1. FeO and TiO2 compositions for our proposed and synthesized glasses, previously made synthetic glasses [7,8], and lunar glass compositions from [9].