Mineralogical characterization of an Apollo 16 sample and lunar simulants through Mid-Infrared spectral analysis

Introduction

Mid-Infrared (MIR) spectral analysis is one of the primary tools to studying the lunar soil mineral samples and providing a better understanding of their geological origin, helping to distinguish a mafic mare from a felsic anorthosite.

In this context, a set of 12 samples has been analyzed: one Apollo 16 sample (62231.44), endmembers (LHS-1 25A, LMS-1, LHS-1, LMS-1D, LHS1D) provided by the Exolith Lab of University of Central Florida, and intimate mixtures with different composition and same grain size (50%LMS1 + 50%LHS, 50%LMS-1D + 50%LHS1D), intimate mixture with same composition and different grain size (50%LMS1 + 50%LMS-1D, 50%LHS-1 + 50% LHS-1D), and intimate mixture with different composition and different grain size (50%LMS1 + 50% LHS-1D, 50% LHS-1 + 50% LMS-1D) under reflectance and in a vacuum environment at ambient temperature.

The primary goal is to characterize variations of spectral features such Christiansen Feature (CF) and Transparency Feature (TF) of these samples and see the relationship among them to understand how intimate mixing influences them. CF is sensitive to the presence of felsic minerals at shorter wavelengths and to the presence of mafic minerals at longer wavelengths, considering its region from 7.5 to 9.5 µm (Lucey et al., 2021). In our data, CF features are identified by the highest peak in this spectral window. The TF is an indicator of fine fraction samples where the mineral’s absorption coefficient reaches a minimum in the region between 11 and 13 µm (Prem et al., 2022).

Methodology

The set of samples has been analyzed in hemispherical reflectance spectroscopy under vacuum using a Bruker Vertex 80V FTIR spectrometer at the Planetary Spectroscopy Laboratory (PSL) at the German Aerospace Center (DLR). The set-up consists of a liquid-nitrogen-cooled MCT detector and KBr beam splitter operating under vacuum to simulate Lunar conditions. The measurements were taken under MIR range (approx. 7 to 25 µm). For each of the 12 samples, 3 measurements of 1000 scans were taken by rotating the sample approximately 120° between each measurement to provide a more accurate statistical analysis.

Pre-processing and data analysis

First, the reflectance data (R) from the spectrometer have been multiplied by a calibration factor of 0.93 to account for detector response. Then, the conversion from Reflectance (R) to Emissivity (E) has been applied as first approximation, according to Kirchhoff’s Law. For each sample, the three collected spectra were averaged. On each, the CF, TF and Reststrahlen Bands (RB) spectral features have been identified. A local polynomial fit (ranging from degree 4 to 12, depending on sample complexity) was applied to the CF window (7.5 - 9.5 µm) to get independence from random noise and maximize measurement precision. Furthermore, the Half Width Band Depth (HWBD) parameter, a physical parameter to quantify the volume scattering, has been identified for each of the TF feature of each sample to quantify the physical broadening of the absorption band, demonstrating the physical mechanism that “pushes” the CF towards a redshift.  These results will focus on the quantification of the impact of fine particulates on mineralogical identification, possibly boosting further research and helping in the study of lunar soil, with a future perspective set on Moon expeditions.

References

Lucey, P. G., Greenhagen, B., Hanna, K. D., Bowles, N., Flom, A., & Paige, D. A. (2021). Christiansen feature map from the lunar reconnaissance orbiter diviner lunar radiometer experiment: Improved corrections and derived mineralogy. Journal of Geophysical Research: Planets, 126, e2020JE006777. https://doi.org/10.1029/2020JE006777

Prem, P., Greenhagen, B. T., Donaldson Hanna, K. L., Shirley, K. A., & Glotch, T. D. (2022). Modeling thermal emission under lunar surface environmental conditions. The Planetary Science Journal, 3(7), Article 180. https://doi.org/10.3847/PSJ/ac7ced