Detection of Rare Earth Element Minerals on the Lunar Surface - Scientific Requirements and Spectral Analysis

DETECTION OF RARE EARTH ELEMENT MINERALS ON THE LUNAR SURFACE – SCIENTIFIC REQUIREMENTS AND SPECTRAL ANALYSIS. Sarah Cader1,2, Gabriele Cremonese1 1Istituto Nazionale di Astrofisica (INAF) OAPd - Astronomical Observatory of Padova, 2EMJM Degree in Planetary Geosciences ([email protected]) Introduction: Rare Earth Elements (REE) are crucial to modern-day scientific research and technological developments. The applications of REEs are widespread, from technological applications in electronics to even medical sciences. Rare Earth Elements consist of 17 elements, which include the lanthanide series and scandium and yttrium. These elements are essential in manufacturing electronic components like capacitors, sensors, batteries, LCD displays, etc. Some of the REEs are also used in medical imaging, especially for magnetic resonance imaging (MRI) systems. Due to their distinctive chemical and electronic properties, REEs are considered important in enabling innovation and technological developments in various sectors of science [1]. This study focuses on the detection of REE minerals using reflectance spectroscopy, using data from the Moon Mineralogy Mapper (M3) instrument. Scientific Requirements: The abundance of REEs on the Earth’s crust raises the question about the need for the detection and extraction of these minerals on the lunar surface. The extraction of REEs on Earth includes various environmental impacts because the separation of these elements from the naturally occurring minerals is a complex process, which includes the production of radioactive waste [2]. This, in addition to the existing geopolitical concerns, pushes the need to extract and mine REEs on the lunar surface [3]. A far more scientific need for the detection of REEs on the lunar surface is to enable future space exploration missions. Extraction of these elements on the lunar surface would support in-situ resource utilization, aiding in the production of various materials for lunar infrastructure and fuel. This would drastically limit the requirement to transport these elements from Earth thus making extended planetary exploration missions, possible. Spectral Analysis: Reflectance spectroscopy is not a common method used for the detection of REEs, and for this reason, these techniques are not very well developed. Applying this method to the lunar surface also poses a problem due to the quality of surface data available. Focussing on the studies of Boesche et. al, (2015) and Turner et al. (2016) [4], Neodymium absorption peaks are taken as primary spectral signatures for REE presence. For neodymium, the peak absorption bands are ~580 nm, ~740 nm, ~800 nm, & ~870 nm. The estimated detection of REEs on the lunar surface has been possible due to various orbital missions. KREEP-rich (Potassium, Rare Earth Elements, Phosphorus) regions are considered to be important in such studies. According to the studies conducted using the Lunar Prospector gamma-ray spectrometer, which mapped the Thorium surface distribution of the lunar surface, the Procellarum-Imbrium region on the near side of the Moon is considered to be the largest reservoir of REEs. This is owing to the fact that Thorium (Th) elemental distribution corresponds to the concentration of REEs. Some distribution of REEs is expected in the South Pole Aitken Basin. Data & Methodology: All the data used in this study was obtained from the NASA Planetary Data System from the Moon Mineralogy Mapper (M3) - Global Mode on board the Chandrayaan – 1. All the data are V01 Reflectance images and the location data (LOC.img) files were used to pin point the necessary regions of interest. Spectra of all Monazite samples and REE samples from the USGS library – M3 Target mode is also obtained for analysis. Based on the Th concentration as provided by the Lunar Prospector Gamma Ray Spectrometer, the regions of interest are chosen using LROC QuickMap. The spectral analysis is performed using ENVI Classic 5.5 software. Results: For the first part of the analysis, 6 Thrich regions within the Procellarum KREEP Terrain (PKT) are chosen and their spectra is collected. Comparing the spectra of these regions with the REE spectra obtained from the USGS library, some similarities between Th-rich region Timocharis crater and Xenotime+Monazite spectra were noticed, which are presented in figure 1. Figure 1: Manual analysis of the spectra from Timocharis and Xenotime+Monazite sample From figure 1, it can be noted that Timocharis region has some similar absorption features as the Xenotime+Monazite sample. But the prominent Neodymium (Nd) band, 800 nm and 870 nm are missing. This can be due to the presence of high FeO in the PKT region of the Moon, as analysed by Chang’e-5 [5]. High FeO content dampens the absorption peaks caused by Nd, as noted in Boesche et. al, (2015). Comparing the noted absorption peaks in Timocharis from figure 1 to their probable elemental origin [4], it can be noted that most of the observed absorption peaks are not caused due to Nd, but rather due to other REEs like Ho, Er, Dy, Yb, and Sm, which are present in both xenotime and monazite minerals as well. Can this be taken as a possible signature for the presence of Xenotime and Monazite within the Timocharis crater? More spectral analysis with better automated methods is needed to confirm this. All the analyses performed in this study were done manually. To further the spectral analysis, the regions of interest are expanded based on their thorium concentration and their proximity to the PKT, and their spectra is obtained. The classified regions are 1. High Th – Inside PKT 2. High Th – Outside PKT 3. Low Th – Bordering PKT 4. Low Th – Far from PKT The spectra of the chosen ROIs within these 4 classifications are compared with the major absorption features observed in the Xenotime+Monazite spectra, from 540 nm – 1000 nm. This comparison is analysed and it is noted that the absorption peaks corresponding to 750.44/770.40 nm and 989.95 nm are both only present in groups 1 and 3. This corresponds to ROIs within the PKT, with both high and low Thorium content. Both these absorption peaks correspond to the absorption peaks of 755.43 – 765.41 nm and 975.01 nm, that is observed in the Xenotime + Monazite Reference spectra from the USGS spectral library. 660.61 nm absorption peak, which corresponds to 655.62 nm in the Xenotime + Monazite Reference spectra, was also observed, but it was observed in all the 4 classified regions, so it was ruled out as a potential signature for REE content. A summary of this observation is currently being developed with further analysis to confirm this observation. As the spectral resolution of the M3 instrument is about 20/40 nm per channel, the difference in the observed absorption peaks can be considered as within the allowed error bars. Considering this, there is a possibility that 750/770 nm and 989 nm absorption peaks can be potentially considered as spectral signatures in the VNIR waveband of REE presence on the lunar surface. Further spectral analysis with better quality data is needed to solidify this claim and move ahead with the detection of REEs on the Moon. The study covers further analysis of these data, with comparison figures using manual spectral analysis. Analysis of the spectra obtained from the sample locations of Chang’e 5 and Chang’e 6 is also compared with the data from elemental chemical analysis performed with the returned samples from these missions. These analyses give a deeper look into the potential spectral signatures that can aid in the detection of REE presence on the lunar surface. References: [1] Boesche, Nina Kristine, et al. "Hyperspectral REE (rare earth element) mapping of outcrops— applications for neodymium detection." Remote Sensing 7.5 (2015): 5160-5186. [2] Su, Jia, et al. "A safer and cleaner process for recovering thorium and rare earth elements from radioactive waste residue." Journal of Hazardous Materials 406 (2021): 124654. [3] Hedrick, Gabrielle. "Towards Mining Rare Earth Elements on the Moon." 2023 IEEE Aerospace Conference. IEEE, 2023. [4] Turner, David J., Benoit Rivard, and Lee A. Groat. "Visible and short-wave infrared reflectance spectroscopy of REE phosphate minerals." American Mineralogist 101.10 (2016): 2264-2278. [5] Yang, Chen, et al. "Comprehensive mapping of lunar surface chemistry by adding Chang'e-5 samples with deep learning." Nature Communications 14.1 (2023): 7554.