3D and 2D clast analysis of Apollo 17 core sample 73002: insights into the Light Mantle dynamics and regolith reworking.

Several lunar samples collected during the Apollo missions were kept sealed and stored in controlled conditions in order to be studied decades later exploiting future, more advanced capabilities. Two of the preserved samples are Apollo 17 double drive tube 73002/73001. The double drive tube extracted a core sample of the Light Mantle deposit at Station 3 in the Taurus-Littrow Valley (Figure 1). As part of the NASA Apollo Next Generation Sample Analysis (ANGSA) program, samples 73002 and 73001 became available to study in 2019 and 2022, respectively [1][2]. The Apollo 17 double drive tube sampled the Light Mantle deposit material down to a depth of 70.6 cm; the effective material length of each tube is 21.3 cm for 73002 and 34.9 cm for 73001 (some material was lost during sampling). This represents an unprecedented opportunity to study the Light Mantle deposit to previously unsampled depths.The Light Mantle deposit represents the only extraterrestrial landslide to have ever been studied in-situ. The Light Mantle is a 5-km-long deposit that formed from debris mobilised from the South Massif, a 2.2-km-high mountain in Taurus-Littrow Valley [3][4]][5]. The origin and hypermobility of the Light Mantle remain debated. The recently opened Apollo 17 double drive tube 73002/73001 provides a new set of samples to investigate the origin and the emplacement mechanisms of the Light Mantle.Prior to dissection and opening of the sample containers, the double drive tube was scanned using X-ray computed tomography (XCT) [6] so that a digital, high-resolution 3D dataset of the whole core sample is available and represents one of the ‘next generation’ capabilities now available to researchers to interrogate the data using novel approaches and obtain new insights into lunar material and processes. Additionally, the 3D dataset preserves the 3D context of all the subsamples extracted from the original core sample.In this work, we used high-resolution X-ray computed tomography (XCT) scans and high-resolution scans of thin sections of the upper 20 cm of the core, sample 73002, and conduct 3D clast-size analysis and investigation of clast morphological fabric. The aims of this work are to:Present a 3D data processing workflow that can be used as a basis for future investigations of lunar core samples. Demonstrate potential scientific information that can be extracted from 3D analysis of lunar core samples. Within this framework, we conduct: (1) 3D grain size analysis and compare the results with grain size analyses conducted on the grains extracted during the dissection of the core sample; (2) 3D analysis of clast size distribution. Additionally, as part of our investigation of the emplacement mechanism of the Light Mantle, we use 2D continuous thin sections (backscattered electron maps) of sample 73002 to search for diagnostic clast fabric similar to those generated during the friction experiments conducted in simulated lunar landslides [7]. The clast fabric is called Clast Cortex Aggregate (CCA) and it’s constituted by a central clast surrounded by nano-scale fine material (Figure 2).The data analysis and visualization of the XCT dataset of core sample 73002 were performed using 3D visualization software Avizo 2022.2 by ThermoFisher. We customized our workflow and established a best-practice protocol so that they can be used as reference for future analysis of 73001. We used backscattered electron (BSE) maps of the sample’s thin sections (73002,6011; 73002,6012; 73002,6013; 73002,6014) [8] to search for (CCAs).The results of clast-size distribution show that the sample is characterised by lack of the largest clast-size fraction in the top 4-5 cm, which we attribute to the fragmentation of larger regolith-hosted clasts and bedrock by space weathering and meteoroid bombardment. The observation of an uppermost layer presenting characteristics of reworked regolith is consistent with results from previous studies of lunar regolith and from other works conducted on 73002 as part of the ANGSA program [8][9]. Moreover, we found extensive presence of CCAs. The formation of CCAs in natural and lab-simulated landslides is attributed to granular flow dynamics, presence of nanoparticles, and adhering forces between such particles. Therefore, we concluded that the presence of CCAs in sample 73002 represents the first evidence that the Light Mantle was emplaced as a granular flow. This work shows that valuable information can be extracted from the 3D analysis of lunar core samples and, more generally, it shows the potential of morphometric and morphological clast analysis using high resolution XCT dataset and thin sections combined.Our work represents the first study to conduct a 3D clast analysis of a lunar regolith core sample. As such, it constitutes an important step in showing the novel information that can be extracted, and presenting a potential workflow for studying lunar regolith core samples that will be collected during future missions to the Moon.  REFERENCES. [1] Shearer et al. (2020). AGU Fall Meeting. Abstract V013-0001. [2] Shearer et al. (2022). 53rd LPSC. Abstract 2546. [3] Schmitt (1973). Science, 182(4113), 681–690. [4] Lucchitta (1977). Icarus, 30(1), 80–96. [5] Kokelaar et al. (2017). JGR:Planets, 122(9), 1893-925. [6] Gross et al. (2023).     https://curator.jsc.nasa.gov/lunar/angsa_attachments/aapreliminary_20catalog/preliminary_73001-73002_catalog.pdf. [7] Magnarini et al. (2023). JGR:Planets, 128(6), e2022JE007520. [8] Bell et al. (2024). Submitted to JGR:Planets – In review. [9] Neuman et al. (2024). Submitted to Science – In review.Figure 1 – a) Oblique view of Taurus-Littrow Valley; the yellow dot shows the location of the Apollo 17 landing site (LROC/NAC image M1266925685L. Image credit: NASA/GSFC/ASU); b) A frame from the original footage recorded from the Lunar Rover Vehicle onboard camera the showing astronaut Gene Cernan extracting the double drive tube containing material from the Light Mantle deposit. c) Double drive tube in the ground prior extraction (AS17-137-20981. Image credit: NASA). The yellow star in the three panel shows the location where the double drive tube core sample 73002/73001 was collected. Figure 2 – Comparison of Clast Cortex Aggregates (CCAs). (a-b) CCAs generated during the friction experiments conducted on anorthosite-bearing gouges by [7]; (c-f) CCAs found in the Apollo 17 core sample 73002. The red dotted lines and the white dotted line in (d) show the corona of finer fragments found around clasts.