Sample Locations for Primordial Crust and South Pole-Aitken (SPA) Basin Material near the Lunar South Pole

SAMPLE LOCATIONS FOR PRIMORDIAL CRUST AND SOUTH POLE-AITKEN (SPA) BASIN MATERIAL NEAR THE LUNAR SOUTH POLE. T. Frueh1, A. Camon2, M. Boyce3, S. Halwa4, G. Ligeza5, M. Lemelin2, B. J. Thomson1, and D. A. Kring6, 1Earth, Environmental, and Planetary Science, University of Tennessee Knoxville ([email protected]), 2Département de Géomatique Appliquée, Université de Sherbrooke, 3School of Earth Sciences, University of Western Australia, 4Department of Environmental and Earth Sciences, University of Manchester, 5Department of Environmental Sciences, University of Basel, 6Lunar and Planetary Institute Introduction: Located at the rim of the South Pole-Aitken (SPA) basin, the lunar south polar region has become the focus of many international landing missions. In contrast to the relatively young, flat, and mostly volcanic Apollo landing sites, the south polar region is a densely cratered and ancient highland terrain [1-3]. During the SPA impact event, the primordial anorthositic lunar crust at the south pole was covered by SPA ejecta. SPA’s ~2,500 km diameter implies deep excavation from the lower crust and potentially the pre-overturn upper mantle [e.g., 4-6]. Thus, the SPA ejecta blanket is expected to be primarily mafic, rich in thorium, iron oxides, and clinopyroxenes [46]. Following the SPA-forming impact, subsequent cratering events redistributed, mixed, and buried SPA-derived ejecta beneath newer layers of debris [7]. Some more recent impact events may have been large enough to excavate beneath the post-SPA ejecta layers, exposing fresh SPA ejecta or primordial crust and providing access to some of the oldest and previously inaccessible lunar rocks [8]. Here, we estimate the thickness of SPA ejecta and post-SPA crater materials to determine what materials craters may have excavated from. The goal is to identify exposures and optimal sites for sampling these materials. Methods: In ArcGIS, we mapped all craters with diameters >10 km within one crater diameter distance to 6° of the south pole on 500 m LOLA hillshade maps [9]. Each crater was assigned a formation epoch based on its morphology [10,11] and stratigraphic relationships. For each epoch, we modeled the cumulative ejecta thickness (CET) of all craters using the MoonPIES model [12]. Transient crater diameters and excavation depths were derived from crater scaling relationships [13,14]. Finally, we compared excavation depths, CET values during the craters` formation epochs, and estimated SPA ejecta thicknesses to determine which stratigraphic layers were excavated. To search for surface exposures of SPA ejecta and primordial crust, we used a regolith composition map. The regolith composition map was constructed from the mineral maps of [15] by classifying 1×1 km pixels based on the proportions of plagioclase, olivine, orthopyroxene, and clinopyroxene. We used the classification scheme of [16] to define an average regolith composition for each pixel. SPA Ejecta Thickness: SPA’s SE to NW elongations implies an oblique impact event from either SE to NW [5,17] or NW to SE [18,19]. Oblique impacts result in preferred ejecta deposition in the down-range direction, whereas in the uprange direction zones of avoidance (ZoAs) can form [5,20]. Therefore, the thickness of SPA ejecta at the south pole is strongly dependent on the impact trajectory. To estimate the present SPA ejecta thickness, we used the 3D hydrocode iSale model from [5]. For a SE to NW trajectory, the results of [5] predict a Zone of Avoidance (ZoA) near the south pole. Due to model constraints and the absence of a modification stage, we cannot determine an exact thickness but infer an SPA ejecta layer of <10 km for this trajectory. For a NW to SE trajectory, we mirrored the model from [5] and estimate an SPA ejecta thickness of 10–20 km. For a non-oblique impact, we adopt a ~10 km thickness based on [21]. Post-SPA Ejecta Layers: We mapped 96 craters >10 km for the CET model, resulting in a minimum post-SPA CET of ~190 m and a maximum of ~1.8 km within 6° of the lunar south pole (Fig. 1). The thickest CET deposits are found towards 90°E, influenced by the presence of several large craters and the two basins Amundsen-Ganswindt and Schrödinger. In the west, Cabeus, Ashbrook, and Drygalski craters contribute the most to CET thickness. Excavated Materials: By considering a crater’s excavation depth, the underlying CET during its formation epoch, and the SPA ejecta thickness, we can infer the deepest excavated material, i.e., primordial crust, SPA, or post-SPA ejecta. We show that all craters >10 km excavated beneath post-SPA crater materials (Fig. 1), suggesting SPA material is widespread in the south polar regolith, increasing the chances of SPA sample return. Because the SPA ejecta thickness at the south pole is unknown, we consider a range of 100 m to 20 km. Assuming a SPA ejecta thickness of <1 km, 64 from 85 classified >10 km craters could have excavated primordial crust (Fig. 1a). A 10 km thick SPA blanket could reduce this number of craters to 6 (Fig. 1b), decreasing the amount of anorthosite in the surface regolith. Only the two basins AmundsenGanswindt and Schrödinger could have excavated material from the primordial crust assuming a SPA ejecta thickness of 15 to 20 km (Fig. 1c). Surface Regolith Compositions: Anorthositic noritic and anorthositic troctolitic regolith compositions dominate the south polar region. However, some regions show higher mafic or anorthositic compositions. Near Shackleton crater, less mafic compositions like troctolitic anorthositic, noritic anorthositic, and anorthositic compositions are present. Increased mafic compositions, such as noritic and gabbronoritic, are spread sparsely throughout the south pole, with the highest concentration around Kocher crater. Large scale processing of the surface regolith by impacts mixed and diluted the regolith, resulting in anorthositic noritic to noritic anorthositc compositions [22]. Most of the south polar regolith is, therefore, indeed intensely mixed and reworked. Deviating compositions may indicate the deposition of fresh material by a nearby crater intersecting a particular layer. The primordial lunar crust is expected to be anorthositic in composition. Therefore, regions with higher anorthositic content, such as those around Shackleton crater, may represent exposures of the primordial crust or crust-rich material. In contrast, SPA ejecta is thought to contain material from the lower crust and upper mantle, suggesting that gabbroic, noritic, gabbronoritic, and anorthositic gabbroic regolith compositions could indicate potential exposures of SPA ejecta, such as the highly mafic compositions observed around Kocher crater. Sampling Locations: Because all craters >10 km were able to excavate from beneath the post-SPA crater materials, we expect SPA ejecta to be widespread near the lunar south pole. However, most of these craters are ancient and their ejecta blankets have undergone reworking, mixing SPA and crustal materials into the surface regolith and diluting the mafic signal. Any remaining SPA material is likely to be fine-grained within the regolith. Therefore, we expect the most promising locations for sampling large quantities of SPA material near the youngest craters identified in the excavation model. Regolith composition near the south pole is the most mafic within the ejecta blanket of the Imbrian to Eratosthenian Kocher crater. Thus, we infer Kocher crater as the freshest and largest exposure of SPA material near the south pole [23]. Shackleton crater’s ejecta blanket marks the most anorthosite-rich surface regolith and potentially largest exposure of primordial crust near the south pole [3,24]. As the crater-forming impact would have excavated SPA material in the stratigraphic excavation model, we interpret the topographic ridge to be a crustal megablock or uplifted primordial crust during the SPA impact event, reducing the local amount of mafic SPA material. References: [1] Wilhelms, D. E. et al. (1979) USGS, 1162. [2] Spudis, P. D. et al. (2008) Geophys. Res. Lett., 35, L14201. [3] Gawronska, A. J. et al. (2020) ASR, 66, 1247-1264. [4] Pieters, C. M. et al. (2001) JGR Planets, 106, 28001-28022. [5] Melosh, H. et al. (2017) Geology, 45, 1063-1066. [6] Hurwitz, D. 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(2025) Submitted to JGR Planets. [24] Yamamoto et al. (2012) JGR Planets, 39. Figure 1. Mapped craters >10 km in diameter, post-SPA cumulative ejecta thickness, and inferred origins of the deepest excavated materials for different SPA ejecta thicknesses. Modified after [23].