A Sample Location for South Pole-Aitken (SPA) Basin Material near the South Pole: Kocher Crater

A SAMPLE LOCATION FOR SOUTH-POLE AITKEN (SPA) BASIN MATERIAL NEAR THE SOUTH POLE: KOCHER CRATER. T. Frueh1, T. Samaddar1, B. J. Thomson1, C. A. Nypaver2, L. Wueller4, and D. A. Kring3, 1Dept. of Earth, Environmental, and Planetary Science, University of Tennessee, Knoxville ([email protected]), 2Smithsonian Institution, National Air and Space Museum, D.C., 3Lunar and Planetary Institute, USRA, Houston, 4Institut für Planetologie, Universität Münster, Germany Introduction: Many international crewed and robotic sample return missions are targeting the lunar south polar region, which is a mostly unexplored, densely cratered, and ancient highland terrain [1-3] with areas of permanent shadow and perpetual illumination [4]. A key scientific attribute is its location on the rim of the South Pole-Aitken (SPA) basin [2,5,6]. SPA’s immense ~2500 km diameter suggests excavation from the lower crust and potentially the preoverturn upper mantle [7-9]. During the impact event, an estimated 2.21 × 10⁶ km² of mantle material was emplaced onto the lunar surface in an ejecta blanket up to ~75 km thick [8]. SPA ejecta is expected to have gabbroic to noritic compositions [7] with increased abundances of clinopyroxene and iron oxides [7-9]. Sampling SPA-derived material is a key science goal [5,6], as it could provide new insights into lunar magma ocean differentiation and the SPA impact [10–11]. In a recent investigation [12-13], we proposed that Kocher crater may have excavated material from the SPA ejecta blanket. To assess its landing site potential, we conducted a study of the regional geology surrounding Kocher crater, identified potential landing sites, and evaluated its exploration feasibility. Here, we present the initial results of that study. Methods: We mapped the regional geology in ArcGIS at 1:100,000 scale using topography, imagery, and radar data. Hillshade maps with varying solar azimuths and slope maps were derived from 20 m/px LOLA topography [14]. We used imagery from the WAC south pole mosaic [15] and the 1 m/px controlled south pole LROC NAC ortho photo mosaic [16], as well as Mini-RF monostatic S-band δCPR and S1 total backscatter images (~30 m/px) [17]. Regolith composition of the mapped ejecta blanket was studied using mineral maps of [18] and M3 [19]. We focused on the FeO and high Ca-pyroxene maps as they represent potential SPA materials. To study the morphology of the ejecta blanket and the potential presence of boulders, we used ShadowCam images (1.7 m/px) [20] and the controlled south pole NAC orthophoto mosaic [16]. We based the exploration feasibility study on LOLA-derived slope maps [14] and Earth and Sun visibility [4]. Durations of Earth and Sun visibility were calculated with the Lunar QuickMap Visibility Analysis tool. Regional Geology: As a heavily cratered ancient highland terrain, the lunar south polar region’s stratigraphy is dominated by ejecta debris. Initially, the region’s surface exposed primordial lunar crust that formed by lunar magma ocean solidification. Rocks within this layer are expected to have a primarily anorthositic composition [10]. At ~4.3 Ga [21], the SPA basin-forming impact covered the south polar region with mafic ejecta and fractured the crust, forming the south polar massifs [1,2]. Mons Kocher, located ~10 km south of Kocher crater, is one such massif. These massifs are likely built from uplifted lunar crust. The thickness of the SPA ejecta layer depends on the impact trajectory and may range from a few hundred meters to 15–20 km [12,13]. Subsequent impact events deposited new ejecta layers, continuously excavating, redistributing, and mixing pre-existing materials [7,22]. This process exposed fresh rocks while forming a megaregolith composed of fragments from all prior layers. Rocks from these deposits are primarily impact breccias with anorthositic noritic to noritic anorthositic compositions [22]. However, regional variability is expected both at the surface and within the stratigraphic column, depending on the materials present in specific ejecta blankets. The post-SPA stratigraphy beneath Kocher crater is primarily influenced by ejecta from the Amundsen-Ganswindt basin, Ashbrook, Drygalski, and two adjacent ~40 km craters further north. Additionally, a heavily eroded ~190–280 km crater toward the farside has contributed ~200 m of ejecta [12,13]. Larger, more distant craters such as Haworth and Schrödinger likely produced less ejecta [12,13]. The estimated cumulative post-SPA ejecta thickness is 0.7 to 0.8 km beneath Kocher crater. Given Kocher crater’s approximate excavation depth of 2.4 km, its impact may have exposed SPA material [12,13]. Geology of Kocher Crater: Kocher crater is a simple crater of Imbrian to Eratosthenian age [23]. Its ejecta blanket extends in lobes up to ~10–30 km from the crater rim (24 km diameter). Within 10 km of the crater, the ejecta blanket appears bright in Mini-RF δCPR and S1 total backscatter images (Fig. 1a), indicating increased surface and subsurface roughness. The visible lobes and high surface roughness confirm the presence of the ejecta blanket and suggest it has not been fully eroded. A positive anomaly of clinopyroxene (Fig. 1b) and iron oxides coincides with the ejecta blanket. The inner radar-bright ejecta blanket contains clinopyroxene with a maximum of 32 wt% and a mean of 5 wt%. Iron oxides reach a maximum of 12 wt% and a mean of 8.4 wt%. Given Kocher crater’s position near the presumed SPA basin rim and the typical association of these mafic phases with SPA material, we suggest that SPA ejecta is exposed within its ejecta blanket [12,13]. Boulders are present throughout the ejecta blanket, with impact melt deposits near the crater rim in the northern ejecta. Initial mapping reveals an uneven boulder distribution, with concentrations primarily within the inner ejecta blanket, where the deepest excavated material would be expected [24]. Exploration Feasibility: Slopes around Kocher crater are generally shallower compared to other proposed south polar landing regions, such as the ejecta blanket of Shackleton crater. Multiple areas on Kocher crater’s ejecta blanket feature slopes of less than 10° extending over several kilometers. Earth visibility conditions at Kocher crater present challenges due to its location on the lunar far side, where the Mons Kocher obstructs direct lineof-sight and communication. However, this issue can be mitigated using relay satellites. Sun illumination is essential for power generation and general visibility. Whereas Sun visibility is not as high as in some other south polar regions, areas on the Kocher crater ejecta blanket experience continuous sunlight for more than 7 days. Assuming an Artemis III style landing mission with 6.5 days of surface operation [5], crewed sample return missions might be feasible. Conclusion: Kocher crater’s ejecta blanket represents the most mafic region within 6° of the lunar south pole, indicating the presence of exposed SPA material [5,16]. Boulders in areas with elevated clinopyroxene and iron oxide abundances may contain substantial amounts of SPA ejecta, making them valuable sampling targets. Therefore, Kocher crater could offer an excellent opportunity to sample large, relatively “pristine” quantities of 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] Mazarico, E. et al. (2011) Icarus, 211. [5] Artemis Science Definition Team (2020) Artemis Science Definition Team Report. [6] NRC (2007) NAP. [7] Pieters, C.M. et al. (2001) JGR Planets, 106, 28001-28022. [8] Melosh, H. et al. (2017) Geology, 45, 1063-1066. [9] Hurwitz, D.M. and Kring, D.A. (2014) JGR Planets, 119, 1110-1133. [10] Pieters, C.M. et al. (2001) JGR, 106(11), 28001-28022. [11] Moriarty D. et al (2021) Nat Commun., 12, 4659. [12] Frueh T. et al. (2025) SUBMITTED TO JGR. [13] Frueh, T. et al. (2025) ELS. [14] Neumann, G.A. (2009) Lunar orbiter laser altimeter raw data set. [15] Robinson, M.S. et al. (2010) SSRv, 150. [16] Archinal, B. et al. (2023) LPSC, LIV, Abstract #2333. [17] Fassett, C.I. et al. (2024) PSJ, 5(4). [18] Lemelin, M. et al. (2022) PSJ, 3(63). [19] Green, R.O. et al. (2011) JGR, 116. [20] Robinson, M.S. et al. (2023) JASS, 40(4), 140-171. [21] Hiesinger, H. et al. (2012) LSPC, 43. [22] Hawke, B.R. et al. (2003) JGR Planets, 108. [23] Krasilnikov, S.S. et al. (2023) Icarus, 394, 115422. [24] Kenkmann, T. et al. (2014) J. Struct. Geol., 62. Figure 1. Maps of Kocher crater highlighting the extent of its ejecta blanket and the radar bright proximate blanket. (A) Unit boundaries superposed on a Mini-RF δCPR map [17], showing higher δCPR values within the inner ejecta blanket. (B) Unit boundaries superposed on the high-Ca pyroxene abundance map of [18]. The inner blanket shows increased high-Ca pyroxene abundance compared to the outer blanket. PSR’s mapped by [4].