講演情報
[PPS02-P16]Sampling Ancient Materials in the South Polar Region of the Moon
*Thomas Frueh1、A. Camon2、M. Boyce3、S. Halwa4、G. Ligeza5、M. Lemelin2、B. J. Thomson1、T. Samaddar1、C. A. Nypaver6、D. A. Kring7,8 (1.Earth, Environmental, and Planetary Science, University of Tennessee Knoxville 、2.Département de Géomatique Appliquée, Université de Sherbrooke、3.School of Earth Sciences, University of Western Australia、4.Department of Environmental and Earth Sciences, University of Manchester、5.Department of Environmental Sciences, University of Basel、6.Smithsonian Institution, National Air and Space Museum、7.Lunar and Planetary Institute, Houston、8.Radcliffe Institute for Advanced Study, Harvard University)
キーワード：
Moon、Artemis、Sample Return、South Pole-Aitken Basin、Impact Cratering、Lunar Exploration
Introduction
The next lunar samples are likely to be returned from the south polar region (90°–84°S). There, future astronauts might collect some of the oldest rocks ever returned from the Moon. The south pole lies on the rim of the South Pole-Aitken (SPA) basin, the largest and oldest (~4.3 Ga) known impact basin on the Moon [1], expressed there as massifs rising to ~10 km above the surrounding terrain. SPA is expected to have excavated from the primordial lunar crust and the upper mantle, emplacing those materials on the paleo-surface [1]. Both sample suites will provide insights into the early Earth-Moon system.
Over the past 4.3 Ga, subsequent impacts buried the SPA ejecta blanket and primordial crust beneath thick impact debris and regolith [2]. However, some more recent impact events may have excavated through post-SPA deposits, re-exposing SPA materials or primordial crust and providing access for future sample return.
Identifying optimal sampling locations requires understanding south polar stratigraphy and the cratering processes that redistributed ejecta across this terrain. In this study series, we estimate the thickness of SPA and post-SPA ejecta, identify potential surface exposures, examine ejecta emplacement over topographically complex terrain, and propose high-interest landing regions.
South Pole Stratigraphy and Exposures of Ancient Materials
SPA’s SE to NW elongation implies an oblique impact event from either SE to NW [3] or NW to SE [4]. Oblique impacts preferentially deposit ejecta downrange [5], implying that SPA ejecta thickness at the south pole depends strongly on impact trajectory. We estimate a potential thickness range of ~100 m to ~10 km in the south polar region.
The cumulative thickness of post-SPA ejecta from >10 km craters within 90° to 84° ranges from ~190 m to ~1.8 km, with the thickest deposits toward 90°E. All craters >10 km in diameter penetrated this post-SPA ejecta sequence, indicating that the south polar regolith is likely a mixture of SPA ejecta and primordial crust.
This mixture produces a generally anorthositic–noritic to noritic–anorthositic regolith signature, with >50% anorthosite (crustal component) and 10–40% mafic material (SPA component). Localized enrichments in anorthositic at Shackleton crater and pyroxene-rich materials at Kocher crater suggest impact-excavated lithologies and represent high-priority sampling targets for crustal or SPA materials, respectively.
Impact ejecta are emplaced on ballistic trajectories across the surface [6], which is usually approximated to be flat for ejecta thickness estimates. However, the high polar massifs intercepted incoming ejecta, creating “ballistic shadows” where ejecta thickness is reduced or absent [7]. Thinning is most pronounced along slopes approaching massif summits, where SPA and crustal materials are expected to lie closest to the surface and may be further exposed by smaller, younger craters.
Conclusions and Sample Sites
Both SPA ejecta and primordial crustal material are likely present in south polar regolith, with >50% crustal and 10–40% SPA components. However, regolith is fine-grained, mixed, and shock-modified, complicating sample provenance. Meter-scale boulders of crustal material may be present within the ejecta blanket of Shackleton crater, and SPA-derived material within the blanket of Kocher crater. In addition, SPA massifs are covered by thinner post-SPA ejecta, increasing the likelihood that small craters have excavated deeper layers and creating promising sampling sites.
References
[1] Melosh, H. et al. (2017) Geology, 45, 1063-1066. [2] Frueh T. et al. (2025) JGR Planets, 130, e2025JE009050. [3] Garrick-Bethell, I. and Zuber, M. T. (2009) Icarus, 204, 399-408. [4] Andrews-Hanna et al. (2025) Nature, 646, 297-302. [5] Gault, D. E. and Wedekind, J. A. (1978) LPSC, 9th, 3843-3875. [6] Shoemaker E. M. (1962) in Physics and Astronomy of the Moon, 283-359. [7] Kring D. A. (2025) ELS, 15463398.
The next lunar samples are likely to be returned from the south polar region (90°–84°S). There, future astronauts might collect some of the oldest rocks ever returned from the Moon. The south pole lies on the rim of the South Pole-Aitken (SPA) basin, the largest and oldest (~4.3 Ga) known impact basin on the Moon [1], expressed there as massifs rising to ~10 km above the surrounding terrain. SPA is expected to have excavated from the primordial lunar crust and the upper mantle, emplacing those materials on the paleo-surface [1]. Both sample suites will provide insights into the early Earth-Moon system.
Over the past 4.3 Ga, subsequent impacts buried the SPA ejecta blanket and primordial crust beneath thick impact debris and regolith [2]. However, some more recent impact events may have excavated through post-SPA deposits, re-exposing SPA materials or primordial crust and providing access for future sample return.
Identifying optimal sampling locations requires understanding south polar stratigraphy and the cratering processes that redistributed ejecta across this terrain. In this study series, we estimate the thickness of SPA and post-SPA ejecta, identify potential surface exposures, examine ejecta emplacement over topographically complex terrain, and propose high-interest landing regions.
South Pole Stratigraphy and Exposures of Ancient Materials
SPA’s SE to NW elongation implies an oblique impact event from either SE to NW [3] or NW to SE [4]. Oblique impacts preferentially deposit ejecta downrange [5], implying that SPA ejecta thickness at the south pole depends strongly on impact trajectory. We estimate a potential thickness range of ~100 m to ~10 km in the south polar region.
The cumulative thickness of post-SPA ejecta from >10 km craters within 90° to 84° ranges from ~190 m to ~1.8 km, with the thickest deposits toward 90°E. All craters >10 km in diameter penetrated this post-SPA ejecta sequence, indicating that the south polar regolith is likely a mixture of SPA ejecta and primordial crust.
This mixture produces a generally anorthositic–noritic to noritic–anorthositic regolith signature, with >50% anorthosite (crustal component) and 10–40% mafic material (SPA component). Localized enrichments in anorthositic at Shackleton crater and pyroxene-rich materials at Kocher crater suggest impact-excavated lithologies and represent high-priority sampling targets for crustal or SPA materials, respectively.
Impact ejecta are emplaced on ballistic trajectories across the surface [6], which is usually approximated to be flat for ejecta thickness estimates. However, the high polar massifs intercepted incoming ejecta, creating “ballistic shadows” where ejecta thickness is reduced or absent [7]. Thinning is most pronounced along slopes approaching massif summits, where SPA and crustal materials are expected to lie closest to the surface and may be further exposed by smaller, younger craters.
Conclusions and Sample Sites
Both SPA ejecta and primordial crustal material are likely present in south polar regolith, with >50% crustal and 10–40% SPA components. However, regolith is fine-grained, mixed, and shock-modified, complicating sample provenance. Meter-scale boulders of crustal material may be present within the ejecta blanket of Shackleton crater, and SPA-derived material within the blanket of Kocher crater. In addition, SPA massifs are covered by thinner post-SPA ejecta, increasing the likelihood that small craters have excavated deeper layers and creating promising sampling sites.
References
[1] Melosh, H. et al. (2017) Geology, 45, 1063-1066. [2] Frueh T. et al. (2025) JGR Planets, 130, e2025JE009050. [3] Garrick-Bethell, I. and Zuber, M. T. (2009) Icarus, 204, 399-408. [4] Andrews-Hanna et al. (2025) Nature, 646, 297-302. [5] Gault, D. E. and Wedekind, J. A. (1978) LPSC, 9th, 3843-3875. [6] Shoemaker E. M. (1962) in Physics and Astronomy of the Moon, 283-359. [7] Kring D. A. (2025) ELS, 15463398.