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Gravity Mapping of Lunar Mantle Material in South Pole‐Aitken Basin Ejecta.

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Title: Gravity Mapping of Lunar Mantle Material in South Pole‐Aitken Basin Ejecta.
Authors: Gowman, Gabriel1 (AUTHOR) gowmang@arizona.edu, Wakita, Shigeru2 (AUTHOR), Johnson, Brandon C.2,3 (AUTHOR), Bottke, William F.4 (AUTHOR), Marchi, Simone4 (AUTHOR), Moriarty, Daniel P.5,6,7 (AUTHOR), Andrews‐Hanna, Jeffrey C.1 (AUTHOR)
Source: Journal of Geophysical Research. Planets. Apr2026, Vol. 131 Issue 4, p1-18. 18p.
Subject Terms: Gravity anomalies, Moon, Meteorite craters, Lunar surface, Impact craters
Company/Entity: United States. National Aeronautics & Space Administration
Abstract: The South Pole‐Aitken basin (SPA) is the largest and oldest impact basin on the Moon and was excavated through the crust and into the mantle early in lunar history. Here, we use gravity data to identify heterogeneities in crustal density that could indicate deeply sourced material in the crust despite a lack of surface spectral signatures. We show the presence of a ∼400‐km‐wide annulus of large‐amplitude, small‐scale gravity anomalies associated with the basin's rim region, which we interpret as the SPA ejecta blanket. We perform a pair of gravity inversions, varying either the thickness of a dense layer or the concentration of dense material in a mixed layer to match the gravity observations, and determine that the SPA ejecta blanket likely contains ≳5 million km3 of mantle‐derived material. Three craters excavate the sources of these anomalies, revealing varied compositions that imply that the lunar mantle may have had orthopyroxene‐ and clinopyroxene‐rich components at similar depths at the time of the SPA impact, with important implications for the timing of mantle overturn. Gravity anomalies also reveal a previously unnoticed 1,440‐km‐diameter peak‐ring within SPA, delineating the transition between the collapsed ejecta and the central melt sheet. The annulus of strong anomalies associated with the ejecta encompasses the south polar region, suggesting a strongly heterogeneous subsurface near several future missions' landing targets, including the upcoming Artemis missions. These results place an important constraint on basin formation, reveal the composition of the mantle, and will guide future studies in targeting this material. Plain Language Summary: The South Pole‐Aitken basin is the Moon's oldest, largest, and most deeply excavating known impact basin. During its formation, the impacting object excavated the local crust and reached the lunar mantle, spraying material across the surface from deep within the Moon. Over time, the surface material has been mixed by later impacts, concealing any sign of this distinct composition in surface measurements. Using gravity measurements, however, we can detect subsurface changes in density that suggest that the basin is surrounded by a 400‐km‐wide ring containing 30‐km‐scale deposits of deeply sourced mantle material that remain buried in the upper crust. Three impact craters lie on top of this dense material and expose that material on the surface where it could be visited and sampled by future missions. We also identify a thickened ring of crust in the basin floor that is not expected based on typical models of giant basin formation. Using these measurements, we can learn about the formation of giant basins, the composition of the Moon's mantle, and possible locations for future missions to target South Pole‐Aitken basin ejecta. These results will address key science goals of NASA's Artemis program as well as major questions about the Moon's origin and evolution. Key Points: Small‐scale gravity anomalies surround the South Pole‐Aitken basin, revealing dense bodies of mantle‐rich ejecta in the upper crustThree craters excavate these anomalies and reveal the composition of the lunar mantle at the surfaceMissions will be able to target locations where mantle‐rich material is close to the lunar surface [ABSTRACT FROM AUTHOR]
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Abstract:The South Pole‐Aitken basin (SPA) is the largest and oldest impact basin on the Moon and was excavated through the crust and into the mantle early in lunar history. Here, we use gravity data to identify heterogeneities in crustal density that could indicate deeply sourced material in the crust despite a lack of surface spectral signatures. We show the presence of a ∼400‐km‐wide annulus of large‐amplitude, small‐scale gravity anomalies associated with the basin's rim region, which we interpret as the SPA ejecta blanket. We perform a pair of gravity inversions, varying either the thickness of a dense layer or the concentration of dense material in a mixed layer to match the gravity observations, and determine that the SPA ejecta blanket likely contains ≳5 million km3 of mantle‐derived material. Three craters excavate the sources of these anomalies, revealing varied compositions that imply that the lunar mantle may have had orthopyroxene‐ and clinopyroxene‐rich components at similar depths at the time of the SPA impact, with important implications for the timing of mantle overturn. Gravity anomalies also reveal a previously unnoticed 1,440‐km‐diameter peak‐ring within SPA, delineating the transition between the collapsed ejecta and the central melt sheet. The annulus of strong anomalies associated with the ejecta encompasses the south polar region, suggesting a strongly heterogeneous subsurface near several future missions' landing targets, including the upcoming Artemis missions. These results place an important constraint on basin formation, reveal the composition of the mantle, and will guide future studies in targeting this material. Plain Language Summary: The South Pole‐Aitken basin is the Moon's oldest, largest, and most deeply excavating known impact basin. During its formation, the impacting object excavated the local crust and reached the lunar mantle, spraying material across the surface from deep within the Moon. Over time, the surface material has been mixed by later impacts, concealing any sign of this distinct composition in surface measurements. Using gravity measurements, however, we can detect subsurface changes in density that suggest that the basin is surrounded by a 400‐km‐wide ring containing 30‐km‐scale deposits of deeply sourced mantle material that remain buried in the upper crust. Three impact craters lie on top of this dense material and expose that material on the surface where it could be visited and sampled by future missions. We also identify a thickened ring of crust in the basin floor that is not expected based on typical models of giant basin formation. Using these measurements, we can learn about the formation of giant basins, the composition of the Moon's mantle, and possible locations for future missions to target South Pole‐Aitken basin ejecta. These results will address key science goals of NASA's Artemis program as well as major questions about the Moon's origin and evolution. Key Points: Small‐scale gravity anomalies surround the South Pole‐Aitken basin, revealing dense bodies of mantle‐rich ejecta in the upper crustThree craters excavate these anomalies and reveal the composition of the lunar mantle at the surfaceMissions will be able to target locations where mantle‐rich material is close to the lunar surface [ABSTRACT FROM AUTHOR]
ISSN:21699097
DOI:10.1029/2026JE009665