2:30 PM - 2:45 PM
[PPS08-16] Relationship between the ejecta thickness from impact basins and surface composition on the Moon
Lunar impact basins, impact structures >300 km in diameters, may have xcavated mantle and crustal material during their formation, and this ejecta extensively covered the lunar surface [e.g., Melosh, 1989]. More than 50 impact basins have been identified on the Moon [Wilhelms, 1987]. Because the formation of these impact basins results in large-scale mixing of surface materials on the lunar surface, understanding the ejecta distribution of impact basins is crucial for reconstructing the surface evolution of the Moon.
The composition of the lunar surface exhibits significant differences between the nearside and farside. For example, thorium (Th) is highly enriched around the Imbrium basin on the nearside but is largely scarce on the farside, except for moderate enrichment in the South Pole-Aitken (SPA) basin [e.g., Kobayashi et al. 2012]. Since Th is a late-stage product of lunar magma ocean solidification, its high concentration suggests that solidification concluded later the nearside. Additionally, Mg# in lunar highland crust decreases from the farside to the nearside [Ohtake et al. 2012], implying that solidification of the lunar magma ocean progressed from the farside toward the nearside.
However, because of the large-scale mixing of surface materials by impact basins, it is essential to account for the present-day compositional distribution and the influence of ejecta. Therefore, it is necessary to determine the global distribution of the thickness of impact basin ejecta. Currently, however, such measurements have only been conducted within ~1000 km of impact basins [Fassett et al., 2011; Yue et al.,2020]. In this study, we focus on the Orientale and Imbrium basins, where they have well-preserved structures to the aim of determining the global distribution of impact basin ejecta.
In this study, ejecta thickness was estimated using crater size frequency distributions (CSFD). First, crater counts were conducted in areas on crater/impact basins around the Orientale and Imbrium basins, and crater size frequency distributions were created for these areas. The resurfacing by ejecta deposition was identified from the kinks in the distributions, and the ejecta thickness was determined using the relationship between the rim height and diameter of lunar craters. Additionally, to determine the source basin of the deposited ejecta, we examined whether the CSFDs of smaller craters followed the isoclines of the Orientale and Imbrium basins.
The ejecta thickness of the Orientale basin, estimated from the kink diameters, ranged from 40 to 260 m within 2,000 km and was estimated to be less than 50 m in the 2,000–5,000 km range. It tended to decrease with increasing distance from the basin. ejecta thicknesses exceeding 50 m were observed, with values reaching up to 360 m near the antipode of the Orientale basin. Wakita et al. [2021] showed that antipodal deposition depends on impact angle. A 45-degree oblique impact, as suggested for Orientale [Melosh et al., 2018], favors ejecta transport to the antipode, supporting our results.
The moderate Th region within the SPA basin lies near the antipodes of Imbrium and Serenitatis. We compared thorium content distribution with thickness distribution of ejecta from the Imbrium and Serenitatis basins. Our analysis revealed that Imbrium ejecta exceeded 100 m in regions without a Th anomaly, while Serenitatis ejecta surpassed 200 m in Th-anomalous areas. This suggests that Th-rich ejecta from the nearside were deposited on the farside. However, since it is unlikely that Imbrium ejecta contained no Th while Serenitatis ejecta contained Th exclusively, the present-day Th anomaly on the farside is more plausibly explained by the excavation of a Th-rich layer beneath the SPA basin rather than by ejecta deposition alone. These results suggest that residual magmatic fluids were also present on the farside, indicating that the solidification did not simply progress from the farside to the nearside.
The composition of the lunar surface exhibits significant differences between the nearside and farside. For example, thorium (Th) is highly enriched around the Imbrium basin on the nearside but is largely scarce on the farside, except for moderate enrichment in the South Pole-Aitken (SPA) basin [e.g., Kobayashi et al. 2012]. Since Th is a late-stage product of lunar magma ocean solidification, its high concentration suggests that solidification concluded later the nearside. Additionally, Mg# in lunar highland crust decreases from the farside to the nearside [Ohtake et al. 2012], implying that solidification of the lunar magma ocean progressed from the farside toward the nearside.
However, because of the large-scale mixing of surface materials by impact basins, it is essential to account for the present-day compositional distribution and the influence of ejecta. Therefore, it is necessary to determine the global distribution of the thickness of impact basin ejecta. Currently, however, such measurements have only been conducted within ~1000 km of impact basins [Fassett et al., 2011; Yue et al.,2020]. In this study, we focus on the Orientale and Imbrium basins, where they have well-preserved structures to the aim of determining the global distribution of impact basin ejecta.
In this study, ejecta thickness was estimated using crater size frequency distributions (CSFD). First, crater counts were conducted in areas on crater/impact basins around the Orientale and Imbrium basins, and crater size frequency distributions were created for these areas. The resurfacing by ejecta deposition was identified from the kinks in the distributions, and the ejecta thickness was determined using the relationship between the rim height and diameter of lunar craters. Additionally, to determine the source basin of the deposited ejecta, we examined whether the CSFDs of smaller craters followed the isoclines of the Orientale and Imbrium basins.
The ejecta thickness of the Orientale basin, estimated from the kink diameters, ranged from 40 to 260 m within 2,000 km and was estimated to be less than 50 m in the 2,000–5,000 km range. It tended to decrease with increasing distance from the basin. ejecta thicknesses exceeding 50 m were observed, with values reaching up to 360 m near the antipode of the Orientale basin. Wakita et al. [2021] showed that antipodal deposition depends on impact angle. A 45-degree oblique impact, as suggested for Orientale [Melosh et al., 2018], favors ejecta transport to the antipode, supporting our results.
The moderate Th region within the SPA basin lies near the antipodes of Imbrium and Serenitatis. We compared thorium content distribution with thickness distribution of ejecta from the Imbrium and Serenitatis basins. Our analysis revealed that Imbrium ejecta exceeded 100 m in regions without a Th anomaly, while Serenitatis ejecta surpassed 200 m in Th-anomalous areas. This suggests that Th-rich ejecta from the nearside were deposited on the farside. However, since it is unlikely that Imbrium ejecta contained no Th while Serenitatis ejecta contained Th exclusively, the present-day Th anomaly on the farside is more plausibly explained by the excavation of a Th-rich layer beneath the SPA basin rather than by ejecta deposition alone. These results suggest that residual magmatic fluids were also present on the farside, indicating that the solidification did not simply progress from the farside to the nearside.