3:45 PM - 4:00 PM
[PPS09-18] Azimuthal Distribution of Ejecta Thickness of the Orientale and Imbrium Basins based on Crater Size-Frequency Measurements.
Lunar impact basins, exceeding 300 km in diameter, ejected large amounts of materials upon formation, leading to extensive mixing of lunar surface materials. It's widely believed that a significant portion of the impact melt breccias returned by the Apollo missions originated from this ejecta [Haskin et al., 1998]. However, debate persists over their exact origins and associated impact basins [Hurwitz & Kring, 2016]. Establishing the thickness distribution of ejecta from individual basins is crucial for interpreting sample origins accurately, especially with upcoming lunar exploration missions. Recent advancements in high-resolution remote sensing data have allowed constraints on ejecta thickness [Fassett et al., 2008; Yue et al., 2020]. Yue et al. [2020] estimated ejecta thicknesses from the Orientale basin based on crater size-frequency measurements. In this study, we aimed to comprehensively determine ejecta thickness and distribution through crater size-frequency measurements over 500 km from the basins.
In this study, we examine the Orientale and Imbrium basins because of their well-preserved structures. Using LROC WAC images, we analyze the crater size-frequency distribution to assess the extent to which preexisting craters, predating the formation of these impact basins, were covered or obliterated by ejecta deposition. Crater counting was conducted at various locations, including the Hertzsprung basin northwest of Orientale, the Mendel-Rydberg basin to the south, and Area 1 (35oN, 265oE), a circular area with the same diameter as Hertzsprung basin, situated about 1800 km north of Orientale and closer to Imbrium. Furthermore, we divided the vicinity of the Orientale basin into eight sectors using the crater catalog [Robbins, 2008], and calculated ejecta thickness for each direction.
The figure depicts the crater size-frequency distribution in the Hertzsprung basin, revealing two inflection points: one at a 10 km diameter and another around 3 km. Craters smaller than at around 3 km exhibit a distribution similar to that of the Orientale basin, indicating that pre-existing craters smaller than 10 km were covered by the Orientale ejecta. This resulted in new craters forming on the Orientale basin's surface at a comparable frequency to pre-existing ones. Utilizing the crater rim height-diameter relationship, ejecta thickness is estimated at around 260 m, thicker than previous estimates using an empirical model for Orientale ejecta thickness [Fassett et al., 2008]. In contrast, inflections in the size-frequency distribution observed in the Mendel-Rydberg basin suggest an Orientale ejecta thickness of around 60 m, about a quarter of that in Hertzsprung. Despite Mendel-Rydberg being closer to Orientale than Hertzsprung, its thickness is significantly less, indicating an azimuthal distribution of Orientale ejecta thickness, with thicker deposits northwest of Orientale and thinner ones to the south. To assess azimuthal dependence, size-frequency distributions were analyzed within 600-1000 km using the crater catalog [Robbins, 2018]. The results indicate a twofold variation in ejecta thickness by azimuth, with thicker ejecta observed west of Orientale compared to the north-south direction. Conversely, closer to the basin, ejecta is thicker in the north-south direction according to Yue et al. [2020], suggesting a distinct azimuthal dependence. The size-frequency distributions of the areas in this study show deflections in a diameter range 2-10 km, suggesting the burial of craters larger than 2 km by the Orientale ejecta. The crater density in the diameter range 2-4 km notably increased with distance from the Orientale basin, consistent with the burial due to the ejecta deposition inferred from inflections in the size-frequency distributions. The rate of this increase varied by direction, with a smaller rate observed in the NW and W directions, where the Hertzsprung basin is located. This might be due to the deposition of the Hertzsprung ejecta.
In this study, we examine the Orientale and Imbrium basins because of their well-preserved structures. Using LROC WAC images, we analyze the crater size-frequency distribution to assess the extent to which preexisting craters, predating the formation of these impact basins, were covered or obliterated by ejecta deposition. Crater counting was conducted at various locations, including the Hertzsprung basin northwest of Orientale, the Mendel-Rydberg basin to the south, and Area 1 (35oN, 265oE), a circular area with the same diameter as Hertzsprung basin, situated about 1800 km north of Orientale and closer to Imbrium. Furthermore, we divided the vicinity of the Orientale basin into eight sectors using the crater catalog [Robbins, 2008], and calculated ejecta thickness for each direction.
The figure depicts the crater size-frequency distribution in the Hertzsprung basin, revealing two inflection points: one at a 10 km diameter and another around 3 km. Craters smaller than at around 3 km exhibit a distribution similar to that of the Orientale basin, indicating that pre-existing craters smaller than 10 km were covered by the Orientale ejecta. This resulted in new craters forming on the Orientale basin's surface at a comparable frequency to pre-existing ones. Utilizing the crater rim height-diameter relationship, ejecta thickness is estimated at around 260 m, thicker than previous estimates using an empirical model for Orientale ejecta thickness [Fassett et al., 2008]. In contrast, inflections in the size-frequency distribution observed in the Mendel-Rydberg basin suggest an Orientale ejecta thickness of around 60 m, about a quarter of that in Hertzsprung. Despite Mendel-Rydberg being closer to Orientale than Hertzsprung, its thickness is significantly less, indicating an azimuthal distribution of Orientale ejecta thickness, with thicker deposits northwest of Orientale and thinner ones to the south. To assess azimuthal dependence, size-frequency distributions were analyzed within 600-1000 km using the crater catalog [Robbins, 2018]. The results indicate a twofold variation in ejecta thickness by azimuth, with thicker ejecta observed west of Orientale compared to the north-south direction. Conversely, closer to the basin, ejecta is thicker in the north-south direction according to Yue et al. [2020], suggesting a distinct azimuthal dependence. The size-frequency distributions of the areas in this study show deflections in a diameter range 2-10 km, suggesting the burial of craters larger than 2 km by the Orientale ejecta. The crater density in the diameter range 2-4 km notably increased with distance from the Orientale basin, consistent with the burial due to the ejecta deposition inferred from inflections in the size-frequency distributions. The rate of this increase varied by direction, with a smaller rate observed in the NW and W directions, where the Hertzsprung basin is located. This might be due to the deposition of the Hertzsprung ejecta.