Ejecta Pattern of Oblique Impacts on the Moon From Numerical Simulations. Issue 11 (1st November 2022)

• Record Type: Journal Article
• Authors: Luo, Xi‐Zi; Zhu, Meng‐Hua; Ding, Min
• Is Part Of: Journal of geophysical research. Volume 127:Issue 11(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-11-01
• Subjects: oblique impact -- impact ejecta -- Moon -- ejecta pattern -- ejecta blanket -- shock physics simulation; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2022JE007333
• Languages: English
• ISSNs: 2169-9097

Abstract

Abstract: The commonly adopted ejecta deposit models always assume vertical impacts for simplification. However, most impacts on planetary surfaces are oblique, which produce significantly different ejecta patterns from vertical impacts. Although the asymmetric ejecta patterns from oblique impacts have been studied in laboratory experiments, they cannot be directly applied to large craters on the planetary scale. Here, we use the three‐dimensional shock physics code iSALE‐3D to systematically simulate impact excavation processes under Moon's gravity with varied impact angles (10°–90° with respect to the horizontal), impactor diameters (1–120 km), and impact velocities (10–20 km/s), which produce craters or basins ranging from ∼5 to ∼1, 000 km. For each model, we record ejecta launch velocities and angles and calculate their integrated patterns after ballistic sedimentation. Our model results show that as the impact angle departs from the vertical, launch velocities of the uprange ejecta decrease, forming a V‐shaped zone devoid of ejecta. For even lower impact angles, the downrange ejecta thickness significantly decreases due to reduced launch angles and smaller deposit distances, forming a butterfly pattern with most ejecta concentrating in crossrange. In addition, our model results indicate that large impactors tend to produce more asymmetric ejecta patterns and the ejecta patterns are insensitive to typical impact velocities on the Moon. Our modeling results provide a semi‐quantitative relation between the ejecta pattern and impact parameters for oblique impacts on the Moon, which can be used to constrain the possible impact parameters for lunar crater or basins according to the observed ejecta pattern. Plain Language Summary: Ejecta are materials expelled out of the crater cavity and deposit around the crater that form a layer of ejecta blanket. Asymmetric distribution of impact ejecta usually indicates oblique impact and can be used to infer the impact parameters. In this work, we use three‐dimensional shock physics codes to model the impact cratering and ejecta deposition processes, and systematically investigate how the impact angle, impact velocity, and impactor size influence the crater ejecta patterns on the Moon. Our results show as the impact becomes increasingly oblique, the ejecta pattern turns more asymmetric until a zone devoid of ejecta appears in the uprange of impact direction. The asymmetric pattern simultaneously becomes elongated in the crossrange, finally forming a highly asymmetric butterfly pattern. The impact velocity has less effect on the ejecta pattern, however, the impactor size greatly influences the asymmetric ejecta pattern: a large impactor produces a more asymmetric ejecta pattern at the same impact angle, which was not seriously considered before. This size dependence can help more precisely constrain the impact angles for large impacts (e.g., Orientale basin) on the Moon and also indicates that the ejecta pattern derived from laboratory experiments cannot be directly used for large impact basins. Key Points: We use three‐dimensional shock physics code to study ejecta pattern variations due to oblique impacts Oblique impacts with decreasing impact angles produce uprange zones devoid of ejecta and then butterfly patterns Transition to asymmetric ejecta patterns is highly dependent on the impact angle and impactor size, but less sensitive to impact velocity