Ejecta Pattern of Oblique Impacts on the Moon From Numerical Simulations

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.