Asteroids pose potential hazards to Earth. The recent NASA Double Asteroid Redirection Test mission successfully demonstrated the change of an asteroid's orbit by a kinetic impactor. This study focuses on impact-induced vertical momentum transfer efficiency (β − 1) considering various impact angles and subsurface boulder arrangements. Utilizing the iSALE-3D shock physics code, we simulate oblique impacts on different subsurface boulder configurations. Our results show that vertical ejecta momentum decreases with obliquity, with buried boulders inducing an anti-armoring effect. We define the direct impact-contacted boulder as the primary boulder and the surrounding boulders as secondary. The anti-armoring effect is most pronounced when the primary boulder is just below the surface, amplifying β – 1 by 50%. Impact angles between 60° and 75° exhibit a critical drop in ejecta momentum. An in-depth exploration of subsurface boulder arrangements reveals that secondary boulders have a minimal effect on vertical momentum transfer efficiency. Varying the size and separation of secondary boulders suggests that these subsurface features can either enhance or diminish the overall β − 1, providing insights into the dynamics of rubble-pile asteroids. In addition, impact melting is explored in our simulations, which suggests a minimal melt retention on Dimorphos's surface. Volumes of retained melt differ by an order of magnitude for impacts on the homogeneous regolith and on targets with buried boulders. In summary, this study provides insights into the effect of subsurface boulders and impact angles on vertical momentum transfer efficiency, which is crucial for understanding asteroid deflection by a kinetic impactor.
