The four largest well-preserved impact basins in the solar system, Borealis, Hellas, and Utopia on Mars, and South Pole–Aitken on the Moon, are all significantly elongated, with aspect ratios >1.2. This population stands in contrast to experimental studies of impact cratering that predict <1% of craters should be elliptical, and the observation that ~5% of the small crater population on the terrestrial planets is elliptical. Here, we develop a simple geometric model to represent elliptical crater formation and apply it to understanding the observed population of elliptical craters and basins. A projectile impacting the surface at an oblique angle leaves an elongated impact footprint. We assume that the crater expands equally in all directions from the scaled footprint until it reaches the mean diameter predicted by scaling relationships, allowing an estimate of the aspect ratio of the final crater. For projectiles that are large relative to the size of the target planet, the curvature of the planetary surface increases the elongation of the projectile footprint for even moderate impact angles, thus increasing the likelihood of elliptical basin formation. The results suggest that Hellas, Utopia, and South Pole–Aitken were formed by impacts inclined at angles less than ~45° from horizontal, with a probability of occurrence of ~0.5. For the Borealis Basin on Mars, the projectile would likely have been decapitated, with the topmost portion of the projectile on a trajectory that did not intersect with the surface of the planet.

This paper is a current status report on a project focused on understanding the formation of large impact basins on terrestrial planetary bodies. A set of preliminary two-dimensional axisymmetric numerical models of collisions of asteroids with diameters from 150 to 800 km with the Moon, Mars, and Mercury illustrates the main mechanical effects of planetary-scale impacts. The target body is modeled on a regular grid with a spatial resolution of 5–10 km. Self-gravity is included in the hydrocode. The main consequence of such an impact is a deep melt pool at the center of the basin. Model results are tentatively compared with known impact basins such as South Pole–Aitken on the Moon and Hellas on Mars.