Elliptical Craters and Basins on the Terrestrial Planets 1 2

The four largest well-preserved impact basins in the solar system, Borealis, Hellas 16 and Utopia on Mars, and South Pole-Aitken on the Moon, are all significantly elongated, with 17 aspect ratios >1.2. This population stands in contrast to experimental studies of impact cratering 18 that predict <1% of craters should be elliptical, and the observation that ~5% of the small crater 19 population on the terrestrial planets is elliptical. We develop a simple geometric model to 20 represent elliptical crater formation, and apply it to understanding the observed population of 21 elliptical craters and basins. A projectile impacting the surface at an oblique angle leaves an 22 elongated impact footprint. We assume that the crater expands equally in all directions from the 23 scaled footprint until it reaches the mean diameter predicted by scaling relationships, allowing an 24 estimate of the aspect ratio of the final crater. For projectiles that are large relative to the size of 25 the target planet, the curvature of the planetary surface increases the elongation of the projectile 26 footprint for even moderate impact angles, thus increasing the likelihood of elliptical basin 27 formation. The results suggest that Hellas, Utopia, and South Pole-Aitken were formed by 28 impacts inclined at angles less than ~45˚from horizontal, with a probability of occurrence of 29 ~0.5. For the Borealis basin on Mars, the projectile would likely have been decapitated, with the 30 topmost portion of the projectile on a trajectory that does not intersect with the surface of the 31 planet. 32 33 INTRODUCTION 34