Asteroids or comets striking a planetary surface at very shallow angles produce elliptical-shaped craters. According to laboratory impact experiments (D. E. Gault and J. A. Wedekind 1978, Proc. Lunar Planet. Sci. Conf. 9th, 3843-3875), elliptical craters result from impact angles within โผ5 โข of horizontal and less than 1% of projectiles with isotropic impact trajectories create elliptical craters. This result disagrees with survey results which suggest that approximately 5% of all kilometer-sized craters formed on Mars, Venus, and the Moon have elliptical shapes.
To explain this discrepancy, we examined the threshold incidence angle necessary to produce elliptical craters in laboratory impact experiments. Recent experiments show that aluminum targets produce elongated craters at much steeper impact angles than sand targets. This suggests that target properties are as important as the projectile's impact angle in determining the eventual ellipticity of the crater. Creating a model which interpolates between impact data produced using sand and aluminum targets, we derive a new elliptical crater threshold angle of 12 โข from horizontal for Mars, Venus, and the Moon. This leads to a predicted proportion of elliptical craters that matches observations within uncertainty given a random projectile population. We conclude that the observed proportion of elliptical craters on these bodies is a natural by-product of projectiles striking at random angles, and that no additional formation mechanisms are needed.