on Interpreting the Elliptical Crater

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 »5 – of horizontal and less than 1 % of projectiles with isotropic impact trajectories create elliptical craters. This result disagrees with survey results which suggest that approx-imately 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 pro-duce 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 – from horizontal for Mars, Venus, and the Moon. This leads to a predicted proportion of ellip-tical craters that matches observations within uncertainty given a random projectile population. We conclude that the observed pro-portion of elliptical craters on these bodies is a natural by-product of projectiles striking at random angles, and that no additional for-mation mechanisms are needed. c ° 2000 Academic Pres