A New Analysis of Oblique Impact Craters on Mars

Asteroids or comets striking a planetary surface at shallow angles typically produce elliptical-shaped craters. If they have isotropic impact trajectories, the proportion of elliptical craters produced should be proportional to the fraction striking at low angles. Based on laboratory impact experiments, where aluminum or pyrex spheres were shot at high velocities into sand (Gault and Wedekind 1978), it was estimated that less th&& 1 % of all projectiles create elliptical craters. This empirically-derived result, however, disagrees with survey results of the Martian surface, which indicate that approximately 5 % of all craters larger than 5 km have elliptical shapes (Schultz and Lutz-Garihan 1982). Thus, if these experimental results are applicable to large craters and the crater survey results are accurate, Mars, at one time, must have had a “special ” population of low-angle impactors, perhaps produced by a number of orbitally decaying moonlets similar to Phobos. To revisit this problem, we tested. each part of the hypothesis mentioned above. First, we reexamined the results of two independent Martian crater surveys. Thoug