The size-frequency distribution of elliptical impact craters

Impact craters are elliptical in planform if the impactor's trajectory is below a threshold angle of incidence. Laboratory experiments and 3D numerical simulations demonstrate that this threshold angle decreases as the ratio of crater size to impactor size increases. According to impact cratering scaling laws, this implies that elliptical craters occur at steeper impact angles as crater size or target strength increases. Using a standard size-frequency distribution for asteroids impacting the terrestrial planets we estimate the fraction of elliptical craters as a function of crater size on the Moon, Mars, Earth, Venus and Mercury. In general, the expected fraction of elliptical craters is ~ 2–4% for craters between 5 and 100-km in diameter, consistent with the observed population of elliptical craters on Mars. At larger crater sizes both our model and observations suggest a dramatic increase in the fraction of elliptical craters with increasing crater diameter. The observed fraction of elliptical craters larger than 100-km diameter is significantly greater than our model predictions, which may suggest that there is an additional source of large elliptical craters other than oblique impact.

► Impact craters are elliptical if the impactor's trajectory is below a threshold angle.
► The threshold angle decreases as the ratio of crater size to impactor size increases.
► Elliptical craters form at steeper angles as crater size or target strength increases.
► 2–4% of craters < 100 km in diameter on the terrestrial planets should be elliptical.
► Large basins are more likely to be elliptical than smaller craters.