Surveys of elliptical crater populations on the saturnian satellites, Mercury, and Mars

Near-horizontal planetary impacts result in elliptical craters. The percentage of elliptical craters on a planet can be used to infer the impact angle at which craters become elliptical. Previous surveys of the Moon, Mars, and Venus indicated that planetary craters become elliptical at more vertical angles than experimental impacts into a strengthless medium, and this was attributed to a higher ratio of crater diameter to projectile diameter. Here we determined the percentage of elliptical craters on the mid-sized saturnian satellites and Mercury, bodies that represent Solar-System extremes of impactor velocity, target density, and target strength. On the saturnian satellites, 7.6% of the craters have ellipticities e (ratio of major to minor axis) greater than 1.2, but only 0.4% have e > 1.5, and no craters have e > 1.75. On Mercury, 3% of the craters have e > 1.2 and 0.5% have e > 1.5. The mercurian percentages are slightly lower than the other terrestrial planets, attributable to a higher crater diameter to projectile diameter caused by the higher impact velocities at Mercury. We attribute the high percentage of moderately elliptical craters on the saturnian satellites to the rugged target terrain on those bodies. We interpret enhanced crater collapse on the icy surfaces of the saturnian satellites as preventing craters with extremely high ellipticities like the lunar crater Schiller. Finally, a reexamination of the martian crater population shows its elliptical crater population to be consistent with the other planets, and we see little evidence for a large population of craters formed by inward-spiraling moonlets.