Comparisons of fresh complex impact craters on Mercury and the Moon: Implications for controlling factors in impact excavation processes

The impact cratering process is usually divided into the coupling, excavation, and modification stages, where each stage is controlled by a combination of different factors. Although recognized as the main factors governing impact processes on airless bodies, the relative importance of gravity, target and projectile properties, and impact velocity in each stage is not well understood. We focus on the excavation stage to place better constraints on its controlling factors by comparing the morphology and scale of crater-exterior structures for similar-sized fresh complex craters on the Moon and Mercury. We find that the ratios of continuous ejecta deposits, continuous secondaries facies, and the largest secondary craters on the continuous secondaries facies between same-sized mercurian and lunar craters are consistent with predictions from gravity-regime crater scaling laws. Our observations support that gravity is a major controlling factor on the excavation stage of the formation of complex impact craters on the Moon and Mercury. On the other hand, similar-sized craters with identical background terrains on Mercury have different spatial densities of secondaries on the continuous secondaries facies, suggesting that impactor velocity may also be important during the excavation stage as larger impactor velocity may also cause greater ejection velocities. Moreover, some craters on Mercury have more circular and less clustered secondaries on the continuous secondaries facies than other craters on Mercury or the Moon. This morphological difference appears not to have been caused by the larger surface gravity or the larger median impact velocity on Mercury. A possible interpretation is that at some places on Mercury, the target material might have unique properties causing larger ejection angles during the impact excavation stage. We conclude that gravity is the major controlling factor on the impact excavation stage of complex craters, while impact velocity and target properties also affect the excavation stage but to a lesser extent than gravity.