A new look at lunar impact craters provides insights into ejecta and melt emplacement on the terrestrial planets

Impact cratering is one of the most fundamental processes responsible for shaping the surfaces of solid planetary bodies. Even on Earth, where the impact record is sparse, it is now apparent that impact events also have played an important role throughout Earth's history, shaping the geological landscape, affecting the evolution of life and producing economic benefits. It is widely acknowledged that the Moon provides a unique natural laboratory for studying the impact processes on planetary scales. One of the most important, but least understood aspects of impact cratering is the generation and emplacement of impact ejecta. In this contribution, we utilize new high-resolution imagery of lunar craters in addition to observations of terrestrial impact structures. Current models of ejecta emplacement do not account for several important observations of ejecta deposits on the terrestrial planets, in particular, the presence of more than one layer of ejecta. On the Moon, this addional ejecta layer takes the form of impact melt ponds on the rim terraces of complex lunar craters and overlying parts of the continuous ejecta blanket. It is also notable that there is also no universal model for the origin and emplacement of ejecta on different planetary bodies. We present a unifying working hypothesis for the origin and emplacement of ejecta on the terrestrial planets, in which the ejecta are emplaced in a multi-stage process. The generation of the continuous ejecta blanket occurs during the excavation stage of cratering, via the conventional ballistic sedimentation and radial flow model. This is followed by the emplacement of more melt-rich, ground-hugging flows - the "surface melt flow" phase - during the terminal stages of crater excavation and the modification stage of crater formation. Minor fallback occurs during the final stages of crater formation. Several factors will affect the final morphology and character of ejecta deposits. The volatile content and cohesiveness of the uppermost target rocks will significantly affect the lateral extent of the ballistically emplaced continuous ejecta blanket, with impact angle also influencing the overall geometry of the deposits. Ejecta deposited during the surface melt flow stage is influenced by several factors, most importantly planetary gravity, surface temperature, and the physical properties of the target rocks. Topography and angle of impact play important roles in determining the final distribution of surface melt flow ejecta deposits with respect to the source crater. This working hypothesis of ballistic sedimentation and surface melt flow provides a framework in which observations of ejecta at impact craters can be compared and placed in the context of the respective terrestrial planets.