Secondary craters formed by the impact of ejecta missiles generally occur outside the lip crest of explosion-induced shock craters. Numerous shallow depressions occurring in the region adjacent to the lunar crater Copernicus have also been interpreted as secondary craters. Missiles which produce secondary craters are:1. Comminuted and shock compacted material.2. Single rock or soil fragments.3. Fragments of structural material.4. Unconsolidated but discrete masses of crater ejecta.Superposition of ejecta layers, exposed of excavation of the large secondary craters near Sedan, indicates that unwinnowed material arrives first followed by the large masses of unconsolidated alluvium. The impact of these masses forms secondary craters that are essentially compressional features. The large masses are followed winnowed ejecta separated by passage through the atmosphere into a coarse fraction deposited as individual grains and a fine fraction that is deposited relatively slowly as long as the dust cloud remains in the area.The secondary craters observed on Earth have all been formed by missile impact ve velocities lower than the acoustic velocity of the medium; however, on the Moon, primary as well as secondary impact craters can be produced at both subsonic and hypersonic velocities. This indicates that the use of shock crater frequency to infer flux of larger meteorites may not be valid.
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