Distribution and Chronostratigraphy of Asymmetric Secondary Craters Associated with Lunar Basins

Many asymmetric lunar craters are inferred to be secondary-impact features related to basin-scale impacts. Ranging in diameter from 10 to 35 km, they have polygonal outlines, are narrow rimmed, shallow (commonly <1.5 km), and occur tens to hundreds of kilometers beyond the outer rim of major impact basins. Some are teardrop shaped, reflecting low-angle impacts; similar morphologies for low-angle impacts have been demonstrated experimentally and are documented for many asymmetric lunar craters. Several criteria exist for differentiating asymmetric secondary craters from morphologically similar, primary craters. They include asymmetric outline; shallow floors; lack of slumps that produce asymmetry in small, complex, main-sequence craters; moderate to high levels of degradation; location beyond outer rings of impact basins; and association with scours and crater chains. Asymmetric secondaries are distinguished from morphologically similar, main-sequence craters such as Lassell and Jansen which have shallow floors owing to lava floor-flooding. Large asymmetric secondaries associated with lunar basins are unique morphological features that can be used to constrain estimated ages of overlapping landforms. However, there are clear differences in morphology and abundance of preserved secondaries for each basin, and additional work should be done to understand the role that impact dynamics (velocity and impact angle) have had on the genesis, distribution, and preservation potential of large secondaries. Additional surveys of large, asymmetric craters of possible secondary origin could be done to infer the presence of previously unrecognized impact basins, although preservation is a limiting factor for recognizing large secondary craters of Nectarian age and older.