Lunar Oddities: Asymmetric, Non-Main-Sequence Craters as Indirect Indicators of Basin-Scale Impacts

William A. Ambrose
Bureau of Economic Geology, Austin, TX

Asymmetric, non-main-sequence lunar craters are inferred to be secondary-impact features caused by major, basin-scale impacts. As with genetically related, radial crater chains, they are useful in estimating the age of features that they overlap. Non-main-sequence lunar craters do not exist within the crater size-morphology continuum. Ranging in diameter from 18 to 35 km, they are polygonal, narrow rimmed, shallow (commonly <1.5 km deep), and occur many tens to hundreds of kilometers beyond the outer rim of major impact basins. Non-main-sequence craters are significant because those with asymmetry can indicate the direction of major impact basins. Many are teardrop shaped, reflecting low-angle impacts. Similar morphologies for low-angle impactors have been demonstrated experimentally by Gault and others (1968). In a study of predominantly nearside lunar craters, criteria were established for differentiating these craters from morphologically similar craters of different or recent origin. Examples of morphologically similar craters of different origin include small, main-sequence craters with slumps and flooded floors. Moreover, some asymmetric craters such as Messier and Messier A are too recent to have been associated with ancient impacts because they are well preserved. Alphonsus B, a teardrop shaped crater possibly associated with the Mare Nectaris impact event, is an example of an asymmetric, non-main-sequence crater used in estimating the age of a lunar feature (Clark, 2006). It overlaps ejecta from Alphonsus, indicating that Alphonsus may be Pre-Nectarian, older than previously interpreted. Many other examples occur on the Moon and are also present throughout the solar system.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas