The Lunar rayed-crater population - Characteristics of the spatial distribution and ray retention

摘要

The global statistics for young impact craters on the Moon is used to unravel potential spatial asymmetries, that may have been introduced by the particular orbital configuration of a synchronously rotating satellite. Only craters that exhibit bright ejecta rays extending for several crater radii were considered in this study. This crater population is younger than about 750. Ma. The shape of the crater size-frequency distribution does not show strong dependence on the target properties (mare vs. highlands). However, slightly lower frequencies indicate a shorter retention of the visibility of rays in mare units when their visibility is purely due to immaturity and not due to composition. Rays of small craters fade away much faster. Large, old, rayed craters sustain their visibility longer than the average crater population because of the compositional contrast between rays and mare material, and thus obscure the cratering record when investigated for spatial variations. Using the existence of rays purely based on optical maturity instead of visibility as marker horizon for the Copernican-Eratosthenian boundary, suggests a shift from 1.1. Ga to 750. Ma.The spatial distribution of lunar rayed craters, namely the latitudinal and longitudinal frequency variations, does not agree with previous analytical and numerical studies. Although there is an apparent hemispherical asymmetry centred close to the apex, the density distribution is patchy and no predicted spatial pattern could be confirmed. Spatial distribution corrections accounting for the lower frequencies in the mare areas did not result in a better agreement with the analytical estimates. Density variations are less than 15% over vast parts of the lunar surface, and the uncertainties for absolute surface ages are similar. However, variations of up to 50% are found even for the more numerous small craters. These extreme values are located at high latitudes. A combination of crater-forming projectile flux distribution and micrometeorite bombardment, which acts on maturation of the ray systems, could prove as an explanation for the contradicting observed rayed crater distribution.An analysis of the older craters is more challenging (on the Moon) because earlier geological processes complicate the setting, and the orbital configuration of the Moon-Earth-Sun-projectile system altered with time.