Thickness of impact crater ejecta on the lunar surface

Abstract

Calculations based on improved models for impact cratering indicate an average thickness (if spread uniformly) of ejecta from craters and basins between 2 and 500km diameters on the visible face of the Moon that ranges from 0.74 to 8.00km (best estimate values between 1. 36-2.39km), depending on combinations of critical parameters utilized.These parameters include: (1) initial effective diameters of craters of excavation, (2) depth/diameter ratios between 0. 05 and 0.35, (3) fractional enlargement of diameters by slumping (from zero to 50%), (4) efficiency of ejection, (5) frequency distribution of craters in the size range used, and (6) appropriate selection of circular structures as impact-generated.Chief uncertainty is the identification of those large basins definitely caused by impact; where mare-filled, the proper choice of diameter becomes critical.Contributions from mascon-related basins versus all roughly circular basins are treated separately.The general thinness of rubble cover ('-1-20 meters) on some mare surfaces implies that most major craters were formed early in lunar history.An anorthositic lunar highlands (indicated by Apollo 11 results) should be covered to varying depths with ejecta derived largely from impact iii basins cut into a pre-mare crust (anorthosite ?) that was continuous around the lunar sphere.Ejecta from earlier (now covered or destroyed) craters plus unknown amounts of volcanic ash would add to the average thicknesses that can be calculated from observable impact craters.