MEASUREMENT AND ANALYSIS OF LUNAR BASIN DEPTHS FROM CLEMENTINE ALTIMETRY

Altimetric profiles from the Clementine LIDAR are used to calculate th e depths of 29 large craters and basins on the Moon. Plotting the dept hs of the best preserved structures together with values for simple an d complex craters measured in pre-Clementine studies reveals an inflec tion in the depth/diameter (d/D) curve in addition to the one revealed by pre-Clementine data. This inflection occurs in the diameter range that corresponds to the morphologic transition from complex crater to basin. The best empirical power law fit for basin depths is log(10)(d) = 0.41 x [log(10)(D)](0.57). This relationship is characterized by a lower slope than that for complex craters, demonstrating that this mor phologic transition corresponds to a further decrease in the depth of an impact structure relative to its diameter with increasing size. Qua litative consideration of possible causes for the second inflection le ads to the conclusion that it is most likely a consequence of a short- term modification mechanism that influences fundamental crater morphol ogy, such as the increasing influence of gravity with diameter. Thickn esses of maria in the major basins are calculated by assuming that the ir unfilled depths would follow the d/D relation. Results are compared with previous estimates and yield thicknesses that are generally grea ter than those determined by studies of flooded craters and less than those obtained from analysis of gravity. (C) 1998 Academic Press.