VARIATIONS IN LUNAR COMPLEX CRATER MORPHOLOGY

Impact craters survive as a major landform on many planetary bodies. Cratered terrain makes up more than 80% of the lunar surface. Craters exhibit progressive morphological changes with increasing diameters. At the smallest sizes, craters are simple, bowl-shaped excavations; at larger sizes, craters develop a complex morphology. Depth-to-diameter trends exhibit a major break (14-18 km diam.) as craters transition from bowl-shaped to flat-floored craters. Slightly different crater transitional diameters are found for mare and non-mare craters.

Complex crater morphology results from instability and slumping of the crater walls. Flattening of crater floors is attributed to the limits of vertical excavation; pooling of impact melt; and by rebound of the crater floor. The apparent diameter of excavation is enlarged by slumping of the crater walls by regolith flow and coherent slump blocks creating terraced the walls. These modifications result in a shallower slope in the depth/diameter scatter plots compared to that of simple craters.

To reconstruct the original crater diameter, measurements are taken across the crater for the inner-most terrace. A correction is made for the inward, horizontal displacement of the scarp face by movement along the dipping slip-surface. Data is compiled for 56 select, eumorphic craters (Copernican and Erastosthenian ages) on the earth-facing side of the moon. Scatter plots for mare craters and terra craters exhibit slightly different trends which might be caused by different rock properties of the target materials.