First-Order Relationships between Lunar Crater Morphology, Degree of Degradation, and Relative Age: The Crater Degradation Index

William A. Ambrose
The University of Texas at Austin, Austin, TX

Although abundant radiometric age data exist for lunar maria samples, similar data are lacking for many individual craters. Age data for the majority of lunar craters are estimated from density of subsequent impacts and overlapping relationships. Statistical information from crater type (USGS main-sequence morphology) versus degree of degradation, in conjunction with crater-density and radiometric data, can also be used to provide an estimate of relative crater maturity. Simple type 1 craters are typically <15 km in diameter, whereas larger type 2 craters are complex and have slumped inner rims. Type 3 craters contain concentric terraces and central peaks. Type 4 craters are large, walled plains, type 5 craters represent small basins, and type 6 craters are multiringed basins. Crater degradation factors include direct impacts and indirect blanketing from the ejecta of younger craters, and extinct processes such as floor fracturing and lava flooding. Statistical trends from the crater degradation index (CDI), determined from a combination of these factors and albedo, suggest greater degradation with increased size and type. In a survey of 704 nearside craters and maria, type 1 craters were observed to have an average CDI value of 2.31, whereas the average CDI of type 2 craters is 3.48. The average CDI for type 3 craters is 4.11, 5.96 for type 4 craters, and 7.90 and 9.60 for type 5 and type 6 craters, respectively. These results are consistent with results from Heiken and others (1991) that suggest diminishing crater sizes and crater-density values thorough time.