From Bureau of Economic Geology, The University of Texas at Austin (
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Bureau Seminar, February 18, 2005

Moon Crater Morphology and Styles of Degradation

William Ambrose


The surface of the Moon records a 4.5-b.y history of bombardment from solar system debris and is pockmarked with millions of craters, ranging in size from microscopic zap pits to multi-ring basins hundreds of kilometers across and several km deep. Although the Moon is our closest planetary neighbor and has been observed telescopically since the early 1600s, the impact origin of craters did not supplant theories of volcanic origin until the 1960s, mainly as a result of the pioneering work of Ralph Baldwin and Eugene Shoemaker, who described analogous impact structures on Earth and who developed the first lunar stratigraphic column based on principles of superposition and crater maturity.

Lunar crater morphology is primarily a function of size: type 1 craters, <15 km in diameter, are simple and bowl-shaped, whereas larger type 2 craters are complex and have slumped inner rims. Type 3 craters (Tycho and Copernicus, 82 and 93 km in diameter respectively), contain classic features such as concentric terraces and central peaks. Type 4 craters are large, walled plains such as Clavius (235 km across), and type 5 craters represent small basins. Type 6 craters occur at the largest possible scale and comprise gigantic multi-ringed basins.

Crater degradation factors include direct impacts and indirect blanketing from the ejecta of younger craters, floor fracturing, lava flooding (either through crater floors or externally from maria), or possible degassing events. Crater-degradation processes such as floor fracturing and lava flooding appear to be extinct on the Moon, although degradation caused by new craters continues to be active. Degradation from degassing processes is controversial and the possible effects are unknown.

This presentation introduces the term crater degradation index (CDI), determined from a combination of these factors and albedo (reflectivity). Bright, young craters such as Tycho and Copernicus typically have CDI values of 0, whereas older, degraded pre-mare craters such as Doppelmayer have CDI values of 6 or more. Crossplots of crater type versus CDI indicate increasing degradation with larger crater type. These results are consistent with the widely accepted hypothesis that most of the large debris in the solar system that produced large craters was swept up via planetary collisions before 3.5 billion years ago, and that the impact rate has diminished through time.