The processes by which craters disappear from the lunar surface have been of principal concern since the first high-resolution pictures of the lunar maria were returned by Ranger VII. Those pictures revealed that craters smaller than a few hundred meters on the lunar maria vary morphologically from sharp and pristine features to shallow, highly subdued depressions. The constancy of the population density of these smaller craters (with diameters of less than 1000 m) was predicted by Moore (ref. 25-40) and Shoemaker (ref. 25-41), who concluded that these craters were in a steady-state condition.
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Orbital-science investigation: Part P: the process of crater removal in the lunar maria