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Interpretation of the diameter-frequency relation for lunar craters photographed by Rangers VII, VIII, and IX

1968; Elsevier BV; Volume: 8; Issue: 1-3 Linguagem: Inglês

10.1016/0019-1035(68)90058-4

1090-2643

C. R. Chapman,

Maritime and Coastal Archaeology

Abstract About 3320 craters from 22 frames of Rangers VII, VIII, and IX were measured, counted, and classified as to morphological appearance (Classes 1 to 4 vary from hard, sharp craters to soft, shallow ones). The constants A and B of incremental diameter-frequency relations of the form N = AD B ( D , the diameter in kilometers; N , the number of craters per square km per km diameter increment) were determined by least-squares methods. The relation for all craters is the sum of several different processes of cratering; these components can be identified by considering the craters separately by class. The observed distribution of Class 1 craters can be explained by meteoroid impacts plus background secondary craters of those primaries, although further data on the characteristics of secondary crater families are needed to confirm the hypothesis. Such secondaries may be responsible for an observed stee section (slope −4.5) of the Class 1 distribution between diameters 0.015 and 0.06 km. On the basis of this steep slope, it is shown that very small craters peppering the lunar surface effectively modify the appearance of Class 1 craters smaller than about 0.015 km to Class 2. Such an overlapping mechanism is discussed which seems capable of explaining the observed incremental diameter- frequency relations for all classes of craters smaller than 0.01 km diameter. The remaining craters of Classes 2, 3, and 4 between 0.02 and 2.0 km diameter are shown to consist, probably, of two types of endogenic craters: (1) Those between 0.25 and 2.0 km cannot be of extralunar origin because of their differing areal densities (of an order of magnitude) between Alphonsus and the general maria. Similarities in their frequency relation to that of volcanic chain craters suggest that they are related in nature; (2) the Class 3 and 4 craters in the 0.01 to 0.1 km range are a distinct group of features, perhaps mostly of the “sink-hole” type described by others. It is shown that erosional processes do not affect the appearances of craters much larger than 0.01 km diameter; the lunar surface is saturated with craters less than 0.001 km diameter in a manner probably consistent with Surveyor photographs, and the surface is churned to a depth of some tens of centimeters.