Small-scale Explosion Craters, Impact Craters, and the Physical Structure of the Moon's Surface

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Abstract
A comparison of the forms of experimental explosion craters and the observed forms of lunar craters leads to the conclusion that parts of the Moon may be covered with a layer of material, having the physical properties of a gravel, to a depth of between 0.3 km and 1–3 km. Predictions are given for the variation of the ratio of the depth of a crater to its diameter, as the diameter varies, for likely models of the Moon's surface layers, and an elementary but critical type of lunar observation, which may be of great assistance in the identification of the broad physical nature of these layers at different points on the Moon, is described. Both explosion-experiments, and experiments involving non-explosives, indicate that the lunar rays were ejected from craters at relatively low angles, and that the rays probably consist of small, secondary craters and many more small, bright particles. The primary ray-craters were probably produced by near-surface meteoritic impacts in which explosive-type violence and crushing both played parts. In the small secondary craters, with velocities only ∼ 1 km sec −1 , crushing would predominate, and this hypothesis is supported by the fact that the rays extending from these small craters are unidirectional. Conditions for dome-formation are stipulated. If domes and conical craters are found to occur together on the Moon, then the domes are probably igneous phenomena rather than the surface effects of camouflets. Experimental conditions required for the formation of craters with central eminences are described, and photographs of bomb craters and explosion craters in lead are compared with the lunar craters. Both small-scale and lunar-size craters often display approximately radial patterns outside their walls, and these patterns indicate that the stresses associated with their formation were of similar type and distribution on the two different scales.

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