The Origin and Nature of the Moon' 



By Harold C. Urey 

 School of Science and Engineering, University of California, La Jolla, Calif. 



[With 5 plates] 



THE ORIGIN OF THE CRATERS 



In 1893 G. K. Gilbert, who appears to be the only student of the 

 moon's surface m the course of the last century who had any sound 

 knowledge of physical geology, published a remarkable paper on 

 "The Moon's Face" [1].^ He reviewed many features of the lunar 

 craters and concluded tliat they were due to great collisions of 

 meteoritelike objects with the moon's surface. In recent times, R. B. 

 Baldwin in his book "The Face of the Moon" [2] reviewed the 

 evidence concerning the origin of the craters, and since then it has 

 not been necessary to reconsider the problem. Baldwin gives many 

 references to the older literature and considers in detail the alternative 

 hypotheses of a volcanic or a collisional origin. The volcanic theory 

 of crater origin was advanced before modern scientists realized that 

 meteorites fell on the earth, and it required nearly a century of dis- 

 cussion before astronomers agreed that most craters resulted from 

 collisions. 



Gilbert concluded that lunar structures are not similar to those 

 of the earth; that the pattern of overlap is that to be expected for 

 chance collisions; and that the moon has no structures similar to 

 terrestrial volcanoes, if account is taken of the absence of erosion 

 there. He recognized, however, that there are small craters which 

 camiot be due to collisions and hence must be of some volcanic type, 

 even though their shape is not that of terrestrial volcanoes. 



Many questions relating to the moon are decisively answered by con- 

 sideration of its overall shape and the nature of one of its principal 

 features, the great Imbrian collision. There are many other subsidi- 

 ary lines of evidence, but we can best begin our account by discussing 

 these two important aspects. 



1 Reprinted by permission from Endeavour, vol. 19, No. 74, April 1960. Acknowledg- 

 ment Is made to the editor of Sky and Telescope for permitting the use of material pub- 

 lished by Professor Urey In that journal in 1956. 



2 Numbers in brackets refer to list of references at end of article. 



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