THE MOON AND RADIOACTIVITY ' 



By V. S. FoRims 

 Chriat't College, Cambridge 



In a study of the surface features of our satellite one is immedi- 

 ately impressed by the remarkable sharpness and freshness which 

 many of those features exhibit. Further, one is struck by the fact 

 that evidences of compressive action are in the minority, whereas 

 the moon's crust shows clearly innumerable clefts, rifts, and frac- 

 tures pointing to an extensional tendency beneath its outer envelope. 

 This is hardly in accordance wilh our ideas of the appearance of a 

 sphere which has lost all its heat; everywhere it should show signs 

 of intense contraction, and arcuate mountain ranges should be more 

 frequent than upon the face of the earth. 



It is now generally accepted that the moon must have cooled to 

 something like its present condition long before the earth came to 

 the state in which its igneously fluid mass was crusted over. Ac- 

 cording to Dr. R. H. Rastall ^ the moon is very probably the missing 

 two-thirds of the earth's sialsphere, which in the liquid state was 

 stripjjed off by some form of tidal resonance. A certain amount 

 of sima was stripped off at the same time, giving the moon its 

 density of 3.46. (Density of sial, about 2.7.) Support is lent to 

 this view by the recent investigations made by Dr. F. E. Wright ' 

 on the optical course of the moon's radiation. He states that " re- 

 sults clearly indicate that at the surface of the moon, no dark rocks, 

 low in silica, nor iron, nor obsidians are exposed in appreciable 

 amounts; but light-colored rocks high in silica and powders of trans- 

 parent substances are possible." In other words, the moon's surface 

 is predominantly of sial. 



Now it seems highly improbable that the many well-preserved 

 surface features of the moon could have been formed at that remote 

 period before the earth had obtained a solid crust. It is true that the 



* Reprinted by permlasion, with author's aUerattons and additions from the Geologi- 

 cal llaRazlne. vol. 6t!. No. 770. February, 1920. 



'The Continents and the Orl^n of the Moon. Nature, May 2. 1926. 



• Carne;,'lc Institution of Washington, War Boolt. No. 28, 192(^-27. p. 883. 



207 



