ORIGIN AND NATURE OF THE MOON — UREY 261 



in these meteorites. Using three radioactive dating methods — based 

 respectively on the rate of change of uranium to lead, of rubidium 87 

 to strontium 87, and of potassium 40 to argon 40 — we find that the 

 time that has elapsed since they were last heated to liigh temperatures 

 is about 4.5 eons. It is probable that the moon, the earth, and in fact 

 the entire solar system were formed at that time. Any theory of these 

 events must explain the curious physical structures and chemical 

 composition of these meteorites. 



It is most curious that the cosmic-ray ages of stone meteorites are 

 only some tens of millions of years, and no certain explanation of this 

 observation is available. However, objects moving m the neighbor- 

 hood of the earth's orbit would be expected to exist in free space for 

 about this length of time before they collide with the earth. Thus 

 for most of geologic time they must have been covered with a layer of 

 screening material, about a meter in thickness, which prevented the 

 cosmic rays from producing special varieties of atoms, in this case 

 the inert gases helium 3, neon 21, and argon 38. They were 

 broken out of this environment as objects so small that cosmic rays 

 could penetrate them. This requirement demands that their diameters 

 should be in the range 30 to 100 cm. Thereafter they traveled in 

 interplanetary space for some tens of millions of years. Could they 

 have been blasted off the moon by the heads of comets? This is a 

 possibility, and if it were so we would know of what some parts of 

 the lunar surface are composed. However, only the transport of some 

 pieces of the moon's surface to the earth can decide whether this sup- 

 position is correct. 



We may conclude that the moon's surface was fashioned mostly by 

 great collisions with its surface some 4.5 eons ago during a relatively 

 shoi-t period of time — probably less than a million years. Since then 

 it has been bombarded by lesser objects which have produced mostly 

 smaller craters. 



THE OTHER HEMISPHERE OF THE MOON 



The great space vehicles of the Soviet Union have given us a 

 glimpse of the previously unknown hemisphere of the moon: this 

 feat must be rated as the greatest exploration since the discovery of 

 America by Christopher Columbus. This first observation (pi. 5) 

 gives us only a vague idea of what the "other side" is like and what 

 we may learn from it. It has fewer maria than the visible hemi- 

 sphere; this is unexpected but not surprising, because the Imbrian 

 collision has supplied so many gray smooth areas to this side, that 

 is to say its own area and probably much of Oceanus Procellarmn, 

 Mare Nubium, and other neighboring areas. If by chance the plan- 

 etesimal had fallen on the other side of the moon the general appear- 



