ORIGIN AND NATURE OF THE MOON — UREY 253 



distribution would keep the temperature in the deep interior above 

 the melting point if it was originally molten. In fact, the entire in- 

 terior out to about 0.8 of its radius would be at the melting point if 

 the initial temperature had been that at which silicates melt. Such a 

 conclusion is, however, inconsistent with the shape of the moon, and 

 it must therefore have been formed at a low temperature. 



We may ask whether the mterior of the moon would be molten at 

 the present time, owing to radioactive heating, even if it had been 

 formed at a low temperature. There are uncertainties in the answer 

 to this question. We do not know the concentrations of the radio- 

 active elements in the moon. Possibly the concentrations in meteorites 

 give the best estimate, though it is not a certain one. Calculations in- 

 dicate that any metallic iron-nickel in the moon's deep interior would 

 be molten, but we have in fact no evidence for the existence of metal- 

 lic iron within the moon. The material of the moon is not a pure 

 substance, and hence would not melt at one temperature : calculations 

 indicate that partial melting of the silicates might occur. All these 

 considerations can be reconciled only with the hypothesis that there 

 is a variation in density with latitude and longitude. This explana- 

 tion is consistent with a partially molten interior today, but incon- 

 sistent with a generally molten condition in the distant past. 



Gilbert said, "During the whole period of growth the body of the 

 moon was cold." This statement was made before the discovery of 

 radioactivity. It has required much argument to come to the same 

 conclusion again, and the problem is a critical one [3]. 



In 1862 Kelvin [4] wrote a paper dealing with the solidification of 

 the earth from a completely molten state. At that time no other 

 source of heat for volcanic processes was known other than residual 

 primitive heat, and he therefore assumed a high-temperature origin 

 for the earth. This belief has found its way into the textbooks, and 

 it is now generally assumed that all objects in the solar system were 

 once at very high temperatures. The discovery of radioactivity at 

 the turn of the century made Kelvin's assumption unnecessary, but 

 no reconsideration of the whole problem was made at that time. Pos- 

 sibly planets and satellites were fonned at high temperatures, but we 

 may now well ask what evidence exists for or against the hypothesis. 



THE IMBRIAN COLLISION 



A very great collision occurred in Mare Imbrium at some time in 

 the past : the evidence for this was discovered and described by Gil- 

 bert. The collision directly modified a large fraction of the visible 

 hemisphere of the moon. The region of Mare Imbrium is shown in 

 plate 1, which is a composite picture of the moon : it is the large gray 

 oval area at the lower right of the picture. It is also shown in plate 2, 



