ORIGIN AND NATURE OF THE MOON — UREY 257 



about 4.5 eons old or slightly older. The intense bombardment which 

 occurred on the moon must have happened more than 3 eons ago, and 

 may well be a record of the final stage of the formation of the moon, 

 the earth, and indeed of the solar system. 



COMPOSITION OF THE MARIA 



It has been commonly supposed that the maria consist of solidified 

 lava. This is a natural assumption, since the maria are level, as we 

 would expect fluids to be in the moon's gravitational field. It has been 

 assumed that the lava flowed from the moon's interior, but a well- 

 known line of reasoning shows that this is doubtful. Terrestrial 

 lavas have temperatures not over 1,200° C, which is a reasonable 

 estimate of the melting points of basaltic liquid in equilibrium with 

 the silicates beneath the earth's surface. This liquid cannot be much 

 above its melting point, because if it was, it would melt the containing 

 walls and hence cool until equilibrium was again reached. If such a 

 liquid flows onto a cold surface the lava begins to freeze, and it is 

 difficult to understand how it could flow very far without producing 

 fairly high terminal walls. It should be noted that proponents of the 

 ideas that the maria are of laval origin have postulated that the liquid 

 flowed very great distances. The Oregon and Deccan plateau lava 

 flows each cover about 500,000 km.^, which is smaller than Mare Im- 

 brium. If the lunar features are indeed lava flows, they are compa- 

 rable to, or even larger than, those on earth. It seems probable that 

 some terminal walls would be preserved if the great smooth areas were 

 m fact of this type [5]. Also, had the moon's surface been underlain 

 by these vast quantities of high-temperature lavas, the great momi- 

 tains would surely have gradually sunk into the moon's surface. On 

 the basis of such arguments Gilbert concluded that the moon was cold 

 when its maria were formed, and we can conclude that it has been cold 

 in its outer parts ever since. There is no evidence that the mountains 

 have settled into the moon's surface, and we can deduce from this that 

 the lavas (if they are lavas) did not come from the moon's interior. 



Gilbert believed that the melting was caused by the collision energy 

 of the objects arriving on the moon. The collision energy per gram 

 of an object arriving with the escape velocity of the moon (2.38 km. 

 per second) is 2,800 joules, and about 2,000 joules per gram are re- 

 quired to heat silicates to the melting point and then to melt them. 

 Some energy would be dissipated as vibration, but some melting might 

 occur. If the velocities were very high, volatilization would occur and 

 something like a great explosion would take place. This is incon- 

 sistent with the unsymmetrical pattern of ridges as mentioned before, 

 but nevertheless a velocity somewhat higher than the escape velocity is 

 possible. 



