260 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



ized" by gas at the time the bombardment ended. A similar argu- 

 ment holds for the absence of moimtainous masses in Mare Serenitatis. 

 Had this mare been formed after Mare Imbrium, one would expect 

 to find the scars of this collision scattered over the radiating ridges of 

 the Haemus Mountains, to the south of the mare, which were obvi- 

 ously produced by the Imbrian collision. Further, the mountains in 

 the region between the two maria are very well preserved, and have a 

 radiating pattern showing that they are part of the Imbrian system 

 (pi. 3). For these reasons I conclude that Mare Serenitatis is older 

 than Mare Imbrium. But in this case the absence of a mountainous 

 mass, similar to the Haemus Mountains, on Mare Serenitatis, shows 

 that this mare was fluid at the time of the Imbrian collision. Lava 

 would very quickly solidify to form a very rigid mass. Dust or sandy 

 material can become rigid also, though it might not for some time have 

 sufficient strength to support a massive mountain. The small craters 

 in the maria are due to collisions that took place during the 4.5 eons 

 that have elapsed since the occurrence of the very great processes 

 which produced the maria. 



These arguments indicate that the major surface features of the 

 moon were fashioned in a very short time. Calculations based on the 

 rate of cooling of solidified rocky materials show that only some tens 

 of thousands of years are required for such pools of lava to cool : if 

 some low-density froth or pumice floated on the surface the time could 

 be considerably longer. (In this connection it must be remembered 

 that the solidification of most liquids does not follow the same course 

 as that of water, whose solid phase floats on the liquid : if a magic 

 wand were to make water behave like all other common substances, 

 the ice of the Arctic Ocean would sink and the ocean would freeze solid 

 from bottom to top.) The consolidation of dust would require still 

 longer. We have little experience on which to base a guess, but pos- 

 sibly much less than a million years would suffice — a surprisingly short 

 time. It is possible that the objects which collided with the moon to 

 produce its gross features were satellites of the earth-moon system. 

 During some thousands of years, such objects would collide with the 

 earth or moon, but objects traveling in orbits about the sun, when in 

 the neighborhood of the earth would collide with the latter within 

 some tens of millions of years, according to calculations of E. J. Opik 

 [7]. Objects moving in orbits about the earth should arrive at its 

 surface with about the escape velocity of the moon. 



The stone meteorites are immensely complicated structures, con- 

 sisting of agglomerated sandy materials whose crystalline minerals 

 were certainly fonned in a body different from those in which they 

 are now found. Some collisional process such as occurred on the 

 moon may have produced the highly fragmented crystals that we find 



