256 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 60 



tributed as isolated objects, mixed with silicates, throughout the mass. 

 They are reminiscent of the iron meteorites, and probably the plan- 

 etesimal was similar to objects which produced the iron meteorites- 

 Other circular maria are Crisium, Nectaris, Hmnorum, and Sereni- 

 tatis. A prominent scarp, the Altai Mountains, to the south and east 

 of Mare Nectaris, is similar to the scarp around Mare Imbrium. The 

 area between the mountains and the smooth mare is covered with great 

 craters : it seems to be analogous to the shelf area in Imbrium. The 

 Eheita and Borda Valleys and other grooves radiate from this mare, 

 and may have been produced by high-velocity missiles. Baldwin 

 believes there are radiating grooves around other maria. Partially 

 covered craters are present in Mare Serenitatis. Probably all these 

 circular maria were produced by great colliding objects which ap- 

 proached at differing angles to the surface. The highly unsymmetrical 

 character of Mare Imbrium shows that in this case the planetesimal 

 approached at a low angle. The other collision maria were probably 

 produced by objects falling more nearly vertically and hence did 

 not produce such a wide-angle fan of ridges and grooves. 



THE TIME OF OCCURRENCE OF THESE EVENTS 



Some of the craters on the moon must have been produced by 

 meteorites, though we have little evidence for really large meteorites 

 hitting the earth. Some of the large lunar craters were formed before 

 the Imbrian collision and others afterward. Ptolemaeus (pi. 4) has 

 walls that have been scarred by missiles from the Imbrian collision and 

 are very low, as though they had been partly shaken down by this 

 energetic event. Mountainous masses have fallen on other craters, as 

 for example in Julius Caesar. Other craters, such as Aristillus and 

 Autolycus lying within Mare Imbrium and Eratosthenes on the south- 

 ern edge of this mare, are more recent. Whether Plato and Archime- 

 des could have survived this great collision is doubtful. Similar rela- 

 tionships are evident in other maria. Theophilus and other craters 

 in the neighborhood are more recent than Mare Nectaris. All these 

 collisions were part of a single series of events; some craters were 

 formed, then a mare, then still more craters, and so on. These craters 

 and maria cover the entire visible surface of the moon so densely that 

 the whole of it has been broken up. One would expect that during the 

 time of this bombardment the earth would have been bombarded even 

 more intensely than the moon, because of the greater energy of the 

 collisions with its surface and because its gravitational field gives the 

 earth a larger collision cross section. Such a bombardment would 

 have destroyed all the terrestrial sedimentary rocks and should have 

 left great scars on the continental shields. The oldest terrestrial rocks 

 have been reliably dated at about 3 eons, while the meteorites are 



