226 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1954 



ray intensity in our part of the Millcy Way (our galaxy) for a period 

 not exceeding about 100 million years as indicated above, then only 

 the newer craters — in general the smaller ones — could have been pro- 

 duced by this material. It is suggested that the larger lunar craters, 

 which, incidentally, were made during the period when the moon was 

 still partly or mostly liquid — that is, during the early history of its 

 formation nearly 4,000 million years ago — must have been made by 

 the impact of large objects from some other source. A possible and 

 logical source of such objects could have been a family of small moons 

 once circulating around our earth. Such a hypothesis seems reasonable 

 from a number of points of view. Such small moons were not only 

 possible, but many were actually formed in the condensation of the 

 cosmic dust within our solar system. For example, one of the moons 

 of Mars is barely 5 miles in diameter. Other moons are quite small. 

 We do not know how many small ones may be circling some of the 

 outer planets. There is ample reason to think Jupiter may have lost 

 a number of her moons as indicated above, because capture by the sun, 

 or collisions, would be more probable in the case of the nonretrograde 

 traveling ones. On the other hand, if there should be capture of 

 moons, by Jupiter, from the material scattered in a planetary collision 

 in the region between Mars and Jupiter, a retrograde direction of revo- 

 lution would be expected to be the more probable. In the case of the 

 earth's possible original system of moons, the capture of the smaller 

 ones by our present satellite would account nicely for its giant size 

 relative to that of the earth as compared with the sizes of other moons 

 relative to their parent planets. Furthermore, little evidence exists to 

 indicate that the earth suffered the extensive bombardment with large 

 objects as was the case with the moon, at least during the last 100 

 million years since the time the meteoric planet ended up in some 

 sort of major catastrophe. However, since at that time the moon 

 was apparently in a semiliquid state, the earth was probably still in- 

 tensely hot so that any collisions with it would simply result in the 

 addition of material which would be integrated throughout the 

 earth's structure. Hence, the old and very large craters on the moon 

 (the seas) might indicate the accumulation of a vast number of plane- 

 toids (Urey, 1951a, b) by the earth in its early history. If such were 

 the case (and it is highly possible and even probable), and since these 

 could not have come from the lost planet as indicated above, their 

 source must have been from an early group of captured moons (by 

 the sun) from Jupiter and the other planets, or else from a multitude 

 of small planetoids once a part of our solar system and which might 

 have been distributed with all sorts of orbital eccentricities more or 

 less throughout its limits. 



