208 ANN-UAL REPOBT SMITHSONIAN INSTITUTION, 1930 



ordinary agents of erosion and degradation are absent on the moon, 

 but there still remain two destructive forces to be reckoned with; 

 forces which should surely have obliterated or dulled the trace of 

 finer markings in the immense period of time at their disposal. The 

 first of these is differential expansion and contraction in the rocks, 

 aided by gravity, and rendered far more potent than on the earth by 

 the tremendous temperature range and the abruptness of thermal 

 changes exjjerienced on the atmosphereless moon. This force should 

 operate to cause exfoliation and crumbling of every exposed rock 

 surface, causing a slow but inevitable degradation of relief. The 

 rock screes would creep downwards through expansion and contrac- 

 tion. The absence of water and organic matter would prevent cemen- 

 tation of the talus, and movement would continue until a low angle 

 was reached. It should be noted that the smaller gravity value on the 

 moon is offset by the fact that this reduces friction, so the capacity of 

 loose material for downward motion is not thereby affected. - 



The fact that this temperature change now occurs but once in 

 every 14 days does not diminish the value of this argument. For if 

 the moon's features were formed before the earth's crust had cooled, 

 these features were then more often exposed to the temperature 

 changes, owing to the moon's then greater rapidity of rotation and 

 revolution. Subjection to this temperature range, even if only once 

 every 14 days throughout the immense period which has elapsed 

 since pre- Archeozoic times, must surely have operated to degrade all 

 the moon's features in a marked degree. 



If during a brief period the moon possessed an atmosphere, the 

 ordinary agents of erosion and denudation were then present to 

 destroy the relief rapidly. Formation of the major part of the 

 original surface features necessarily preceded the loss of the atmos- 

 phere, since as long as volcanic action occurred, gases would be 

 emitted to compensate wholly or partly for the diffusion of the 

 atmosphere into space. It is indeed surprising that evidences of this 

 period of active degradation as well as the subsequent and exceedingly 

 long period of slow degradation, are so conspicuously absent in many 

 of the moon's surface features. If, on the other hand, the original 

 surface features were not of volcanic formation, as assumed above, 

 an explanation must be sought for a phase of violent vulcanicity at 

 a later stage in its history. 



The second destructional force likely to obliterate the moon's sharp 

 relief is the continuous bombardment of meteors and meteorites to 

 which it is no doubt subjected. The moon being destitute of any 

 protective gaseous envelope, all meteorites and meteors entering its 

 gravitative field will come into contact with its surface. The impact 



