MOON AND RADIOACTIVITY — FORBES 209 



of these bodies will l)v tlieir shock, aiul also by hittiiijj: screes directly, 

 greatly accelerate the downward luotion of rock i)articles. 



Again, the moon exhibits on its surface a color-range between the 

 dark lavas of the " seas " or maria and the dazzling white ray systems 

 which surround the largest craters or vulcauoids. One can not be- 

 lieve that this gradation in coloring could have persisted since the 

 moon's formation, under an incessant hail of meteors. For if a 

 meteor's force of impact be suflicient, its constituents will be imme- 

 diately volatilized by the resultant heat.* The meteoric material 

 will thus be scattered, but will settle down uj:)on the crust. In time 

 tiie whole surface of the satellite will be covered by a homogeneous 

 layer of meteoric dust which will give the lunar features a uniform 

 hue. If on the other hantl the meteoric mass merely pits the surface 

 without being volatilized, the eil'ect will be to change the color for the 

 area of that pit. Continuous pitting of the surface will ultimately 

 reduce the whole surface to the same hue. 



Against this it may be said that a certain number of meteorites 

 will fail to reach the face of the moon visible to us, owing to the 

 earth's superior gravitative attraction, and that most meteorites will 

 fall upon the hidden face. But again it nmst be remembered that 

 the moon has not alwaj's rotated at its present speed, and that dur- 

 ing the initial stages of its history it probably exposed every part 

 of its surface equally to meteoric attack. If it ever possessed an 

 atmospheric envelope, its loss must have been an early occurrence, 

 and it can ]iot have acted as a protective mantle for long. If, on 

 the other hand, it be assumed that the loss of the atmosphere is of 

 recent date, then it is the more difficult to explain the absence of 

 marked degi'adation by weathering in numy of the surface features. 



It ma}' be argued that it is possible that the earth and moon have 

 but recently reached a part of space where meteors exist, and that 

 therefore there has not yet been time for the falling bodies to pro- 

 duce any marked effect upon the moon's surface. However, even if 

 one or other of these arguments be accepted to account for the still- 

 existing diversity of color evident upon the moon, there still remain 

 to be exi)lained (a) why our satellite shows signs of expansion rather 

 than of contraction, (b) why many of its features are so clear in 

 spite of degradation due to differential contraction and expansion 

 of its surface materials. 



Question (b) induces the idea that those features of the moon 

 which are clear and sharp are so because the}' have not loTjg been 

 exposed to the degradational forces; in other words that (hey are of 

 comparatively recent origin. Both questions seem to be satisfactorily 



*G. K. GUbcrt. Bulletin Philosophical Society of Washington. XII, 1893. p. 250. 



