264 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



west of Copernicus, but others exist in the land areas and in the regions 

 near the center of the moon's disk. These cracks are probably due to 

 some general phenomenon, such as a gradual expansion of the moon 

 due to increasing temperature; the escape of gases may well be due 

 to this same circumstance. 



The big valleys have not yet been discussed. The Alpine Valley is 

 the most prominent of these and is about 130 km. long. It points in 

 the direction of the great collision in Mare Imbrium and has been 

 ascribed to the effects of an iron-nickel object plowing through the 

 surface. It has also been suggested that it is due to a crack in the 

 lunar surface. It is certainly much straighter than the other features, 

 which all agree are cracks. The Kheita and Borda Valleys in the 

 southwest regions are even longer. All are remarkably straight; if 

 they are in fact cracks plowed out by iron-nickel objects it is evident 

 that these were very large and were moving with high velocity. 



SOME PROBLEMS OF LUNAR EXPLORATION 



On all grounds, the most immediate task of lunar exploration should 

 be to determine the composition and physical character of the maria. 

 Quite simple observations on a few samples of material by methods 

 that permit the study of the crystalline structure would unequivocally 

 answer questions raised in this paper. Such observations cannot be 

 made from a distance of 384,000 km. : obtaining better photographs 

 that will resolve smaller craters will give but little additional infor- 

 mation. The color of the rocks is determined by their composition 

 and by the action of ultraviolet and particle radiation in a near-perfect 

 vacuum during the last 4.5 eons. 



Radioactivity could be measured from a space vehicle flying some 

 hundreds of kilometers above the surface. If the radioactivity is 

 similar to that of the earth, much differentiation of the limar surface 

 by melting processes must have occurred. If the concentration of 

 radioactive elements is low, we must conclude that the composition is 

 similar to that of meteorites and that little differentiation has oc- 

 curred. The former result would mean that the moon has had a very 

 high-temperature history, and the latter that it has had a low-tem- 

 perature history. 



The moon's density is less than that of the earth, even when allow- 

 ance is made for the compression in the latter due to its great interior 

 pressures. This means that the moon contains less iron than the earth 

 or more low-density substances, most probably water. If the reason 

 is more water, this would be expected to be concentrated in the surface 

 rocks as water of crystallization, and the percentages required are 

 large — probably 10 percent or more. This could be detected easily if 

 simple chemical analyses could be made. If the reason for the low 



