262 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



ance of the two hemispheres would have been largely reversed. Cer- 

 tainly removing Maria Imbrimn and Serenitatis would produce this 

 general effect. The number of craters on this side is so great that it 

 would not be possible by any stretch of the imagmation to suppose 

 that the great "land" areas of the other side would not be covered 

 by similar craters. Though we might suppose that an equivalent 

 of Mare Imbrium would be fomid on the far hemisphere, it would 

 not be surprising if this did not occur. 



On the basis of pictures so far made available to the West by the 

 scientists of the Soviet Union we are not able to draw any very im- 

 portant conclusions about the structure and history of the moon. The 

 maria on the limb of the moon extend to the other side, and Mare 

 Moskva (Moscow Sea) occupies the approxmiate middle of the fur- 

 ther hemisphere. Again there is an indication that it has a neighbor- 

 ing smooth area which might be due to a flow of lava from the area 

 of a collision or to the fall of dust and rubble produced by such a 

 collision. It is quite evident that general erosional effects from sun- 

 light and particle radiation from the sun have not been veiy eff'ective 

 in shaping the moon's features. 



OTHER FEATURES OF THE LUNAR SURFACE 



Over the years, many features of the lunar surface have been re- 

 corded in considerable detail by many observers, and some new de- 

 tails have been added by recent observations. The rays fiom some 

 of the craters cross all other features, both the mountainous and de- 

 pressed areas. They are due undoubtedly to particles thrown out 

 over the surface as a result of the explosive collisions wliich produced 

 the craters. In the liigh vacuum existing on the moon's surface, 

 each dust particle would travel in an elliptical path, and if the par- 

 ticles from a crater moved in similar but not identical paths, tliey 

 would fall in a pattern which might or might not be that of the ob- 

 served rays. If such particles were moving with a velocity of 1.7 km. 

 per second or greater, and if the initial direction were parallel to tlie 

 Imiar surface, they could travel completely round the moon and ar- 

 rive back at their starting point in somewhat more than 108 minutes. 

 The moon m this time would have rotated to the west, and the parti- 

 cles would, therefore, miss the original crater and fall on its eastern 

 run, if the orbit were in a north-south direction. Two rays of Tycho 

 do just this. It has recently been suggested that, if particles were 

 thrown vertically to a great height and fell back to the surface in 

 about the same length of time, the same displacement would be ob- 

 served. The rays of Copernicus are veiy irregular, and various ex- 

 planations have been given of this. Possibly a slight atmosphere was 

 present at the time when they were formed. Many well-formed 



