174 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 5 



recent craters are more sharply outlined and are lighter in color, as 

 a general rule. Not far above Tycho in plate 4 i^ located Clavius, 

 a magnificent crater 142 miles in diameter, 17,000 feet deep, and 

 containing smaller craters, one of which is larger than any terres- 

 trial crater. In this figure also a fault scarp is shown in the mare 

 below Tycho which is called the " Straight Wall "; it is 70 miles long 

 with a downthrow of nearly 1,000 feet on the east. 



Study of the mountainous areas in the photographs and on other 

 parts of the moon shows that they are unlike terrestrial mountains 

 and are for the geologist and the astronomer exceedingly difficult 

 to interpret. The heights of the mountains reach 25,000 feet in 

 isolated cases; the deepest crater has a depth of 24,000 feet. The 

 lunar mountains are extremely rough and would be difficult to trav- 

 erse, even if there were water and air present to support life. This 

 is not the place to discuss the many hypotheses which have been 

 suggested to account for the mode of formation of the different 

 types of lunar surface features. Suffice it to state that no single 

 hypothesis has been adequately proved so that it can be accepted 

 without reservations. Each hypothesis contains certain elements of 

 truth. With reference to the volcanic theory of the origin of the 

 craters, the observed intimate relationship between lunar crustal 

 structure and the occurrence of craters indicates that some of the 

 craters, at least, are due to volcanic action. On the other hand, the 

 translational energy of a meteor impmging unimpeded on the moon 

 with a velocity of 20 to 40 kilometers a second and penetrating into 

 the surface for some distance is able not only to produce the crater 

 form, but also, on transformation of the residual kinetic energy into 

 heat, to melt and even to volatilize the country rock and thus set up 

 actions which in their effects would closely resemble volcanic phe- 

 nomena. In this connection the low lunar gravity is an important 

 factor. 



From a geological standpoint the absence of water and air on the 

 moon together with its low gravity are factors favorable to the 

 development and maintenance of extremes in surface forms. One of 

 the results of low gravity and the lack of air resistance is the greatly 

 increased length, twenty-five to fifty fold, of trajectories of materials 

 thrown out of lunar craters as compared with the trajectories of 

 materials ejected at the same initial velocity and angle of elevation 

 on the earth. For a muzzle velocity of 1,600 meters (5,250 feet) per 

 second, equal to that of the Big Bertha gun which the Germans used 

 against Paris during the World War, the terrestrial range for an 

 elevation angle of 50° is 75 miles; on the moon the maximum range 

 for this initial velocity is 2,200 miles, or more than one-quarter of 

 the distance around the moon. The rays from Tycho have been 



