ORIGIN AND NATURE OF THE MOON — UREY 255 



stood. Vertical deceleration of the top of the object would occur, 

 and this part could move off horizontally at some 1.7 Ian. per second : 

 probably some would spray out sideways to produce a fanlike dis- 

 tribution. The object may have been a satellite of the earth-moon 

 system or one moving in an orbit similar to that of the earth. 



AVliatever its nature, the object most probably produced Sinus 

 Iridum, and hence passed between the two promontories of Laplace 

 and Heraclides : the distance between these is about 230 km., and this 

 is therefore the maximum diameter of the planetesimal. Gilbert sug- 

 gested 100 miles for its diameter : other suggestions have been made, 

 but on less direct grounds. Using 200 km. as a likely value, calcula- 

 tions of the object's kinetic energy can be made. Assuming a density 

 of 3.5 g./cm.^ and 2.38 km. per second (the velocity of escape from the 

 moon) for the velocity, the kinetic energy is 4.15 XlO^^ ergs. This is 

 equivalent to 4.6 X 10^^ atomic bombs, equivalent to one for each 1,100 

 square meters of the earth's surface. The largest earthquakes are 

 estimated to expend about 10^* ergs, and thus the Imbrian collision 

 dissipated an energy more than 10^ times as gi'eat. 



A collision of this magnitude is completely beyond any observa- 

 tions which we have made, and deductions made from any scaling up 

 of terrestrial o])servations would be most doubtful. It is better to 

 take this collision as an observed fact and to try to learn something 

 about large collisions from it. The object plowed in through Sinus 

 Iridum ; flattened out in the collision area ; and raised a great bulging 

 wave in the moon's surface in all directions, but particularly in the 

 forward direction, that is, toward the center of the moon's visible 

 disk. The affected area was badly broken up, perhaps even to the 

 consistency of fine sand, and after the collision, part of t\\Q material 

 subsided again, producing the shelf area between the inner and outer 

 rings shown m plate 2. 



It is possible that part of the lunar surface was lifted and then 

 dropped as big blocks, forming the Straight Range, Piton, Pico, Spitz- 

 bergen, and the other mountainous mass indicated by the arrows in 

 plate 2. The Alps, Caucasus, Apennines, and Carpathian Mountains 

 may also be formed of this kind of material; the first two, in particu- 

 lar, look like fragments of this kind. It is, of course, possible that 

 those mountains were part of the planetesimal. The Haemus Moim- 

 tains must consist of fragments of the colliding body. The long 

 grooves must have been produced by high-velocity and high-density 

 objects plowing through the surface. Because of their high density, 

 these materials must have been metallic iron-nickel, and they must 

 have been part of the colliding object in spite of their great distance 

 from the collision area. Such iron-nickel objects could hardly have 

 formed a core of the colliding planetesimal : probably they were dis- 



