PLANETARY EVOLUTION 



tory of the solar system might have been ex- 

 pected to occur, it did occur. 



Supposing this explanation to be sound in 

 principle, it is quite easy to show why such an 

 event has not occurred subsequently. The next 

 ring — the one which gave rise to Mars — 

 must have been more than twice as thick as the 

 genetic ring of the asteroids, and consequently 

 better fitted to resist a strain from without. 

 And, moreover, being 1 1 5,000,000 miles far- 

 ther removed from Jupiter, the latter planet 

 could exert upon it only four ninths of the dis- 

 turbing force which it had exerted upon the 

 asteroid-ring. Thus the Mars-ring was per- 

 mitted to develop into a planet. In turn, the 

 small size of Mars prevented him from exert- 

 ing any disastrous perturbing force upon the 

 ring which gave rise to the earth, though his 

 distance from that ring could not have exceeded 

 50,000,000 miles. A simple computation will 

 show that Mars could exert upon the earth- 

 ring not much more than one hundredth part 

 of the attraction exercised by Jupiter upon the 

 ancestral ring of the asteroids. On the other 

 hand, had the mass of Mars been one twenty- 

 fifth as great as that of Jupiter — that is, thir- 

 teen times as great as the mass of the earth — 

 he might have prevented the formation of the 

 planet on which we live. And had the mass of 

 Mars been equal to that of Jupiter, he might 



vol. 11. -^73 



