FUNDAMENTAL PROBLEMS OF GEOLOGY. 221 



1 . There is a certain necessary limitation to the size of tenuous 

 bodies in the presence of more massive bodies. The principle in- 

 volved is one of vital importance in the study of planetary evolu- 

 tion. Within the field of the effective attraction of a dominant body 

 like the sun, or the ancestral nebular center, small bodies exercise 

 differential gravitative control over a limited sphere only, known 

 technically as the " sphere of activity." This sphere for the earth, 

 with its present mass, reaches out about 620,000 miles.* If the 

 earth has grown at all its primitive sphere of control must have been 

 smaller than this. The earth nucleus could, therefore, only have 

 embraced such matter as lay within this limited sphere. If the 

 original knot could be supposed to have extended beyond this limit 

 the outljang portion would have been drawn away by the solar mass 

 into independent planetesimals, and must have been gathered in, if it 

 became a part of the earth at all, by some other means than direct 

 attraction. The moon controls, as against the attraction of the 

 earth at its present distance, a sphere whose radius is about 25,000 

 miles, and considerably less than 25,000 miles as against the joint 

 attraction of the earth and sun. Its primitive nucleus, if it has 

 grown at all, was confined to smaller dimensions. Attenuated nuclei 

 of indefinite size can not, therefore, be supposed to maintain them- 

 selves permanently in the fields of attraction dominated by larger 

 bodies. Bodies of gas, subject to the dispersive effects of their own 

 molecular velocities, in addition to the competitive attractions of 

 the dominant bodies, have still narrower limits, and, below a certain 

 mass, are inevitably dispersed. In such a system as ours gases 

 must, for the most part, either join themselves to the dominant 

 bodies or be scattered into molecular planetesimals. None of the 

 smaller knots of the solar nebula could probably have been gaseous 

 in any large measure. Gases were probably attached to and oc- 

 cluded in the aggregated or solid planetesimals, and may have 

 been held in a free gaseous state in the interiors of the larger nuclei. 

 The sun is, of course, presumed to have been gaseous throughout 

 the evolution. 



2. Quite a definite indication of the size of the nuclei of the planets 

 may perhaps be deduced from the very remarkable fact that Phobos, 

 the inner satellite of Mars, revolves around the planet in less than 

 o7ic-ihirdoi the time of the planet's rotation, and from the analogous 

 fact that the little bodies which make up the inner part of the inner 



* "The Spheres of Activity of the Planets," by F. R. Moulton, Pop. Astron., 

 No. 66, p. 4. 



