THE ORIGIN OF THE EARTH. 63 



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 through- 

 out 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 

 one-third of 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 

 ring of Saturn revolve about that planet in a little more than one-half 

 the time of the planet's rotation. 1 These are exceedingly troublesome 

 facts from the view-point of the Laplacian hypothesis, for under it 

 the contraction of these planets, after they had shed their secondaries, 

 should have greatly accelerated their rotations, and these should have 

 become much shorter than the revolutions of the secondaries. Before 

 Moulton's citation of the second case, an attempt was made to explain 

 the case of Phobos by a supposed tidal retardation of the planet's 

 rotation, but the Saturnian case appears to render this explanation 

 incompetent. 2 



Under the hypothesis of growth from a nucleus by the addition 

 of planetesimals, the rotations of the planets were dependent largely 

 on the special phases of the impacts of the infalling planetesimals, 

 and no necessary relations between the rotation of a planet and the 

 revolution of its satellite are assignable. But if this be neglected, and 

 the rotation-period of the planetary nucleus be assumed to have been 

 originally the same as the revolution-period of the satellites' nucleus, 

 the growth of the mass of the planet must have drawn the satellite nearer to 

 itself, and shortened the time of its revolution. 



If the whole of the periodic difference between Mars and Phobos 

 be due to this cause, the growth of the nucleus of Mars is deducible 

 from it. Under this view, the matter of the rings of Saturn may 

 have been satellite nuclei at the outset, and have been drawn 

 within the Roche limit by the growth of Saturn, and then disintegrated 

 by tidal action and distributed into the ring form. All other satellites 

 should, under this view, have been drawn towards their primaries 



1 For a discussion of these phenomena, see " An Attempt to Test the Nebular 

 Hypothesis by an Appeal to the Laws of Mechanics," Moulton, Astrophys. Jour., 

 1900, p. 109. 



2 See Moulton's discussion, loc. cit., pp. 109, 110. 



