PLANETESIMAL THEORIES OF THE EARTH* S ORIGIN. 195 



collecting zones and presumably swept them thoroughly, the reduc- 

 tions of eccentricity are subequal. For the very small bodies that 

 presumably grew but little, the eccentricities remain large, for the- 

 greater part. For example, the eccentricity of Mercury, the- 

 smallest of the planets, remains more than twice that of any other 

 planet. Mars, the next smallest in size, comes next in eccentricity 

 among the planets, while the asteroids, which probably grew but 

 little, have high eccentricities, as a rule. . 



"To bringoutthegeological bearingsof the planetesimal hypothesis*. 

 I have given considerable time to a study of the probable stages of 

 growth of the early earth, of the time and mode of introduction of 

 the atmosphere and hydrosphere, and of the initiation of the great 

 topographic features, together with the leading modern processes. 



"Following the postulates of the previous sketch, a nebular knot 

 is assumed to have been the nucleus of the growing earth. . . 

 Assuming that the nuclear mass was quite small, it is inferred that 

 it was composed chiefly of matter of high molecular weight, since 

 light molecu 7 es would be liable to escape because of their velocities _ 

 The nucleus is supposed to have been originally an assemblage of 

 planetesimals grouped together by their mutual gravity, and to 

 have passed gradually into a solid mass in connection with the- 

 capture of outside planetesimals. . . . 



" As the solid nucleus thus formed may not have been massive' 

 enough to control a gaseous envelope in its earlier stages, a possible 

 atmosphereless stage is to be recognized. Just how massive a 

 planetary body must be to hold permanently an appreciable atmo- 

 sphere is not accurately computable at present, because of the 

 uncertain value of some of the factors involved. A fairly safe 

 conclusion may perhaps be drawn from known celestial bodies. 

 The moon . . . has no detectable atmosphere, nor has any 

 smaller body, whether satellite or asteroid, so far as known. Mars 

 . . . has an appreciable, but apparently quite limited, 

 atmosphere. The limit between atmosphereless and atmosphere- 

 bearing bodies probably lies between the two, i.e., roundly between 

 one-eightieth and one-tenth of the earth's mass. . . . 



" When the growing earth reached a mass sufficient to control the 

 flying molecules of atmospheric material, there were two sources 

 from which these could be supplied for the accumulation of an 

 atmosphere, an external and an internal one. 



"In the later stages of organisation, and thence down to the- 

 present time, the molecules discharged from all the bodies of the 

 solar system were possible sources of atmospheric accretion. Of 

 these the most important were probably volcanic, and similar 

 discharges from the small bodies that could not hold gases per- 

 manently, and discharges from the sun by virtue of the enormous 

 explosive and radiant energies that are there resident. 



