370 CARNEGIE INSTITUTION OF WASHINGTON. 



etesimals. If the mass of the nucleus were large and much concentrated 

 toward the center, its future career would be secure; but if small, 

 unless very highly concentrated, its persistence would be critically 

 dependent on special conditions. Scrutiny of the conditions that 

 affected the smaller pla,netary types was therefore imperative. The 

 motions imparted by the eruptive and projective action of the sun 

 were the first source of hazard.^ Turbulent, vortical, and rotatory 

 motions would be almost inevitably inherited from the solar expulsion. 

 Turbulent motions prevail in the sun and additional turbulences must 

 have been generated by unbalanced thrusts and drags incident to 

 eruption. More or less of vortical or eversive motion almost inevitably 

 arises from eruptive action. The mass was ah-eady rotating as a part 

 of the sun, and doubtless received additional impulse toward rotation 

 in the eruptive act. All these motions more or less critically condi- 

 tioned the early evolution of the smaller nuclei. Convective action, as 

 it developed in each nucleus after its expulsion, was necessarily built 

 upon these inherited motions and took form in adaptation to them. 

 The convection currents were not, therefore, simple vertical cycles, 

 as naturally enough pictured in static quiescent bodies. They must 

 have been highly complex, more or less gyratory, and even tortuous. 

 In so far as they were specially eversive, they were much more effective 

 in discharging internal heat in the smaller order of bodies than simple 

 thermal convection. 



II. The Sifting of the Solar Gases. 



When the material for the planetary nuclei was ejected from the 

 sun, it consisted essentially of highly mixed hot gases, doubtless 

 essentially alike in all nuclei, except perhaps for some systematic 

 gradations. The giant planets seem to have remained essentially 

 gaseous, as implied by then* densities: Jupiter 1.25, Saturn 0.63, 

 Uranus 1.44, and Neptune 1.09. The material for the terrestrial 

 group, however, was so changed as to form essentially stony or metallic 

 solids with notably high densities: earth 5.53, Venus 4.85(?), Mars 

 3.58, and Mercury 4.48(?). Their atmospheres formed only a negli- 

 gible proportion of their masses. The planetoids and smaller satellites 

 seem to be quite without appreciable atmospheres and are doubtless 

 formed wholly of stony or metallic material. It is clear, therefore, 

 that in the formative process there was much sifting out of the lighter 

 molecules that were mixed with this stony and metallic material as it 

 came forth from the sun. Such sifting is precisely what the kinetic 

 theory of gases requires under such conditions. The escape of the 

 light molecules from the smaller nuclei should take place with great 

 facility as long as they were still in the hot, expanded condition that 



'Respecting the mode of propulsion, the discoveries of Petti t relative to the eruptive promi- 

 nences of May 29 and July 15, 1919, arc very instructive. Edison Pettit, The Great Eruptive 

 Prominences of May 29 and July 14, 1919, Astrophys. Jour., vol. 50, 206-219. Oct. 1919. 



