282 The Origin and Evolution of the Solar System [OH. xn 



of Mars were born in the liquid or solid state out of a liquid or solid primary, 

 they ought to have been comparable in mass with that primary. 



This apparent difficulty may arise largely from our having assumed that 

 a body may be accurately labelled either as gaseous or as liquid or solid. The 

 masses we have under consideration must all have been at low temperatures, 

 and the pressure must have been considerable in their central regions. We 

 must consider the possibility of formations which are liquid in their central 

 regions where the pressure is highest, and gaseous in their outer regions. 

 Such masses will be represented with tolerable accuracy by the composite 

 Roche's model, of which the behaviour under tidal forces was discussed in 1 62. 



We can now perhaps account for the formation of the system of Mars by 

 supposing Mars to have started condensing in its central regions during or 

 immediately after birth, and so assuming the structure of a dense nucleus 

 surrounded by a light atmosphere. A further tidal cataclysm would result in 

 a jet of the atmosphere being ejected, and if this immediately started to con- 

 dense into the liquid state the final result might be two small planets of the 

 kind observed. 



The satellites of Uranus and Neptune may be explained in the same way. 

 The earth-moon system admits of a similar explanation, but may also admit 

 of explanation in terms of a wholly fluid earth ; pending further mathematical 

 investigation it is hardly possible to say whether the masses of the earth and 

 moon are too unequal for the system to have originated out of a wholly fluid 

 mass. The question reduces ultimately to one of degree only; the earth at 

 birth was probably more largely fluid than the planets whose satellites are 

 relatively smaller. 



301. The foregoing considerations have shewn that four at least of the 

 eight planets must have been partially fluid at, or shortly after, their birth. 

 If once this conclusion is admitted and it seems inevitable on almost any 

 theory of cosmogony then there is no justification left for assuming, as we 

 momentarily did in 300, that the two biggest planets, Jupiter and Saturn, 

 were wholly gaseous at their birth, although the calculation of 297 shews it 

 to be quite possible that Jupiter at least may have always been gaseous. 



302. Let us examine how the tidal theory stands if we admit the 

 possibility of all the planets having been partially fluid at birth. 



We picture the primaeval sun throwing out a jet of matter under the 

 influence of a passing star. The calculations of 296 have already suggested 

 that the conditions of the tidal encounter must have approximated to those 

 we have described as " slow/' The tide-generating star must have described 

 an orbit passing within a sphere of radius equal to 2'2 (M'/M) mean radii 

 of the sun. As soon as the star came within this sphere the tidal ejection of 



