296-soo] The Tidal Theory 281 



for the planets themselves. The systems of Saturn and Jupiter are so like 

 that of the sun that any hypothesis which assigned different origins to the 

 system and its sub-systems would be condemned by its own artificiality. 



The first five satellites of Saturn all have masses comparable with 5 x 10 23 

 grammes. Assuming these to have been formed by gaseous condensation, 

 the range of molecular velocities (7= 4 x 10 4 to = 4 x 10 5 would give a range 

 of density from 8 to 8,000,000 if calculated by the method of 284. The 

 obvious inference is that either these satellites were not formed by gaseous 

 condensation, or that they are mere remnants of larger masses. Similar con- 

 siderations apply to the satellites of Mars and to some at least of the satellites 

 of Jupiter. It is improbable that these satellites are all remnants of much 

 larger masses ; their present uniformity of size is opposed to any such hypo- 

 thesis. Thus we are driven to supposing that they have been solid or liquid 

 from their birth *. 



299. This conclusion is quite independent of the tidal theory, or of any 

 other theory of cosmogony. The small bodies we are considering are even 

 now too small to retain an atmosphere ; if they were suddenly transformed 

 into a gaseous state, so that gravity was largely reduced at their surface, they 

 would be still less able to retain an atmosphere, and their outer layers would 

 rapidly dissipate into space. Whatever theory of cosmogony we hold, it seems 

 comparatively certain that most of the asteroids, the majority of the satellites 

 of the planets, and of course the particles of Saturn's rings, have been solid or 

 liquid from birth. 



It is fairly safe to assume that the satellites of Mars, Saturn and Jupiter 

 originally formed part of these planets, for in each case the plane of rotation 

 of the planet almost coincides with the plane of the orbits of the innermost 

 planets. If the satellites had been " captured " or otherwise picked up from 

 outside, it is improbable that they should have almost unanimously stumbled 

 into the plane of rotation of the planet for the planes of their orbits. If we 

 suppose that Jupiter and Saturn have always been gaseous we must suppose 

 that solid or liquid satellites were born out of a gaseous planet. This might 

 have happened if the outer layers were at a temperature not far above their 

 boiling point; the jet of matter thrown out by tidal action might cool still 

 further by radiation and immediately condense into a solid or liquid mass. 



300. The satellites of the smaller planets are not so easily accounted for. 

 To take a definite instance, the satellites of Mars are too small to have started 

 life in the gaseous form ; they seem likely to have originated out of Mars ; 

 Mars itself is too small to have formed as a gaseous condensation out of the 

 same nebular arms as produced Jupiter and Saturn ; and yet if the satellites 



* Cf. Jeffreys, Monthly Notices R.A.S. 78 (1918), p. 424. 



