324 



[CH. XVII 



gives the values of M, a and ga in terms of the corresponding quantities for 

 the earth : 



Superior Planets. From this table it appears that for all the superior 

 planets except Mars, ga is greater than the case of the earth, so that we may 

 conclude that all constituents of the atmosphere would be retained at 

 temperatures up to 230C. Indeed, the lowest value of ga is 3'5 times that 

 of the earth, so that the absolute temperature of the outer atmosphere would 

 have to be more than 3'5 times as great as 230C., say 1500C., before the 

 loss even of hydrogen would be perceptible. A temperature as high as this 

 is highly improbable, so that we may say that, so far as the Kinetic Theory 

 is concerned, the superior planets except Mars ought to retain all constituents 

 of their atmospheres. 



There is general agreement amongst astronomers as to the existence 

 of atmospheres on Jupiter and Saturn. As regards Uranus, spectroscopic 

 evidence (absorption bands) suggests an atmosphere. The spectrum of 

 Neptune is too faint for any such inference to be possible, but it has been 

 suggested that its green appearance is due to absorption bands at the red end 

 of the spectrum, similar to those in the spectrum of Uranus. 



The Sun. For the Sun, ga is 3060 times as great as for the earth. The 

 temperature of the outer atmosphere could therefore be about 3060 x 500 

 absolute, say 1,530,000C., before any appreciable loss, even of hydrogen, 

 would occur from the causes now contemplated. 



Venus. For Venus the value of ga is '8 times as great as for the earth, so 

 that the critical values of are V'8 times as great as the critical values for 

 the earth, and are therefore 



313 x 10 5 and 2-24 x 10". 



