391] Rate of Escape 325 



For hydrogen, the value C = 2'24 x 10 5 corresponds to a temperature of 

 about 130C. It therefore seems probable that Venus retains the whole of 

 her atmosphere. There is conclusive astronomical evidence of the existence 

 of an atmosphere on Venus. 



Mars. For Mars, ga is only one-fifth as great as for the earth. The 

 critical velocities are accordingly 



1-6 x 10 5 and 1-1 x 10 5 . 



From the table on page 113 it appears that at a moderate temperature 

 hydrogen and helium would escape from the planet, while water vapour, air, 

 and heavier gases would be retained. 



Mercury. The case of Mercury is exactly similar to that of Mars, as 

 regards the evaluation of the critical velocities, but as Mercury is much 

 nearer the Sun, the temperature of the outer atmosphere would probably be 

 much higher, and therefore the chance of an atmosphere of any kind being 

 found on Mercury would be much smaller. The evidence of astronomy leaves 

 little doubt that Mercury is entirely devoid of atmosphere. 



The Moon. For the moon the critical velocities are found to be about 

 7-8 x 10 4 and 5'6 x 10 4 , 



so that if the moon ever possessed an atmosphere at a temperature much 

 above zero, all constituents, except possibly carbon-dioxide and denser gases, 

 would rapidly escape. 



The bearing of the Kinetic Theory on the question of the dissipation of 

 planetary atmospheres was first brought into prominence by the writings 

 of Dr Johnstone Stoney*. An adequate mathematical treatment was first 

 given by Professor G. H. Bryan in the paper to which reference has already 

 been made. In the present chapter I have borrowed largely from Professor 

 Bryan's paper, although not following his treatment in detail. The results 

 arrived at are practically identical. 



Dr Stoney now maintains that the Kinetic Theory is not competent 

 to decide on the question, being unable to take full account of the physical 

 conditions of the problem, and that the conclusions of the Kinetic Theory 

 are inconsistent with the facts. His present position is stated in a recent 

 letter "Escape of Gases from Atmospheres^." 



* "Atmospheres upon Planets and Satellites," Trans. Roy. Soc. Dublin, vi. p. 305, and 

 earlier papers. 



t Phil. Mag. vn. p. 690 (June, 1904). 



