568 NORMAN LOCKYER LECTURE 



in its atmosphere provides further confirmation, if any lingering doubt 

 remains, for, if Jupiter were hot, these gases would be dissociated. The 

 ultra-violet radiation from the Sun gradually breaks up the molecules 

 both of ammonia and of methane even at low temperatures. In the 

 absence of oxygen, the break-.up is followed by a natural recombination. 

 From the quantity of ammonia observed to be present in the atmosphere 

 of Jupiter, Dunham has concluded that the temperature cannot be lower 

 than about — 120° c, if there is a large excess of hydrogen in the atmo- 

 sphere. This is in close agreement with the directly observed value. 



The ammonia in the atmospheres of Jupiter and Saturn must be nearly 

 on the point of condensation and the clouds over these planets may consist 

 of droplets of liquid ammonia or even small crystals of frozen ammonia. 



The mean temperatures of Uranus and Neptune due to solar radiation 

 alone are about — 200° c. and — 220° c. respectively. At the temperature 

 of Neptune the methane must be nearly ready to condense. 



The nature of the planetary atmospheres, about which so little was 

 known until recently, seems now to have been solved in its broad outlines. 

 There are many details still not understood, such as the nature of the 

 disturbances that continually occur in the atmosphere of Jupiter, and the 

 cause of the colorations ; and it still remains a puzzle whether there is 

 water or water-vapour on Venus. As a brief summary we find that we 

 can divide the planets and their satellites into three groups : the small 

 ones, entirely devoid of atmospheres ; the middle-sized ones, with atmo- 

 spheres of moderate extent, devoid of hydrogen or hydrogen compounds 

 but containing oxygen or compounds of oxygen ; and the large ones, with 

 very extensive atmospheres, devoid of oxygen or compounds of oxygen 

 but containing hydrogen and compounds of hydrogen. 



12 JAN 1959 



