120 SIMPLER ORGANIC SUBSTANCES 



The temperatures are far lower on the surfaces of Uranus 

 and Neptune, which are still further from the Sun. The 

 ammonia is completely solidified but, on the other hand, a 

 very large amount of methane is present in their atmospheres. 



Thus we find carbon in combination with hydrogen on all 

 the large planets. The discovery of methane in the atmo- 

 sphere of Titan, a satellite of Saturn, by G. P. Kuiper in 

 1944*^ is of very gi^eat interest. Titan is one-third of the 

 size of the Earth and has one-fortieth of its mass. It is only 

 the extremely low temperatures which prevail in the neigh- 

 bourhood of Saturn (— 180° C) which enable Titan to retain 

 its atmosphere of methane. It is clear that there can be no 

 question of biogenic formation of hydrocarbons here any 

 more than on the large planets. 



In the atmospheres of the planets belonging to the same 

 group as the Earth the carbon is mostly oxidised and exists 

 in the form of cOg. Thus the proportion of this gas in the 

 atmosphere of Venus is many times greater than in that of 

 the Earth. According to Kuiper, there is reason to believe 

 that a certain quantity of methane and other hydrocarbons 

 of the acetylene and ethylene series are present in the 

 atmospheres of Venus and Mars. Here, however, one cannot 

 completely exclude the possibility that both the carbon di- 

 oxide and the organic substances have arisen biogenically. 



The study of meteorites is of particular interest in connec- 

 tion with the problem under discussion ; in the first place 

 because meteorites which have fallen on to the Earth may 

 be submitted to direct chemical analysis and, further, to 

 mineralogical investigation. These are the only * non-terres- 

 trial ' bodies of which the composition may be established 

 with completeness and certainty. In the second place, a study 

 of meteorites shows us more and more convincingly that their 

 chemical composition is very close to that of the Earth as 

 a whole, and that their formation was related to that of our 

 own planet. 



Long ago the attention of scientists was directed towards 

 the origin of the Earth and the meteorites. Many prominent 

 geochemists of the twentieth century, including F. W. 

 Clarke,'^ H. S. Washington," V. M. Goldschmidt,'« and I. and 

 W. Noddack," have studied the structure and composition 



