DISTRIBUTION OF ORGANIC SUBSTANCES 1 IQ 



which also includes Venus and Mars. Mercury occupies a 

 somewhat special position, in that it is a naked rocky mass 

 without an atmosphere, similar in some respects to our Moon 

 and Pluto, about the chemical composition of which we still 

 know very little. 



When they were formed the large planets retained the 

 quantitative relationship between the various elements which 

 is characteristic of the galaxy as a whole. Thus the elements 

 which predominate in their composition are, first hydrogen, 

 and then the other light elements ; this is what causes their 

 characteristically low specific gi'avity and chemically reduced 

 state. 



For a long time spectroscopic studies of these planets led 

 to no definite results. The bands which had been observed 

 in their spectra remained a puzzle and it was not until 1932 

 that R. Wildt showed that some of these bands in the spec- 

 trum of Jupiter corresponded with the bands of ammonia 

 and others with those of methane. This was soon confirmed 

 by T. Dunham, and then A. Adel and V. M. Slipher^^ suc- 

 ceeded in identifying all the bands characteristic of methane. 



There could thus be no doubt as to the presence of the 

 hydrocarbon, methane, in the atmosphere of Jupiter. H. C. 

 Urey has shown that this methane must be converted photo- 

 chemically to other higher hydrocarbons, both saturated and 

 unsaturated. In particular, he showed that cuprene, a hydro- 

 carbon of high molecular weight having a red colour, would 

 arise by the polymerisation of acetylene. According to Urey 

 the presence of this substance would account for the colour 

 of the red spot on Jupiter. Owing to the temperature of 

 the surface of Jupiter, ^shich is very low compared to that 

 on the Earth (- 140° C), only methane can exist there in 

 the gaseous state. Even such hydrocarbons as ethane, ethy- 

 lene and acetylene are liquids under such conditions. 



Saturn has an abundant atmosphere which, like that of 

 Jupiter, contains methane and ammonia, but as the distance 

 of this planet from the Sun is far greater, the temperature on 

 its surface is even lower than that on Jupiter. A considerable 

 proportion of the ammonia on Saturn is therefore in the solid 

 state, as may be seen from the spectrum, in which the 

 methane bands stand out very clearly. 



