208 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1950 



of methane, ammonia, water, and rare gases, 56 parts by weight of 

 helium, and 180 parts by weight of hydrogen. 



If we assume the above composition, then it appears likely that 

 Uranus and Neptune, the two planets of size intermediate between the 

 terrestrial planets on the one hand, and Jupiter and Saturn on the 

 other, captured substances of intermediate condensability and molec- 

 ular weight. This capture process enormously increased the masses 

 of the j)lanets, owing to the rather high abundances of water, methane, 

 and ammonia. 



Saturn and Jupiter condensed sufficiently rapidly and grew to a 

 size sufficiently large to permit their capturing the very abundant 

 gases hydrogen and helium. The preponderant abundances of these 

 elements permitted the two planets to develop into the giants of the 

 solar system. 



A careful study of the Jovian planets utilizing theoretical studies on 

 the behavior of matter under very high pressures demonstrates that 

 the above picture of the compositions of the planets in relation to the 

 composition of the sun is essentially correct. In order of increasing 

 size we have first the terrestrial planets composed of metal and rock. 

 We next have Uranus and Neptune with earthlike cores composed of 

 metal and rock, surrounded by very thick layers of ice, liquid ammo- 

 nia, and methane, and thin atmospheres of hydrogen and helium. 

 Following this, we have Saturn and Jupiter composed of Uranuslike 

 cores surrounded with thick layers of hydrogen and helium. Indeed, 

 approximately 90 percent of the mass of Jupiter appears to be com- 

 posed of these gases ! 



Studies of planetary atmospheres by Kuiper and others substantiate 

 the general picture. Methane, which is a very volatile substance, is 

 detected in considerable concentration in the atmospheres of Jupiter, 

 Saturn, Uranus, and Neptune. Ammonia has been observed in the 

 atmosphere of Jupiter (the warmest of the four). Presumably the 

 vapor pressure of ammonia is too low in the other three to permit its 

 detection. Water has such a low vapor pressure at the temperatures 

 of the Jovian planets that it cannot exist as an atmospheric constituent. 



There is still much to be done if we are to have a clear picture of the 

 composition of our cosmos. There must be increased precision in the 

 determination of the composition of stars and interstellar matter. 

 There must be increased precision in the determination of the composi- 

 tion of meteorites and the earth's crust. Theoretical studies must be 

 continued on the relationship between stars, interstellar matter, plan- 

 ets, and meteorites. But already, in spite of the meager data, a pattern 

 is unfolding that suggests strongly that our cosmos is remarkably 

 uniform in chemical composition. It is to be hoped that by the time 

 another decade has passed, we will know the composition reasonably 

 accurately. 



