Primitive Planetary Atmospheres 

 and the Origin of Life 



HAROLD C. UREY 



University of Chicago, Urbana, Illinois 



Haldane [i] pointed out that life probably originated under anaerobic con- 

 ditions. He argued that the fermentative metabolic processes of Uving organisms 

 of widely different structure are very similar while the oxidative reactions are 

 very different, thus indicating that the anaerobic metaboHsm is the most 

 primitive. He suggested that the source of energy for the metabolism was 

 metastable molecules produced by ultraviolet light from the sun. Oparin [2] in 

 an interesting book comes to very similar conclusions. He has discussed in some 

 detail the possible ways by which organic substances of inorganic origin may 

 combine to produce the most primitive organisms. These arguments as well as 

 others by Pirie [3] and Bernai [4] seem to me to be more conclusive evidence for 

 the primitive conditions on the Earth than any that can be dravni from geo- 

 chemical and cosmochemical studies. However, studies of the latter kind confirm 

 the biochemical conclusions. 



The most remarkable feature of the Earth's atmosphere is its content of 

 elementary oxygen, and it is the only planet on which free oxygen has been 

 detected. It has been recognized, ever since Jeans derived formulae for the 

 escape of gases from atmospheres, that hydrogen would escape more readily 

 than other gases and that this must result in an increased state of oxidation of 

 the atmosphere and perhaps in the presence of free oxygen [5, 6]. To-day we 

 have much more information in regard to the Earth's atmosphere than previously 

 and hence are in a better position to discuss its probable history. From the ages 

 of the meteorites as determined from the lead isotopic abundances and from the 

 **^Rb-8^Sr and "^^K-'^^A dating methods, we know that some very involved 

 physical and chemical processes took place 4-5 x 10^ years ago. Presumably 

 this occurred at the time the solar system originated and we can take this as the 

 age of the system. Also, we now know more about the temperature distribution 

 and convection in the terrestrial atmosphere and hence we are able to supplement 

 the theory of Jeans for the escape of atmospheres. These studies make possible 

 a better estimate of the history of planetary atmospheres generally. 



HISTORY OF THE EARTH'S ATMOSPHERE 

 We do not know the temperature at which the Earth's surface was formed, 

 but it can be stated firmly that the only feasible source of heat for producing a 

 high-temperature primitive earth was its own gravitational energy of accumu- 

 lation [7]. If it accumulated so rapidly that this energy could not be radiated 

 into space, it could have originated in a melted condition. However, in view of 



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