61 



of Venus is distinctly different. It thus seems that the fractionation 

 processes were acting in the solar nebula prior to the formation of 

 the planets, and affected all of the solid bodies in the inner solar sys- 

 tem in different ways. We have also learned that the T-Tauri phase of 

 solar history was probably not cataclysmic enough to have blown 

 gases away from the surfaces of planets. 



To reconstruct the Earth's early atmosphere, we must therefore 

 turn this argument around. This fractionation of the noble gases in 

 the solar nebula was presumably accompanied by a fractionation of 

 other gases as well. Therefore, the maximum amount of hydrogen 

 that the Earth could have captured can be calculated by using the 

 neon in the present atmosphere as an index. In other words, if we 

 assume that neon was captured, the cosmic hydrogen-to-neon ratio 

 would give us the maximum value for early atmospheric hydrogen. 

 This turns out to be about 10 millibars, or 1/100 of our present 

 atmospheric density. Such an atmosphere would be lost by escape 

 in less than 10,000 years. 



Methane and ammonia would have had abundances 1,000 times 

 smaller than that of hydrogen. Ammonia is particularly unlikely as a 

 long-term atmospheric constituent, since this small amount would be 

 destroyed in less than 40 years by solar ultraviolet light. Ammonia 

 would also have been out of equilibrium with crustal rocks. 



Thus, the only hope for a highly reducing early atmosphere 

 would seem to reside in the possibility of producing it by degassing 

 from the early Earth, either by internal melting processes caused by 

 radioactivity or by external processes - bombardment by meteorites 

 and comets that incidentally may have contributed reduced volatiles 

 themselves. The first of these possibilities requires the presence of 

 some reducing agent in the upper mantle. Free iron has been sug- 

 gested as a candidate for this role, assuming that this early degassing 

 took place prior to formation of the Earth's core. Some investigators 

 are unhappy with this picture, however, arguing that the core should 

 have occurred as the planet itself formed, since the energy of accre- 

 tion would have been sufficient to initiate the melting of iron once 

 the embryo-Earth attained 10% to 25% of its present size. 



At this stage of our knowledge, there seems no way to rule out 

 a transient, early atmosphere rich in hydrogen, methane, and carbon 

 monoxide and containing ammonia. Because of the difficulties 

 referred to above, however, interest is shifting toward the possibility 



