62 



that the early atmosphere of our planet was only weakly reducing, 

 consisting of a mixture of nitrogen, carbon dioxide, carbon monox- 

 ide, water, and a few percent hydrogen. Such an atmosphere would 

 provide a sufficient greenhouse effect to keep the Earth warm even if 

 the Sun were 25% less luminous (a model which has some support) 

 during that period than it is today. There is no need for ammonia or 

 some other reduced gas to provide this effect as long as the partial 

 pressure of C0 2 is on the order of 200 millibars. This large amount 

 of atmospheric C0 2 would gradually diminish as a result of rock- 

 weathering and the consequent production of carbonates. 



Everything that we have described for the Earth should apply to 

 Mars and Venus as well. Why then has our planet turned out so dif- 

 ferently from our neighbors? Let us return to our basic criteria for 

 planetary differences — size and distance from the Sun. 



We first consider distance from the Sun. Suppose we could 

 move the Earth to the position occupied by Venus. What would 

 happen? The increased intensity of sunlight would cause the mean 

 temperature of our planet to rise. Model calculations show that this 

 increase in temperature would cause increased evaporation of sea 

 water and would lead to a larger amount of water vapor in the atmo- 

 sphere that could increase the greenhouse effect and the mean sur- 

 face temperature further, leading to more evaporation, etc. In 

 other words, the Earth's climate would go into a positive feedback 

 loop that would lead to a condition known as a runaway greenhouse. 

 The end result would be that the oceans would boil, putting all of 

 the surface water into the atmosphere. The atmosphere itself would 

 be so hot that there would no longer be a cold trap to confine water 

 vapor to lower levels. Photodissociation by ultraviolet light would 

 then become very efficient, and the hydrogen atoms resulting from 

 this process would escape into space. 



This is one explanation for the absence of water on Venus. The 

 process we have just described would have occurred on that planet 

 shortly after it formed. The oxygen left over from H 2 O dissociation 

 would have combined with the crust and available volatiles; we see 

 one of the results in the dense carbon dioxide atmosphere that now 

 blankets the planet (fig. IV-5). Plausible as this scenario seems, it has 

 not yet been rigorously proven. A demonstration that deuterium is 

 enriched on Venus would provide a strong supporting argument. If 

 indeed there were once oceans that boiled with the subsequent 



