20 HAROLD C. UREY 



require that some permanent surface features exist. These might be continents 

 awash with the oceans so that erosion is not effective. Or, water clouds may 

 exist in the high atmosphere but no liquid water is now present in its surface. 



It has been suggested that the clouds are due to dust, but in this case the dust 

 must be remarkably white. No abundant white solids are present on the Earth 

 and no method for their production on Venus has been suggested. Moreover, 

 the partial clearing indicates the presence of phenomena such as rain or evapor- 

 ation. Dust clouds would indicate the presence of strong convection and of very 

 small particles. Settling of dust during a short period of time would be impossible 

 under these conditions. 



Water and carbon dioxide appear to be the only stable substances likely to be 

 present on a planet having an oxidation state such as that of Venus. Lyot [22] 

 observed the polarization of the light from the clouds and found it to be very 

 similar to that produced by small droplets of water. Altogether it seems most 

 likely that the clouds are due to water, as Menzel and Whipple have argued, but 

 it is not certain that the surface is covered with oceans. 



Since Venus is nearer the sun and had a smaller mass, the escape of hydrogen 

 from its surface should be greater than that from the Earth on the basis of 

 Jeans' formula. However, the escape of hydrogen from the Earth is limited by 

 condensation of water at the tropopause and the diffusion of hydrogen in the 

 high atmosphere and not by the conditions imposed by Jeans' escape equation. 

 We can make only very uncertain estimates in regard to detailed structure of 

 the atmosphere of Venus. Since the planet probably rotates only once in its 

 year, a rising current at the mid solar point probably exists similar to the rising 

 current at the Earth's equator. For purposes of approximate calculation we 

 estimate this temperature as 225 °K as compared with 187 °K for the tempera- 

 ture of the Earth's equatorial tropopause. If the atmosphere of Venus above 

 this point is saturated with water at this temperature, the concentration of 

 hydrogen above the tropopause will be about 145 times as great as that above 

 the Earth and, other conditions being similar, the rate of loss of hydrogen will 

 be about 1-45 x 10^ atoms /sec/cm^. This is equivalent to 3000 g of water/cm^ 

 during 4-5 x 10^ years. This would be sufficient to oxidize 1000 g of carbon cm^ 

 from the zero oxidation state to carbon dioxide. This estimate has no great 

 certainty at all. 



The decomposition of water below the tropopause cannot occur because of 

 the absorption of hght in the Schumann region by carbon dioxide. This absorp- 

 tion will be substantially complete if 100 cm atmospheres of carbon dioxide is 

 present above the tropopause. Thus, the history of the atmospheres of Venus 

 and the Earth must have been different because the carbon dioxide remains in 

 the atmosphere of Venus and not in that of the Earth. As soon as 100 cm atmos- 

 pheres of carbon dioxide remained above the tropopause of Venus the rapid 

 loss of hydrogen ceased, but in the case of the Earth this occurred only when 

 some 2 cm atmospheres of oxygen gas appeared above its tropopause. Hence, the 

 oxidation of the Earth has probably progressed to a much more advanced stage 

 than did that of Venus. The general course of chemical change should have 

 been similar to that outlined for the Earth. 



