Evolution of the Earth's Atmosphere 55 



This question is very important in consideration of the history of the atmos- 

 phere of the Earth and it is of the utmost complexity. The accumulation of new 

 scientific data and generahzations from them have forced people to change the 

 ideas which were originally put forward. 



One must reckon that at the earliest period of the formation of the Earth the 

 amount of radioactive substances and, therefore, the intensity of their radiations 

 and the amount of heat dissipated by them was considerably greater than it is 

 now, mainly owing to the higher concentrations of actinouranium (235U) and 

 40K. The calculations made by some workers show that, 3000-4000 million years 

 ago, the amount of radioactive heat given off was so great as to have caused the 

 whole Earth to be molten [6]. Such calculations are, however, only of suggestive 

 value, the more so since the question of the amount and distribution of radio- 

 active elements in the crust of the Earth has, as yet, been insufficiently studied. 

 It has been stated that the amounts of radioactive elements in the Earth are 

 considerably less than those assumed in the calculations mentioned above. 



It would appear that the original temperature of the protoplanetary cloud of 

 dust and gas was low. The lowest calculated value is 10-20 °K [5]. The heat 

 given out by the radioactive elements in this dispersed material was well dissi- 

 pated. When the Earth was formed, as an aggregation of this cloud of dust and 

 gas, heat was given off on account of its condensation. Furthermore, during the 

 whole course of its history radiogenic heat was being given off at a gradually 

 dechning rate corresponding to the rate of decay of the radioactive elements as 

 well as heat derived from chemical reactions. 



The general trend of the mean temperature of the Earth may be supposed to 

 have followed a curve, the first part of which corresponds to the low temperature 

 already mentioned. It then rose gradually, reached a maximum and later 

 decUned. 



The thermal conditions of the crust of the Earth and the inner parts of the 

 terrestrial globe were not, however, uniform, and it must not be thought that 

 they were in thermal equihbrium. There can be no doubt that the temperature 

 of the outer layers of the Earth must have been far lower than those of the 

 deeper layers. At present the mean temperature of the surface of the Earth is 

 about o °C. As one goes deeper the temperature rises (i °C for every 20-50 m) 

 till, at some depth, it reaches 1000 °C or higher, as is evident from volcanic 

 eruptions. 



The question as to what was the temperature of the surface of the Earth in 

 earher epochs, e.g. 2000-3000 million years ago, remains unsolved. There is 

 reason to suppose that the temperature was higher then than it is now. However, 

 since the temperature of the surface of the Earth has not changed significantly 

 during the last several hundred million years, it may be supposed that 2000- 

 3000 miUion years ago it can scarcely have exceeded 100-200 °C, more Hkely it 

 was below 100 °C. This was the time of the extensive development of volcanic 

 activity. Large masses of magma were erupted on to the surface of the Earth 

 at that time. The problem of the mean maximal temperature of the surface of 

 the Earth at an even earher period — 3000-4000 miUion years ago— remains 

 obscure. 



