68 B. YU. LEVIN 



formation into the planets of the present, was accompanied by strong heating 

 on account of the gravitational energy thus released. A similar idea is now 

 advocated by V. G. Fesenkov [3, 4]*. If this idea of Fesenkov were correct 

 it would mean that the simplest organic compounds which existed in the pre- 

 planetary cloud must have disintegrated during the hot stage and only after the 

 Earth had become sufficiently cool could their formation begin again. In this 

 case the compounds which existed in the cloud cease to interest us and the 

 problem of the original formation of the simplest organic compounds is exactly 

 the same as it was at the time when the hypotheses of Jeans and his associates 

 prevailed. 



The other extreme case, in which there is supposed to be minimal heating of 

 the Earth during the process of its formation, is that envisaged in the theory of 

 O. Yu. Shmidt [6-8]. According to this theory the Earth was formed by the 

 gradual accumulation of dust particles of the protoplanetary cloud taking place 

 at low temperatures. An essential intermediate stage in this process was the 

 formation of a multitude of bodies of the size of asteroids. The collision of these 

 bodies naturally led to heating, but this was localized. As well as the non- 

 volatile stony materials which constituted the main part of the mass of the Earth, 

 there must have entered into it various gases, both in the sorbed state and in 

 the form of frozen bodies formed in cold parts of the cloud far away from the 

 Sun and entering the zone where the Earth was being formed. In this way 

 organic compounds existing in the preplanetary cloud could perfectly well 

 enter the Earth and be present throughout the whole depth of its substance. 

 Contemporary organic substances on the Earth are, if one may put it so, the 

 direct descendants of the organic compounds of the protoplanetary cloud. 



The ideas of Urey [9-14] are, to some extent, intermediate between those of 

 Fesenkov and Shmidt. According to Urey, the main process consisted in the 

 accumulation of cold solid particles, but he considers that, at some stage, there 

 was strong superficial heating of the asteroid-hke bodies. We shall return later 

 to the question as to why Urey needed to postulate this heating. In the mean- 

 while it is important for us to note that, according to Urey, the composition of 

 the internal parts of the asteroidal bodies remained unchanged during heating. 

 According to Urey, therefore, as well as to Shmidt, organic compounds from 

 the preplanetary cloud must have entered into the composition of the Earth. 



Without entering into detailed criticism of Kuiper's and Fesenkov's hypothesis 

 from the astronomical point of view we will only point out the implausibility and 

 baselessness of their supposition that, when gravitational instability developed 

 within the preplanetary cloud and it began to disintegrate into separate aggre- 

 gates, there was formed just one protoplanet for each planet. In fact, gravitational 

 instability must have led to the development of many aggregates (protoplanets), 



* Recently, in a book written in collaboration with A. I. Oparin [5], V. G. 

 Fesenkov has expressed ideas which are essentially similar to those of O. Shmidt. 

 In particular, he considered that the Earth was formed by gradual growth and that, in the 

 first place, it was relatively cold. Nevertheless, he postulated a large original mass, 'the 

 proto-Earth', with a high hydrogen content. This involved an internal inconsistency. In 

 the address which he presented to the present symposium, V. G. Fesenkov has reverted 

 to his earlier views which are similar to those of Kuiper. 



