4 V. G. FESENKOV 



must have been widespread chiefly in the distant past, it is all the more reasonable 

 that similar phenomena should have occurred on planets with a greater mass, 

 such as Mars and the Earth, As observations show [9], an atmosphere on the 

 Moon should be non-existent due to the very small force of gravity on its surface 

 and to the comparatively high temperature, which exceeds 100 °C at the centre 

 of the lunar disc during full Moon. On Mars water vapour may easily escape 

 into interplanetary space at a temperature of approximately 200 °C (according 

 to Spitzer). This temperature might easily have been attained in the past during 

 the first stages of the existence of this planet. On the Earth, water vapour 

 escaped only during the period of dissipation of the mass of the original proto- 

 planet, but later eruptions from the interior were totally retained because of 

 various tectonic processes. The production of water vapour presupposes a 

 considerable temperature in the interior of the Earth, and this requires an 

 explanation. 



In this connection, it is essential to dwell on the problem of the early thermal 

 history of the Earth. Speaking generally, one should presuppose the existence 

 of four different sources of heat during the formation of our planet: the con- 

 version of potential energy into kinetic energy, in other words, the release of the 

 energy of condensation as is the case in the gravitational compression of any 

 gaseous-dust nebula; the origin of different mineral compounds, for example 

 olivine and others, that is the transition of the chemically comparatively simple 

 protoplanetary medium into complex compounds, all of which take place chiefly 

 in conditions where oxygen is insufficient. These geochemical reactions should, 

 in the main, also be of the exothermic type. Then there develop processes of a 

 more protracted character, those that take place throughout the whole existence 

 of the planet, namely radioactive heating that depends essentially on the distri- 

 bution of radioactive substances within the mass of the Earth, and the redistri- 

 bution of the internal masses of the planet according to molecular weight. Both 

 processes depend on the intensity of the first two. If even during the very first 

 stage of its existence the planetary mass reaches a sufficiently high temperature, 

 then, owing to the convective currents that arise, radioactive substances are 

 quite rapidly transported to the surface layers, in which case radioactive heating 

 is confined essentially to these surface layers and does not in general attain any 

 great magnitude. If, however, the planetary mass remained in the cold state 

 from the very beginning, the distribution of radioactivity may be considered as 

 being even throughout its whole mass in the course of the greater part of all 

 past history, in which case radioactive heating might be very considerable though 

 not capable of causing the Earth to become hquid. The detailed calculations of 

 the radioactive heating were made by E. A. Lyubimova, J. A. Jacobs, and others 

 for different models of the Earth [9, 10, 11]. To avoid these uncertainties in our 

 discussion of the early thermal history of the Earth, we must consider the most 

 indicative facts as a whole. As may be supposed, of greatest importance in this 

 respect are the data of seismology and terrestrial magnetism, according to which 

 the central core of the Earth (at least in its external part) is in a Hquid state and 

 does not transmit transverse waves. Consequently, the temperature of the 

 central parts of the Earth should be at least several thousand degrees. The 



