In addition, during recent years the techniques for collecting meteoric and cosmic dust in the Earth's atmosphere have been refined considerably. Samples of this dust have been taken using jet aircraft flying at altitudes of 7 to 16 km, and they have also been obtained from the snow covers of glaciers in high mountains and in the Antarctic.
Consequently, the growth of the Earth as a result of the accretion of meteoric matter is now estimated as being from 13,000 to 80,000 tons daily, which gives a yearly average growth of about ten million tons: We might ask ourselves at this point why the discrepancy with the previous estimates is so great. The answer is obviously that the quantity of very fine meteoric dust in interplanetary space was previously underestimated. For example, it was assumed that the quantity of meteoric particles increases in inverse proportion to the mass of the individual particles, so that the total mass of particles of any given size remains constant. In reality, the total mass of meteoric matter increases with a decrease in the mass and size of the individual particles, as has been verified by recent, more careful, comparisons of the results of visual, radar, and rocket observations for a number of meteors.
An amount of meteoric accretion to the tune of 10 million tons grams) yearly certainly should be taken into account with respect to the life of our planet, the more so since (as we noted above) this factor was even more effective during previous stages in the evolution of the Earth. Even though the mass of the accumulated meteoric matter grams) is infinitesimal in comparison with the total mass of the Earth (6 grams), it is nevertheless significant in comparison with the mass of the Earth's crust. Deep seismic soundings have made it clear that the mean thickness of the Earth's crust is 40 km on the continents and 6 km in the oceans. If the density of the crust is taken as 2.8 and if the percentage of the Earth's surface occupied by the continents and oceans is taken as 29 and 71 % respectively, then the total mass of the Earth's crust is about 2. grams. Thus, even if the accretion rates throughout the geological past had been as insignificant as they are at present, the total amount of meteoric matter deposited on the Earth during the 4 billion years of its existence would still amount to grams, or 1 / 500 of the present mass of the crust. However, this is only a lower limit for the "cosmic" contribution to the composition of the outer crust of the Earth. In reality, this contribution was considerably larger. Meteoric accretion played a very important part in the formation of deep-water deposits in the open sea, where other deposit-producing factors are less effective than they are at other places on the Earth's surface. Iron globules containing admixtures of nickel, which have been found in core samples of deep water deposits, attest to the fact that meteoric matter has been falling onto the Earth at least throughout the entire Tertiary period.
It should be noted at this point that the accumulation of cosmic matter on the Earth, the significance of which has been established by the studies of recent years, is accompanied by the opposite process of the escape of terrestrial matter into space.
Gas molecules and atoms continually leave the Earth's atmosphere, provided that their velocities exceed, for some reason or other, the "escape velocity". The latter is the critical velocity required for a body to extract itself from the sphere of terrestrial gravity. An even greater amount of matter is ejected into space as a result of volcanic eruptions, and also during the explosions caused when the Earth collides with gigantic meteorites. Moreover, the artificial explosions which have taken place since man discovered atomic energy also have been on a cosmic scale. Geologists are unanimous in the opinion that, during past epochs in the history of the Earth, volcanic activity was even more intense than it is at present. As yet it is impossible to evaluate the loss of matter by the Earth quantitatively, and it is not even known which exchange process predominates, the accumulation of matter on the Earth or the escape of it from the Earth.
One thing is certain: an exchange of matter continually takes place between the Earth and space, and the scale of this exchange is such that it must be taken into account when considering the present state and the past history of the Earth. In the past this exchange was much more intensive, so that the struggle between the two opposing factors must have been more pronounced than it is now. The present example thus confirms the correctness of the remarkable ideas held by the prominent natural scientists Vernadskii and Fersman, the founders of Soviet geochemistry. The Earth and space were considered by Vernadskii (1863-1945) to be regions of transient physicochemical equilibrium, between which a continuous exchange of chemical elements takes place /4/. In his book "Geochemistry", Fersman (1883-1945) stressed the idea of the exchange of matter when he wrote: "... in general the force of dispersal and the force of attraction, ... mutually compensating one another, equalize the chemical composition of the universe. There is no basis for assuming that a chemical equilibrium exists and that the present distribution of elements in space is stationary. On the contrary, everything goes to show that the chemical elements are shifted and regrouped according to definite laws, first combining into chemical compounds and then decomposing again" /5, p. 217/ .
Launchings of artificial satellites and space rockets have shed new light upon the problem of the exchange of matter between the Earth and space. They have provided the physical basis for a more profound treat ment of this problem, which involves astronomy, geology, geochemistry, astronautics, and other fields as well.