SELF

O. N. Karavashkina and S. B. Karavashkin.

26

Some aspects of the Earth evolution

V.S. Safronov enhanced Schmidt's model by adding to it the gas-dust cloud gravitation differentiation already at the early stage of its existence. He interpreted this composite model as follows. "The Sun and planets originated in a single process of a gas-dust (98- 99% of gas) nebula, during 1 million years for Sun and less than 100 000 years for planetesimals (primary condensations which further transformed into planets - Authors). In the near-Sun nebula compression, there first formed more dense hot nucleus. Then due to the instability at the periphery of the nucleus, in its equatorial plane, there separated a small amount of substance from which a plane-like gas-dust cloud disk has formed. The substance that went on falling onto disk promoted disk growing up to the size like the contemporary planet system. The nucleus from which the angular momentum was transmitted turned into the Sun, when compressed. According to the computations that have been carried out by the scientists from different countries, this stage amounts one million years. In the produced before-planet disk the particles of dust felt through gas to the central plane and formed a layer enriched with dust. They showed such layer gravitationally unstable and having to break down into multitude dust clots. Merging in collisions, the clots contracted and turned into continuum solid bodies about 10 km across in the region of Earth-like planets. This stage lasted no more than one hundred thousands years. Then more lasting evolution of the swarm of before-planet bodies followed. The relative speeds of bodies were determined by their gravity disturbances in approach and initially were small. The bodies mainly united in collisions. With it their speeds, according to calculations, increased proportionally to the radiuses of largest bodies. When these last achieved the size of Moon, the speeds increased almost up to 1 km/s and the bodies began braking down in collisions. Large bodies held the splinters by their gravity and went on growing, joining others… Most large bodies grew relatively faster than others and became the nuclei of future planets. They effectively absorbed the substance which they met in their way. With it the regions of their absorption and gravitation affection broadened, they became located too tight, and smaller bodies turned from the absorbing into absorbed. By the end of this process there remained only 9 planets distanced one from another so that their motion remained stable during milliards years. Nearby the Earth the bodies accumulation into the planet lasted about 100 million years. Under the corpuscular 'wind' affection and short-wave radiation of young Sun, gas was removed from the Earth region during about 10 million years, so the final stage of its growth took place probably in the absence of gas. But in the regions of Jupiter and Saturn, the gas was trapped longer, and quite massif solid nuclei of these planets (as different authors evaluated, the initial masses of these nuclei were about 3- 5 Earth masses) added the rest of gas.

The small bodies of Sun system were a side product of the basic process… During the planets accumulation, around some of them the satellite swarms formed, because the particles encountering the inelastic collisions were captured by their gravity field. The satellites accumulation in these swarms occurred like the planet accumulation. In the regions of giant planets their speeds were so large that a part of bodies has been thrown away from the Sun system, and a part formed at a far periphery, forming so-called Oort comet cloud. In the asteroids region, the planets formation has been interrupted by larger bodies flying from Jupiter region. Colliding with the asteroids, these bodies captured the overwhelming part of asteroids, and approaching the rest of them, they increased their relative speeds by their gravity disturbances. As a result, the between-asteroids collisions caused them not uniting but crushing. The meteorites falling upon the Earth are mainly the products of crushing asteroids …

The main factor determining the initial state of the Earth was the size of bodies of which it formed. The larger the falling bodies were the more effectively they heated the Earth. The computations showed the initial temperature of central region of the Earth was about 1500 K - few times lower than that of melting for those depths. But as the Earth grew, its outer envelop heated more, and at depths 100- 1000 km the temperature could reach that of near-the-melt. The falling bodies disintegration caused the emission of water and gas which constituted the primary atmosphere and hydrosphere. The largest bodies felt on the Earth produced in it considerable thermal and chemical inhomogeneities extending for thousands kilometres and undoubtedly exerting an essential effect on the pattern of early evolution" [36, p. 10- 14].

After V.S. Safronov, O.Yu. Schmidt's and V.I. Vernadsky's premises of radioactive heating of depths almost up to 3000 ^oC is hardly possible, it should be enough only to heat the central region up to 200 ^oC [35, p. 46]. "That is why it is thought at present that the principal source of heating for the growing Earth were the strokes of particles and bodies forming it… The earth heating caused heavier substances descending, and lighter substances ascending… This process of gravitational differentiation gradually layered the Earth, so its dense nucleus and less dense surrounding envelops formed" [35, p. 47]. (This is an important point in the context of our following computations).

Though the Safronov's hypothesis is very bright and detailed, it also is quite contradictive, as it supposes that the bodies then unite, then crush under affection of the same forces and laws. Under the same conditions and events, the asteroids crush, while the planets accumulate their material; the bodies accumulating the dust component lose that gaseous, and up to the statement that during the accumulation a part of bodies has been thrown away from the Sun system. The second considerable discrepancy is the statement that when the body achieved the size of Moon, the speed of planetesimals exceeds 1 km/s, and it causes already not the accumulation but decay. It follows from such statement that the bodies larger than the Moon basically could not form. Further, the mineralogists state that practically all meteorites burn in the dense layers of atmosphere when falling on the Earth, and the rests that achieved the surface are considerably melted. Thus, the meteorites enrich the atmosphere well more than the solid body, even of such small planet as the Earth, and with all similarity of chemical constitution, they mechanically clearly differ from the Earth minerals. The fourth important contradiction is, according to this group of hypotheses, the accretion heating did not cause the rocks melting. If so, the planet has to consist of caked heterogeneous lumps. With all variety of rocks and minerals, this is not so; on the contrary, we see the depths hot, and this reveals in the volcanism which is observed on all planets accessible for our studying and even some on satellites, and for the initial stage of planets the giant-gauge volcanism has been discovered.

Chusiro Hayashi endeavoured to clear this discrepancy. "In the works by the Hayashi's school, the planets formation process is modelled also on the basis of idea of their accumulation from solid bodies and particles. The difference is, Japanese scientists suggest that during the entire process of growth, the Earth remained surrounded with gas of primary nebula, which, by their opinion, volatilised much later. They found that at such conditions the initial state of the Earth has to be also another - around it there should form a powerful (2000 times more than contemporary) opaque primary atmosphere, because of which the surface temperature at that stage had to exceed 4000 K. With such approach we encounter considerable difficulties with the following withdrawing such giant atmosphere with the help of sun wind or sun ultraviolet radiation. Japanese scientists assume this primary atmosphere to be withdrawn during first 500 million years of the Earth existence. However recent observations of the young stars belonging to the Sun spectral class point out that the necessary sun wind and short waves intensities (orders more than those contemporary) are inherent only in the earliest stages of the star evolution - during first dozen millions of its existence. The specimens of Moon regolith retaining the memory of the first milliards years of the Sun system also evidence of only five-fold increase of the sun wind at that stage. Thus, the idea of massif atmosphere on the Earth is not supported by contemporary astrophysical data. It is also unclear, how can we match so high temperature of the Earth in this model with the data of the Earth sciences. As the investigation conducted by Dr. A.I. Tugarinov showed, the water basins (though not so vast as today) surely existed already in the first hundreds million years of the Earth existence" [36, p. 13- 14]. Though, if after Hayashi the ancient atmosphere was 2000 times denser, with the increased pressure the boiling point also elevates, so this argument of Safronov seems unconvincing. We will speak of the Hayashi hypothesis more specifically in the following chapters.