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• W2072792809 endingPage "104" @default.
• W2072792809 startingPage "41" @default.
• W2072792809 abstract "The terrestrial planets are believed to have formed by accretion from an initially cold and chemically homogeneous cloud of dust and gas. The iron occurring in the dust particles of the cloud was present in a completely oxidised form. Either before or during accretion of dust into planets, partial reduction of oxidised iron to metal occurred. The role of oxidationreduction equilibria during the formation of terrestrial planets is discussed and it is concluded that the differing zero-pressure densities of the planets are caused dominantly by differing mean states of oxidation which were established during the primary accretion processes. This interpretation avoids the necessity for assuming the occurrence of physical fractionation of metal from silicates in the solar nebula before accretion. A detailed study is made of the evidence shed by chondritic meteorites upon oxidationreduction equilibria occurring early in the history of the solar system. The significance of the widely varying oxidation states of chrondrites is discussed. It is concluded that the different classes of chondrites have formed by an autoreduction process operating upon primitive material similar in composition to the Type I carbonaceous chondrites. Reduction occurred when this material accreted into parent bodies which were heated internally, perhaps by extinct radioactivities. Under these conditions, trapped carbonaceous material reacted with oxidised iron to produce a metallic phase in situ. Such a process explains the primary oxidation-reduction relationships in chondrites established by prior. The chemistry of the reduction process which operated in chondrites is studied. The evidence strongly indicates that the principal reducing agent was carbon and not hydrogen. Furthermore, reduction occurred in a condensed environment and not in the dispersed solar nebula. The origins and chemical evolution of other terrestrial planets are discussed in the light of evidence yielded by the chondrites. The hypothesis is advanced that each of the terrestrial planets formed by a single-stage autoreduction process operating upon primitive material similar to the Type 1 carbonaceous chondrites. In the case of the earth, autoreduction occurred at higher mean temperatures than in chondrites because of the large gravitational energy source which was involved. Accordingly, it is suggested that selective volatility played a more important part than in chondrites, and that many relatively volatile elements were lost from the earth. On the other hand the earth may have retained essentially the primordial abundances of elements which are not readily volatile under high-temperature, reducing conditions. A detailed study of the earth's chemical composition supports this hypothesis. It is possible to construct a self consistent model from the primordial abundances of elements which are not readily volatile under high-temperature reducing conditions. The model implies the presence of silicon as an important component of the earth's core. Independent evidence supporting this implication is cited. The distribution and fractionation of oxyphile non-volatile elements imply that much or all of the mantle has been subjected to complete or partial melting at some stage in its history. In contrast to the non-volatile elements, it appears that the earth has suffered strong depletion in a large number of volatile elements—Na, K, Rb, Cs, Zn, Cd, Hg, Bi, Tl, Pb, Cl, S and many others. It is suggested that loss of these elements by volatilization occurred during the primary accretion of the earth from primitive oxidised material, and that reduction, complete melting, formation of the core, and fractionation of the mantle occurred during and immediately after the primary accretion process. Studies of the abundance of siderophile elements in the mantle, of the mean oxidation state of the mantle and of the nature of the volatile components which have been degassed from the mantle show that the mantle is not and never has been in equilibrium with the core. This conclusion places an important constraint on the core-formation process. It is shown to be incompatible with the currently accepted theory that the material from which the earth accreted was composed of an intimate mixture of silicate and metal particles similar to ordinary chondrites. The formation of the earth by direct accretion and autoreduction of primitive material resulted in the generation of an enormous primitive atmosphere composed principally of CO and H2, together with the volatile elements mentioned above. It is suggested that this atmosphere subsequently escaped from the earth carrying the volatile elements mentioned above. Possible mechanisms of escape are discussed. In the terminal phase of accretion, the temperature was sufficiently high to reduce and volatilise magnesium and silicon monoxide from the infalling planetesimals and dust. At this stage, the condensed matter accreting on the earth consisted principally of metallic iron and calcium and aluminum silicates. When the primitive atmosphere was disrupted and escaped, the accompanying expansion and cooling caused precipitation of the non-volatile silicate components of the atmosphere in the form of planetesimals and smoke. Precipitation of this material, mainly as iron-poor magnesian silicates occurred in a sediment-ring around the earth. This material became mixed with primitive planetesimals possessing the composition of Type 1 carbonaceous chondrites, which had not accreted upon the earth. The sediment-ring of highly reduced magnesian silicate planetesimals and primitive oxidised planetesimals became unstable and coagulated to form the moon. The properties of the moon are discussed in terms of its formation from such material. Possible explanations of the moon's density, luminescent properties, surface heterogeniety, thermal history and stress history emerge. A possibility that stoney meteorites are derived from the moon is also discussed. It is distinctly possible that ordinary chondrites may have formed by autoreduction and fractionation which occurred when primitive Type I carbonaceous chondrite planetesimals collided with the moon during its terminal period of formation. Other theories of lunar origin are briefly reviewed. The origins and internal constitution of the other terrestrial planets are discussed. Mercury is believed to have accreted from the solar nebula at an initially high temperature, maintained by an early stage of high solar luminosity. As a result, Mercury suffered depletion of magnesian silicates which were reduced and volatilised under these conditions. The abundance of iron, which was not volatilised was correspondingly increased, resulting in a high mean density for this planet. The evolution of Venus was very similar to that of the earth. Its mean state of oxidation may be slightly higher. The material from which Venus accreted possessed a higher C/H ratio than the source material of the earth. Differing atmospheric compositions are attributable to this factor. Mars is composed of highly oxidised primordial material, with little or no metal phase. Lack of reduction is attributed to the small content of carbonaceous material in the primordial material from which Mars accreted. Physical properties and the thermal history of Mars are discussed in terms of the proposed chemical constitution and the possibility of a self consistent solution is demonstrated." @default.
• W2072792809 created "2016-06-24" @default.
• W2072792809 creator A5045652400 @default.
• W2072792809 date "1966-01-01" @default.
• W2072792809 modified "2023-10-14" @default.
• W2072792809 title "Chemical evolution of the terrestrial planets" @default.
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