The earth was born in the solar system about 4500 million years ago. Since then it has undergone such changes as the formation of the core and atmosphere and mantle-crust differentiation, finally evolving into its present state. It would be no exaggeration to say that the intrinsic direction of this evolution had already taken shape when the earth broke away from the solar nebula to become a planet. For instance, suppose that in line with conventional thought the formation of the earth was completed after the dissipation of the solar nebula. In this case heat would have been easily released from the earth into space as infrared radiation. It is predicted that under these conditions the earth would have had a relatively low temperature. Suppose on the other hand that the formation of the earth reached completion before the dissipation of the solar nebula, as a Kyoto group of scientists headed by C. Hayashi has maintained recently. This would mean that the gravitational force of the earth would attract a thick primary atmosphere from the solar nebula. It is predicted that this primary atmosphere would have prevented infrared radiation from the earth, and that the earth would have reached quite a high temperature in its early stages. An understanding of the environment that produced the earth, i.e., an understanding of the early solar system, is essential to understand the evolution of the earth.
KeywordsSolar System Terrestrial Planet Parent Body Solar Nebula Carbonaceous Chondrite
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