Ocean and Atmosphere on Early Earth
Where did Earth’s ocean water come from? Scientists agree that a large amount of water must have arrived during planet, accretion, which is the process of collision and sticking together of the material in orbit around the Sun by which Earth and the other planets in the solar system formed 4.6 billion years ago. Perhaps significant volumes were added during the period of heavy bombardment, during which Earth was being hit by a large number of comets (whose nuclei are composed mostly of ice) and other objects left over from the formation of the planets. The volume of water eventually found on Earth may be related to the formation of Earth’s core (innermost layer). When the iron- and nickel-rich core formed, most of the water in the forming planet was consumed in oxidation processes whereby the oxygen component of water was used to make iron and nickel oxides. It is the residual water that make up the oceans. Perhaps that residual quantity was significantly enhanced by water carried by comets after Earth’s initial formation, perhaps not .1n either case, the oceans reached approximately their present volume by 3.8 billion years ago. But this does not mean they were at their present area. Geologist Don Lowe has estimated that before 3 billion years ago, less than 5 percent of Earth’s surface was land. Earth’s atmosphere was also very different from that of today. There was no oxygen, and there was a great d.1 more carbon dioxide (Co2)—perhaps 100 to 1,000 times as much as today.
完整版题目和答案请付费后查阅
