【正文】 e crust. With the help of bombardment by ets, whose many impacts scarred and heated Earth39。s surface, the lightest silicates rose to Earth39。s surface, where they cooled more rapidly than the better insulated materials in Earth39。s interior. These lighter materials, such as the rocks we call granites, formed a layer of continental crust about 35 kilometers thick. Relative to Earth as a whole, this is as thin as an eggshell. Seafloor crust is even thinner, at about 7 kilometers。 thus, even continental crust reaches only about 1/200th of the way to Earth39。s core. Much of the early continental crust has remained on Earth39。s surface to the present day. The lightest materials of all, including gases such as hydrogen and helium, bubbled through Earth39。s interior to the surface. So we can imagine the surface of the early Earth as a massive volcanic field. And we can judge pretty well what gases bubbled up to that surface by analyzing the mixture of gases emitted by volcanoes. These include hydrogen, helium, methane, water vapor, nitrogen, ammonia, and hydrogen sulfide. Other materials, including large amounts of water vapor, were brought in by etary bombardments. Much of the hydrogen and helium escaped。 but once Earth was fully formed, it was large enough for its gravitational field to hold most of the remaining gases, and these formed Earth39。s first stable atmosphere. Paragraph 1: In order to understand the origin of Earth39。s atmosphere, we must go back to the earliest days of the solar system, before the plas themselves were formed from a disk of rocky material spinning around the young Sun. This material gradually coalesced into lumps called plaesimals as gravity and chance smashed smaller pieces together, a chaotic and violent process that became more so as plaesimals grew in size and gravitational pull. Within each orbit, collisions between plaesimals generated immense heat and energy. How violent these processes were is suggested by the odd tilt and spin of many of the plas, which indicate that each of the plas was, like a billiard ball, struck at some stage by another large body of some kind. Visual evidence of these processes can be seen by looking at the Moon. Because the Moon has no atmosphere, its surface is not subject to erosion, so it retains the marks of its early history. Its face is deeply scarred by millions of meteoric impacts, as you can see on a clear night with a pair of binoculars. The early Earth did not have much of an atmosphere. Before it grew to full size, its gravitational pull was insufficient to prevent gases from drifting off into space, while the solar wind (the great stream of atomic particles emitted from the Sun) had already driven away much of the gaseous material from the inner orbits of the solar system. So we must imagine the early Earth as a mixture of rocky materials, metals, and trapped gases, subject to constant bombardment by smaller plaesimals and without much of an atmosphere. 1. The word coalesced in the passage is closest in meaning to ? Collided ? Joined ? Changed ? Shrank 2. The word chaotic in the passage is closest in meaning to ? Rapid ? Disanized ? Intense ? Longlasting 3. All of the following are true of the plaesimals mentioned in paragraph 1 EXCEPT: ? They were formed of rocky material spinning around the early Sun. ? They collided violently with each other. ? They gradually grew in size. ? They lost their atmospheres as they were hit by larger bodies. 4. The word retains in the passage is closest in meaning to ? Reveals ? Acquires ? Hides ? Preserves 5. The author discusses the Moon in paragraph 1 in order to ? Help explain why Earth had fewer meteoric impacts than other plas in the solar system ? Show why it is difficult to understand how the first plaary atmospheres developed ? Help explain the processes that took place in the formation of large plaary bodies in the solar system ? Illustrate why the Moon39。s spin and tilt are unique among other plaary bodies in the solar system 6. The word constant in the passage is closest in meaning to ? Considerable ? Unpredictable ? Continual ? Violent Paragraph 2: As it began to reach full size, Earth heated up, partly because of collisions with other plaesimals and partly because of increasing internal pressures as it grew in size. In addition, the early Earth contained abundant radioactive materials, also a source of heat. As Earth heated up, its interior melted. Within the molten interior, under the influence of gravity, different elements were sorted out by density. By about 40 million years after the formation of the solar system, most of the heavier metallic elements in the early Earth, such as iron and nickel, had sunk through the hot sludge to the center giving Earth a core dominated by iron. This metallic core gives Earth its characteristic magic field, which has played an extremely important role in the history of our pla. 7. Paragraph 2 answers which of the following questions about early Earth ? What caused materials on Earth to bee radioactive ? What percentage of Earth39。s core was nickel ? What internal pressures caused Earth to heat up as it grew in size ? What caused Earth39。s magic field 8. According to paragraph 2, Earth39。s core is mostly iron because, pared to most other elements on early Earth, iron ? was denser ? melted more easily ? was more radioactive ? was more plentiful Paragraph 3: As heavy materials headed for the center of Earth, lighter silicates (such as the mineral quartz) drifted upward. The denser silicates formed Earth39。s mantle, a region almost 3,000 kilometers thick between the core and the crust. With the help of bombardment by ets, whose many impacts scarred and heated Earth39。s surface, the l