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  3. "Four Stages of Planetary Development" Planetary Development The planet Earth has passed through four-stages of planetary

"Four Stages of Planetary Development" Planetary Development The planet Earth has passed through four-stages of planetary

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问题 "Four Stages of Planetary Development"
   Planetary Development
   The planet Earth has passed through four-stages of planetary development. All terrestrial planets pass through these same stages to some degree, but some planets evolved further or were affected in different ways.
   The Four Stages
   The first stage of planetary evolution is differentiation, the separation of material according to density. Earth now has a dense core and a lower-density crust, and that structure must have originated very early in its history. Differentiation would have occurred easily if Earth were molten when it was young. Two sources of energy could have heated Earth. First, heat of formation was released by in-falling material. A meteorite hitting Earth at high velocity converts most of its energy of motion into heat, and the impacts of a large number of meteorites would have released tremendous heat. If Earth formed rapidly, this heat would have accumulated much more rapidly than it could leak away, and Earth was probably molten when it formed. A second source of heat requires more time to develop. The decay of radioactive elements trapped in the Earth releases heat gradually; but, as soon as Earth formed, that heat began to accumulate and helped melt Earth. That would have helped the planet differentiate.
   While Earth was still in a molten state, meteorites could leave no trace, but in the second stage in planetary evolution, cratering, the young Earth was battered by meteorites that pulverized the newly forming crust. The largest meteorites blasted out crater basins hundreds of kilometers in diameter. As the solar nebula cleared, the amount of debris decreased, and after the late heavy bombardment, the level of cratering fell to its present low level. Although meteorites still occasionally strike Earth and dig craters, cratering is no longer the dominant influence on Earth’s geology. As you compare other worlds with Earth, you will discover traces of this intense period of cratering, on every old surface in the solar system.
   The third stage, flooding, no doubt began while cratering was still intense. The fracturing of the crust and the heating produced by radioactive decay allowed molten rock just below the crust to well up through fissures and flood the deeper basins. You will find such flooded basins with solidified lava flows on other worlds, such as the moon, but all traces of this early lava flooding have been destroyed by later geological activity in Earth’s crust. On Earth, flooding continued as the atmosphere cooled and water fell as rain, filling the deepest basins to produce the first oceans. A   Notice that on Earth flooding involves both lava and water, a circumstance that we will not find on most worlds.   B
   The fourth stage, slow surface evolution, has continued for the last 3.5 billion years or more. C   Earth’s surface is constantly changing as sections of crust slide over each other, push up mountains, and shift continents. D   Almost all traces of the first billion years of Earth’s geology have been destroyed by the active crust and erosion.
   Earth as a Planet
   All terrestrial planets pass through these four stages, but some have emphasized one stage over another, and some planets have failed to progress fully through the four stages. Earth is a good standard for comparative planetology because every major process on any rocky world in our solar system is represented in some form on Earth.
   Nevertheless, Earth is peculiar in two ways. First, it has large amounts of liquid water on its surface. Fully 75 percent of its surface is covered by this liquid; no other planet in our solar system is known to have such extensive liquid water on its surface. Water not only fills the oceans but also evaporates into the atmosphere, forms clouds, and then falls as rain. Water falling on the continents flows downhill to form rivers that flow back to the sea, and in so doing, the water produces intense erosion. You will not see such intense erosion on most worlds. Liquid water is, in fact, a rare material on most planets. Your home planet is special in a second way. Some of the matter on the surface of this world is alive, and a small part of that living matter is aware. No one is sure how the presence of living matter has affected the evolution of Earth, but this process seems to be totally missing from other worlds in our solar system. Furthermore, the thinking part of life on Earth, humankind, is actively altering our planet.
   Glossary
   meteorite: a mass that falls to the surface of a planet from space planetology: the study of planets
The word its in the passage refers to
选项 A、meteorite
B、Earth
C、velocity
D、motion
答案A
解析 "A meteorite hitting Earth at high velocity converts most of its [the meteorite’s] energy of motion into heat." The pronoun "its" does not refer to Choices B, C, or D.
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