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  3. "Four Stages of Planetary Development" Planetary Development → In our study of the planet Earth, we will find a four-sta

"Four Stages of Planetary Development" Planetary Development → In our study of the planet Earth, we will find a four-sta

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问题               "Four Stages of Planetary Development"
    Planetary Development
    → In our study of the planet Earth, we will find a four-stage history of planetary development. The moon and all the terrestrial planets have passed through these stages, although differences in the way the planets were altered by these stages have produced dramatically different worlds. The moon, for example, is much like Earth, but its evolution has been dramatically altered by its smaller size. As we explore the solar system, we will discover not entirely new processes but rather familiar effects working in slightly 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.
    Differentiation would have occurred easily if Earth were molten when it was young. Two sources of heat could have heated Earth. First, heat of formation would be created by in-falling material. A meteorite hitting Earth at high velocity converts most of its energy of motion into heat, and the in-falling of a large number of meteorites could release tremendous heat. If Earth formed rapidly, this heat would have accumulated much more rapidly than it could leak away, and Earth may have been 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 would have begun to accumulate and could have helped melt Earth to facilitate differentiation. Most of Earth’s radioactive elements are now concentrated in the crust, where they continue to warm and soften the rock layers.
    Earth formed by material falling together, but meteorites could have left no trace until a crust solidified. Once Earth had a hard surface, the meteorites could form craters. This second stage in planetary evolution, cratering, was violent. The heavy bombardment was intense because the solar nebula was filled with rocky and icy debris, and 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 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 we compare other worlds with Earth, we will discover traces of this intense period of cratering, the heavy bombardment, 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 caused by radioactive decay allowed molten rock just below the crust to well up through fissures and flood the deeper basins. We will discuss such flooded basins 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 activecrust and erosion.
    Earth as a Planet
    All terrestrial planets pass through these four stages, so in that respect, Earth is a good basic reference planet for comparative planetology. Some planets have emphasized one stage over another, and some planets have failed to progress fully through the four stages. Nevertheless, Earth is a good standard of comparison. Every major process on any rocky world in our solar system is represented in some form on Earth.
    On the other hand, 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 and no other planet in our solar system is known to have such extensive liquid water on its surface. Furthermore, some of the matter on the surface of this world is alive, and a small part of that living matter is aware. We do not know 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.
    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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