2019年10月12日托福阅读真题+题目+答案：The Heavy Bombardment and Life on Earth

It is estimated that Earth formed around 4.5 billion years ago, yet life-even the simplest sort of microscopic life that must have started things off_did not begin right away. Until about 3.9 billion years ago, Earth was subject to an intense barrage of large objects from space, a time called the "heavy bombardment." Radiometric dating of Moon rocks shows that most of the Moon's visible impact craters, or holes formed on its surface from impacts, must have formed during this early period of the solar system's history. This is not surprising: According to our modern theory of solar system formation, the planets were built as larger and larger chunks of rock (sometimes mixed with metal or ice). or planetesimals, collided with one another. When a colliding planetesimal stuck to a growing planet, the planet got larger, increasing its gravity and allowing it to draw in even more planetesimals. Even after the planets had reached essentially their current sizes, there must still have been many planetesimals floating around; some of them still remain today, as the objects we call asteroids and comets. Those planetesimals that had orbits intersecting the orbits of the planets were doomed to eventual collisions. and most of those collisions must have occurred early in the solar system's history, when the number of planetesimals was still large. In other words, the heavy bombardment was the period of time during which impacts were most common, and the evidence from the Moon tells us that this period ended by about 3.9 billion years ago.

1.The word "estimated" in the passage is closest in meaning to

O roughly calculated

O suggested

O assumed

O proven beyond doubt

2.Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.

O There were many planetesimals left after the planets reached their current sizes, and the ones still remaining are called asteroids and comets

O The objects that we call asteroids and comets were floating around when the planets reached their current sizes

O Some of the planetesimals that remain today have reached essentially the same sizes as the planets that they are floating around

O The current sizes of the asteroids and comets that float around today are essentially the same as those of the earliest planetesimals

3.The discussion of radiometric dating in paragraph 1 suggests which of the following about the Moon?

O It may have once supported a very simple form of microscopic life

O It could not have formed the way that most other planets formed

O Its composition was originally similar to that of a comet.

O Most of its craters had been formed by about 3.9 billion years ago.

Some of the planetesimals were quite big. We have good reason to think that the Moon itself was created when a planetesimal the size of the planet Mars struck the young Earth within just 20 to 30 million years after Earth's formation. This "giant impact" is thought to have blasted rock from Earth's outer layers into space, where some of it settled into Earth's orbit and then was collected together by gravity to make the Moon.

4.According to paragraph 2. which of the following probably occurred within 20 to 30 million years of Earth's formation?

O Two planetesimals about the size of Mars collided to form the Moon.

O A planetesimal the size of Mars collided with the Moon.

O Rock blasted from Earth's outer layers collided with a planetesimal orbiting Earth.

O A major collision separated a significant amount of material from Earth, leading to the formation of the Moon.

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