2019年10月13日托福阅读真题+题目+答案：Planetary Atmospheres: Venus, Earth, and Mars

2019年10月13日托福阅读真题+题目+答案：Planetary Atmospheres: Venus, Earth, and Mars

Planetary scientists have struggled to understand how three rocky planets could have turned out so differently. Why is the atmosphere on Venus 100 times as dense as Earth's, which is in turn 100 times denser than the air on Mars? During the early stages of planetary formation, all three embryonic worlds were probably very similar, their growing cores all gathering dust and asteroids (large rocks that circle the Sun) from those swirling around our young Sun. But Earth and Venus, being farther away from the gravitational pull of Jupiter, began to grow bigger than Mars. As the planets grew, their gravitational pull got stronger and the asteroids and lumps of dust rained down at higher and higher speeds until they were partially vaporized (turned into gas) on impact, probably along with a fair proportion of the planets' surface. These impacting objects were predominantly a mixture of rock and water ice. At first, the steam released on impact would drift off into space. But as the planets swelled in size, so did their gravitational pull, and eventually the gas could not escape. Soon the developing Earth and Venus were surrounded by a thick layer of hot steam. At a temperature of around 1,500 degrees centigrade, these were the first atmospheres of the inner solar system.

By this stage, Mars had already gone down a different path. With only one-tenth the mass of Earth or Venus, Mars never managed to cause impacting asteroids to bombard it fast enough for them to be vaporized. Early Mars would have had a small amount of steam and some other gases in its thin air, and we know it must have had water on its surface, but most of the volatile chemicals were still in the form of underground ice. As the red planet cooled, much of its early atmosphere ended up as a layer of frost trapped in the surface.

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1. It can be inferred from paragraph 1 that Mars ended up being smaller than Earth and Venus because

A. Venus and Earth started with denser cores

B. Mars experienced more asteroid impacts, which broke it apart

C. Venus and Earth were closer to the dust and asteroids swirling around the Sun

D. Mars's nearness to Jupiter limited its growth

2. The word "predominantly" in the passage is closest in meaning to

A. mainly

B. initially

C. usually

D. probably

3. 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.

A. The growing planets eventually pulled objects at such high speeds that the objects and probably the planets' surfaces partially vaporized on impact.

B. The planets grew more and more rapidly, with their surfaces consisting partially of dust, which had rained down together with asteroids, and gas.

C. As the planets grew in size, asteroids and lumps of dust flew towards them at higher and higher speeds, often vaporizing before they reached the planets' surface.

D. The planets grew quickly despite asteroid impacts that vaporized a fair proportion of their surfaces.

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