TPO63阅读真题+题目+答案下载：Structure and Composition of Comets

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TPO63阅读真题+题目+答案下载：Structure and Composition of Comets

[ Paragraph 1] Ast ronomers now have a fai rly good idea of what a comet really is. When it is far from the Sun, it is a very small object only a few ki lometers across It consists mainly of ices (water, methane, ammonia) with bits of dust embedded in it -a kind of dirty ice ball. As it approaches the Sun, radiation from the Sun vaporizes the icy matter and releases some of the dust. This forms a gigantic halo around the ice ball. This halo- ca lled the coma-extends out tens of thousands of kilometers from the icy core, which is the nucleus of the comet. Sunlight reflected off the dust particles makes the coma visible to observers on Earth. Ultraviolet radiation from the Sun breaks down the vapor molecules into their constituents. These components can be excited by absorbing radiation from the Sun. In returning to lower-energy states, the excited atoms and ions emit light, contributing to the luminosity of the coma.

1. Which part of a comet is described in the passage as a "dirty ice ball " 7

A. The nucleus

B. The coma

C. The dust tail

D. The ion tail

2. Select the TWO answer choices from paragraph 1 that describe changes that occur as a comet approaches the Sun. To receive credit, you must select TWO answers.

A. Water, methane, and ammonia condense into a core.

B. The icy content of the comet begins to vaporize

C. The comet' s halo decreases in size

D. Dust particles and vapor form a coma

3. The word "luminosity" in the passage is closest in meaning to

A. immensity

B. development

C. display

D. brilliance

[ Paragraph 2] When the comet gets even closer to the Sun, one of its most spectacular parts begins to fo rm-the tai l. Actually, there are two kinds of tai ls the dust tail and the ion tail. The dust tail is produced by the light from the Sun refl ecting off the dust particles in the coma. A photon ca rries momentum. In bouncing off a dust particle, it imparts a tiny, but perceptible, momentum change to the dust particle, driving it away from the coma. As the comet sweeps along its orb it, it leaves a curving trail of dust behind in its path. This visible dust tail can extend for tens or hundreds of mill ions of kilometers out from the nucleus. The dust tail is characterized by its gently curving shape and its yellowish color.

4. The word "imparts" in the passage is closest in meaning to

A. shows

B. gives

C. confirms

D. maintains

5. According to paragraphs 2 and 3, all of the following statements about comet tails are true EXCEPT:

A. Ion tails are created by ultraviolet radiation

B. Photons are active in creating both ion ta ils and dust ta ils.

C. Dust tails fo llow in the path of a comet.

D. Ion tails have a curved shape and yel lowish color.

[ Paragraph 3] A different mechanism is responsible for the ion tail. Near the Sun, ultraviolet radiation from the Sun (solar wind) ionizes and excites the atoms in the coma. As the solar wind sweeps through the coma, the high-velocity charged particles of the solar wind interact with the electrically charged excited ions in the coma, driving them away from the head of the comet. In returning to lower-energy states, these excited ions emit photons and form a luminous, bluish- colored ta il extending out from the comet directly away from the Sun. Since both kinds of tai ls are produced by radiation streaming out from the Sun, they extend out from the coma in the general direction away from the Sun. A comet may exhibit several ta ils of each kind.

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