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  Structure and Composition of Comets

  [Paragraph 1] Astronomers now have a fairly good idea of what a comet really is. When it is far from the Sun, it is a very small object only a few kilometers 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-called 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.

  [Paragraph 2] When the comet gets even closer to the Sun, one of its most spectacular parts begins to form-the tail. Actually, there are two kinds of tails the dust tail and the ion tail. The dust tail is produced by the light from the Sun reflecting off the dust particles in the coma. A photon carries 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 orbit, it leaves a curving trail of dust behind in its path. This visible dust tail can extend for tens or hundreds of millions of kilometers out from the nucleus. The dust tail is characterized by its gently curving shape and its yellowish 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 tail extending out from the comet directly away from the Sun. Since both kinds of tails 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 tails of each kind.

  [Paragraph 4 ] Although the nucleus is of the order of a few kilometers in size, the diameter of the coma may be tens or hundreds of thousands of kilometers, the tails typically extend out tens or hundreds of millions of kilometers away from the coma.

  [Paragraph 5] A comet leaves a trail of matter behind it as it moves through the inner solar system. Some of this debris may get strewn across Earth's orbit around the Sun. When Earth passes through this part of its annual path, it sweeps through the dust trail. The particles enter Earths atmosphere at high velocity. The air friction can cause one of these bits of matter to produce a brief streak of light as it burns up in the atmosphere.

  [Paragraph 6] Since a comet loses matter on each pass by the Sun, eventually it will be depleted to the point where it is no longer visible. Comets that approach the Sun have finite lifetimes. Given the typical sizes of comets and the typical rates at which they lose matter, astronomers have concluded that the lifetimes of comets with orbits that bring them near enough to the Sun to be seen from Earth are very much shorter than the age of the solar system. Where do the new comets come from to replace the old ones that dissipate and vanish from view?

  [Paragraph 7] Dutch astronomer Jan Oort proposed that a giant cloud of matter left over from the formation of the solar system surrounds the Sun and extends out to about 50,000 astronomical units. This cloud contains large chunks of matter like the nuclei of comets. The gravitational influence of a passing star can be sufficient to perturb the orbit of one of these chunks to send it toward the inner solar system and bring it near the Sun.

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

  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

  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 tails and dust tails.

  C. Dust tails follow in the path of a comet.

  D. Ion tails have a curved shape and yellowish color.

  6. According to paragraph 3, the tails of a comet

  A. flow in the same direction as the radiation that produces them

  B. become straighter as the comet moves away from the Sun

  O C. are pulled toward the Sun

  D. can become detached from the comet head if the solar winds are strong enough

  7. 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. [Paragraph 4] Although the nucleus is of the order of a few kilometers in size, the diameter of the coma may be tens or hundreds of thousands of kilometers, the tails typically extend out tens or hundreds of millions of kilometers away from the coma.

  A. The diameter of a coma is usually larger than the tails that extend away from it while the nucleus is much smaller.

  B. The diameters of the nucleus, coma, and tail of a comet exhibit minor differences in size.

  C. The nucleus is reduced in size, while the coma and tails extend far into space

  D. Comet tails are much larger than the coma, which is much larger than the nucleus

  8. The word "strewn" in the passage is closest in meaning to

  A. combined

  B. damaged

  C. observed

  D. scattered

  9. According to paragraph 5, what happens when Earth passes through the dust trail of a comet?

  A. Dust particles burn up in Earth, s atmosphere.

  B. The comet7 s orbit is changed.

  C. The dust trail is completely destroyed

  D. The comet's tails change direction.

  10. What does paragraph 6 imply about comets that are visible from Earth?

  A. The visible comets grow brighter as they grow older

  B. The comets that are visible from Earth today have not been orbiting close to the Sun throughout the history of the solar syste m

  C. There were fewer visible comets during the early history of the solar system than there are today.

  D. The average lifetime of the visible comets is decreasing.

  11. Why does the author end paragraph 6 with a question?

  A. To indicate that certain information about comets is unknown

  B. To introduce a topic that will be clarified in the next paragraph

  C. To propose an area that requires further research

  D. To challenge theories that were presented earlier in the passage

  12. According to paragraph 7, how did astronomer Jan Oort contribute to our understanding of comets?

  A. He theorized that comets form when passing stars push chunks of debris from the outer to the inner solar system

  B. He proposed a way of comparing comets in our own solar system with those that orbit other stars.

  C. He explained that the lifetime of a comet depends on its distance from the Sun at the time the solar system was formed.

  u. oeners

  13. Look at the four squares 【】that indicate where the following sentence could be added to the passage

  For example, the so-called mystery religions, some of which originated in the formerly independent countries of Egypt, Anatolia, and Persia, attracted a number of followers in Rome.

  Where would the sentence best fit? Click on a square [] to add the sentence to the passage.

  [Paragraph 7] Although the Romans had gods of their own, they also adopted many Greek gods and myths and assimilated Greek religious beliefs and practices into a form of state religion. [A] To these gods they added their own deified emperors, in part to maintain the allegiance of the culturally diverse populations of the empire. [B] Worship of ancient gods mingled with homage to past rulers, and oaths of allegiance to the living ruler made the official religion a political duty-increasingly ritualized, perfunctory, and distant from the everyday life of the average person. [C] As a result, many Romans adopted the more personal religious beliefs of the people they had conquered. [D] These unauthorized religions flourished, despite occasional government efforts to suppress them.

  A. A

  B. B

  C. C

  D. D

  14. There were a number of different reasons for the greatness of the Roman Empire.

  Answer Choices

  A. The Romans allowed the legal and administrative structures of conquered lands to operate independently of Rome.

  B. The Romans developed a wide variety of techniques and institutions to efficiently administer their territories.

  C. Landscape and nature were not represented in the decorations inside Roman homes

  D. Greek culture had a significant impact on the art and religious beliefs of the Romans.

  E. The Romans built architectural structures that are still functional today.

  F. The Roman government made attempts to eliminate unauthorized religions

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