The Sun is the hub of a huge rotating system consisting of nine planets, their satellites, and numerous small bodies, including asteroids, comets, and meteoroids. An estimated 99.85 percent of the mass of our solar system is contained within the Sun, while the planets collectively make up most of the remaining 0.15 percent. The planets, in order of their distance from the Sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Under the control of the Sun's gravitational force, each planet maintains an elliptical orbit and all of them travel in the same direction.
The planets in our solar system fall into two groups: the terrestrial (Earth-like) planets (Mercury, Venus, Earth, and Mars) and the Jovian (Jupiter-like) planets (Jupiter, Saturn, Uranus, and Neptune). Pluto is not included in either category, because its great distance from Earth and its small size make this planet's true nature a mystery.
The most obvious difference between the terrestrial and the Jovian planets is their size. The largest terrestrial planet, Earth has a diameter only one quarter as great as the diameter of the smallest Jovian planet, Neptune, and its mass is only one seventeenth as great. Hence, the Jovian planets are often called giants. Also, because of their relative locations, the four Jovian planets are known as the outer planets, while the terrestrial planets are known as the inner planets. There appears to be a correlation between the positions of these planets and their sizes.
Other dimensions along which the two groups differ markedly are density and composition. The densities of the terrestrial planets average about 5 times the density of water, whereas the Jovian planets have densities that average only 1.5 times the density of water. One of the outer planets, Saturn, has a density of only 0.7 that of water, which means that Saturn would float in water. Variations in the composition of the planets are largely responsible for the density differences. The substances that make up both groups of planets are divided into three groups—gases, rocks, and ices—based on their melting points. The terrestrial planets are mostly rocks: dense rocky and metallic material, with minor amounts of gases. The Jovian planets, on the other hand, contain a large percentage of the gases hydrogen and helium, with varying amounts of ices: mostly water, ammonia, and methane ices.
The Jovian planets have very thick atmospheres consisting of varying amounts of hydrogen, helium, methane, and ammonia. By comparison, the terrestrial planets have meager atmospheres at best. A planet's ability to retain an atmosphere depends on its temperature and mass. Simply stated, a gas molecule can "evaporate" from a planet if it reaches a speed known as the escape velocity. For Earth, this velocity is 11 kilometers per second. Any material, including a rocket, must reach this speed before it can leave Earth and go into space. The Jovian planets, because of their greater masses and thus higher surface gravities, have higher escape velocities (21-60 kilometers per second) than the terrestrial planets. Consequently, it is more difficult for gases to "evaporate" from them. Also, because the molecular motion of a gas depends on temperature, at the low temperatures of the Jovian planets even the lightest gases are unlikely to acquire the speed needed to escape. On the other hand, a comparatively warm body with a small surface gravity, like Earth's moon, is unable to hold even the heaviest gas and thus lacks an atmosphere. The slightly larger terrestrial planets Earth, Venus, and Mars retain some heavy gases like carbon dioxide, but even their atmospheres make up only an infinitesimally small portion of their total mass.
The orderly nature of our solar system leads most astronomers to conclude that the planets formed at essentially the same time and from the same material as the Sun. It is hypothesized that the primordial cloud of dust and gas from which all the planets are thought to have condensed had a composition somewhat similar to that of Jupiter. However, unlike Jupiter, the terrestrial planets today are nearly void of light gases and ices. The explanation may be that the terrestrial planets were once much larger and richer in these materials but eventually lost them because of these bodies' relative closeness to the Sun, which meant that their temperatures were relatively high.
1. According to the passage, each of the following statements comparing terrestrial planets with Jovian planets is true EXCEPT:
○Terrestrial planets are closer to the Sun than Jovian planets.
○Terrestrial planets have smaller diameters than Jovian planets.
○Terrestrial planets have smaller masses than Jovian planets.
○Terrestrial planets travel in a different direction than Jovian planets do.
Paragraph 4: Other dimensions along which the two groups differ markedly are density and composition. The densities of the terrestrial planets average about 5 times the density of water, whereas the Jovian planets have densities that average only 1.5 times the density of water. One of the outer planets, Saturn, has a density of only 0.7 that of water, which means that Saturn would float in water. Variations in the composition of the planets are largely responsible for the density differences. The substances that make up both groups of planets are divided into three groups—gases, rocks, and ices—based on their melting points. The terrestrial planets are mostly rocks: dense rocky and metallic material, with minor amounts of gases. The Jovian planets, on the other hand, contain a large percentage of the gases hydrogen and helium, with varying amounts of ices: mostly water, ammonia, and methane ices.
2. The word markedly in the passage is closest in meaning to
○Essentially
○Typically
○Consistently
○noticeably
3. Paragraph 4 mentions which of the following as a reason why terrestrial planets are dense?
○They are made up of three groups of substances.
○They are composed mainly of rocky and metallic materials.
○They contain more ice than Jovian planets.
○They contain relatively small amounts of water.
4. Paragraph 4 supports each of the following statements about Saturn EXCEPT:
○It is less dense than any of the terrestrial planets.
○It contains no rocky material.
○It contains ices.
○It contains a large percentage of gases.
Paragraph 5: The Jovian planets have very thick atmospheres consisting of varying amounts of hydrogen, helium, methane, and ammonia. By comparison, the terrestrial planets have meager atmospheres at best. A planet's ability to retain an atmosphere depends on its temperature and mass. Simply stated, a gas molecule can "evaporate" from a planet if it reaches a speed known as the escape velocity. For Earth, this velocity is 11 kilometers per second. Any material, including a rocket, must reach this speed before it can leave Earth and go into space. The Jovian planets, because of their greater masses and thus higher surface gravities, have higher escape velocities (21-60 kilometers per second) than the terrestrial planets. Consequently, it is more difficult for gases to "evaporate" from them. Also, because the molecular motion of a gas depends on temperature, at the low temperatures of the Jovian planets even the lightest gases are unlikely to acquire the speed needed to escape. On the other hand, a comparatively warm body with a small surface gravity, like Earth's moon, is unable to hold even the heaviest gas and thus lacks an atmosphere. The slightly larger terrestrial planets Earth, Venus, and Mars retain some heavy gases like carbon dioxide, but even their atmospheres make up only an infinitesimally small portion of their total mass.
5. The word meager in the passage is closest in meaning to
○rich
○thin
○unique
○complex
6. According to paragraph 5, which of the following statements is true of both Jovian and terrestrial planets?
○ The thicker the atmosphere, the smaller the planet’s mass
○ The more varied the gases in the atmosphere, the higher the temperature
○ The higher the surface gravity, the higher the escape velocity
○ The less the atmosphere contributes to the total mass, the lower the temperature
7. According to paragraph 5, what is a major reason that Jovian planets have much thicker atmospheres than terrestrial planets do?
○ Jovian planets have lower surface gravities
○ Jovian planets have lower temperatures
○ Jovian planets have lower escape velocities
○ Jovian planets’ gas molecules have higher average speeds
8. Paragraph 5 supports which of the following statements about the ability of planets to retain gases?
○More-massive planets are less able to retain gases than less-massive ones.
○Planets are more likely to retain heavy gases than light gases.
○Jovian planets are unlikely to retain the lightest gases.
○Only terrestrial planets have been able to retain carbon dioxide.
Paragraph 6: The orderly nature of our solar system leads most astronomers to conclude that the planets formed at essentially the same time and from the same material as the Sun. It is hypothesized that the primordial cloud of dust and gas from which all the planets are thought to have condensed had a composition somewhat similar to that of Jupiter. However, unlike Jupiter, the terrestrial planets today are nearly void of light gases and ices. The explanation may be that the terrestrial planets were once much larger and richer in these materials but eventually lost them because of these bodies' relative closeness to the Sun, which meant that their temperatures were relatively high.
9. In calling the cloud of gas and dust from which the Sun and all the planets are thought to have condensed "primordial,' the author means that the cloud was
○immense in size
○composed of similar particles
○present at the very beginning of our solar system's formation
○created from a great variety of different materials
10. The word eventually in the passage is closest in meaning to
○over time
○long ago
○simply
○certainly
11. According to paragraph 6, what is a possible explanation for the lack of light gases and ices on terrestrial planets?
○The location of terrestrial planets caused them to lose some of the materials they once contained.
○Terrestrial planets were formed much later than Jovian planets.
○The composition of terrestrial planets was different from that of Jupiter.
○Terrestrial planets were formed out of different material than the Sun was.
Paragraph 4: Other dimensions along which the two groups differ markedly are density and composition. The densities of the terrestrial planets average about 5 times the density of water, whereas the Jovian planets have densities that average only 1.5 times the density of water. One of the outer planets, Saturn, has a density of only 0.7 that of water, which means that Saturn would float in water. Variations in the composition of the planets are largely responsible for the density differences. ■The substances that make up both groups of planets are divided into three groups—gases, rocks, and ices—based on their melting points. ■The terrestrial planets are mostly rocks: dense rocky and metallic material, with minor amounts of gases. ■The Jovian planets, on the other hand, contain a large percentage of the gases hydrogen and helium, with varying amounts of ices: mostly water, ammonia, and methane ices. ■
12. Look at the four squares [■] that indicate where the following sentence could be added to the passage.
This explains their relatively low densities.
Where would the sentence best fit?
13. Directions: From the seven answer choices below, select the two phrases that correctly characterize the terrestrial planets and the three phrases that correctly characterize the Jovian planets. Drag each phrase you select into the appropriate column of the table. Two of the phrases will NOT be used. This question is worth 3 points.
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terrestrial planets
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Jovian planets
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●
●
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●
●
●
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Answer Choices
○Have relatively small sizes
○Are grouped in the same category as Pluto
○Contain relatively high proportions of ices
○Have relatively high temperatures
○Have densities that are generally lower than the density of water
○Have relatively high escape velocities
○Have a composition closer to that of the cloud from which they condensed terrestrial
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參考答案：
1. ○4
2. ○4
3. ○2
4. ○2
5. ○2
6. ○3
7. ○2
8.○2
9. ○3
10. ○1
11. ○1
12. ○4
13. ○1,4 ○3, 6,7
九大行星、它們的衛(wèi)星以及數(shù)以億計(jì)的小天體，包括小行星、彗星和隕星，共同構(gòu)成了一個(gè)巨大的公轉(zhuǎn)系統(tǒng)而太陽(yáng)是這個(gè)公轉(zhuǎn)系統(tǒng)的核心。太陽(yáng)系中，太陽(yáng)的質(zhì)量占大約99.85%，而所有行星的質(zhì)量加起來(lái)占剩下的0.15%。九大行星按照距離太陽(yáng)的遠(yuǎn)近依次為：水星、金星、地球、火星、木星、土星、天王星、海王星和冥王星。在太陽(yáng)引力的作用下，每個(gè)行星都沿著橢圓形的軌道，按照相同的方向公轉(zhuǎn)。
太陽(yáng)系中九大行星分為兩類(lèi)：類(lèi)地行星（和地球類(lèi)似，包括水星、金星、地球和火星）和類(lèi)木行星（與木星類(lèi)似，包括木星、土星、天王星和海王星）。冥王星不屬于這兩類(lèi)中的任何一個(gè)，因?yàn)樗嗟厍蚝苓h(yuǎn)且體積較小，所以目前冥王星的真實(shí)形態(tài)仍然是個(gè)謎。
類(lèi)地行星和類(lèi)木行星最為明顯的差別就在于它們的體積。比如最大的類(lèi)地行星地球的直徑僅僅是最小的類(lèi)木行星海王星的四分之一，而質(zhì)量更是只有海王星的1/17。因此，類(lèi)木行星通常又被稱為巨行星。又因這四顆類(lèi)木行星與地球的相對(duì)位置，它們也被稱為外行星，而類(lèi)地行星則相應(yīng)被稱作內(nèi)行星。這表示行星的位置與體積之間是有關(guān)聯(lián)的。
兩類(lèi)行星其它方面的區(qū)別中，比較顯著的是密度和構(gòu)成成分。類(lèi)地行星的平均密度大約為水的5倍，而類(lèi)木行星的平均密度大概只有水的1.5倍。外行星中土星的密度只有水的0.7倍，也就是說(shuō)土星可以浮在水上。行星的構(gòu)成成分不同很大程度上是因?yàn)槊芏炔町�。兩�?lèi)行星的構(gòu)成物質(zhì)根據(jù)熔點(diǎn)可以劃分為三類(lèi)——氣體、巖石和冰。類(lèi)地行星多數(shù)由巖石（致密巖石和金屬材料）以及少量氣體構(gòu)成。而類(lèi)木行星恰恰相反，包含較大比例的氣態(tài)氫和氦，以及各種形態(tài)的冰（大部分是水、氨和甲烷冰）。
類(lèi)木行星有非常致密的大氣層，主要由變化量的氫、氦、甲烷和氨組成。相比之下，類(lèi)地行星的大氣層則要稀薄得多。一個(gè)行星保持大氣的能力取決于其溫度和質(zhì)量。簡(jiǎn)單來(lái)說(shuō)，如果氣體達(dá)到逃逸速度，那么氣體分子可以從行星上“蒸發(fā)”。地球的逃逸速度大約為11千米/秒。任何物質(zhì)，包括火箭，要離開(kāi)地球進(jìn)入太空就必須達(dá)到這個(gè)速度。由于類(lèi)木行星的質(zhì)量較大并因此產(chǎn)生更高的表面引力，因此，類(lèi)木行星的逃逸速度（21～60千米/秒）要比類(lèi)地行星高得多。所以，氣體從類(lèi)木行星的表面“蒸發(fā)”就更為困難。同時(shí)又因?yàn)闅怏w分子運(yùn)動(dòng)取決于溫度，所以在類(lèi)木行星這樣的低溫環(huán)境下，即使是最輕的氣體也無(wú)法達(dá)到所需要的逃逸速度。而從另一個(gè)角度講，一個(gè)相對(duì)溫暖表面引力很小的天體，比如月球，甚至無(wú)法留住最重的氣體，因此沒(méi)有大氣層。體積稍大的類(lèi)地行星，比如地球、金星和火星，保持了二氧化碳等一部分較重的氣體，但即便如此，大氣構(gòu)成也只占它們總質(zhì)量的很小一部分。
太陽(yáng)系有序的性質(zhì)使得大部分天文學(xué)家得出結(jié)論：行星基本形成于同一時(shí)間并且構(gòu)成物質(zhì)與太陽(yáng)相同。天文學(xué)家們推測(cè)，所有行星原始狀態(tài)的塵埃和氣體形成的云狀物凝聚，合成物與木星的有些類(lèi)似。然而，和木星不同的是，如今類(lèi)地行星上的輕質(zhì)氣體和冰極度缺乏。有一種解釋認(rèn)為，類(lèi)地行星曾經(jīng)體積更大并且物質(zhì)構(gòu)成上更為豐富多樣，但因?yàn)樗鼈兙嗵?yáng)較近致使溫度相對(duì)較高而最終失去這些物質(zhì)。
冥王星于2006年被降級(jí)為矮行星，九大行星修訂為八大行星，海王星仍屬太陽(yáng)系。
