TPO 16 -3Planets in Our Solar System（自然科学--天文）

字数：719

建议做题时间 20分钟

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.

题目

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答案解析

1. 事实信息否定题

解析：terrestrial, Jovian在第2段中才出现，根据3-5段的开头句可知这两段在说这两者之间的差异。A对应第3段Also, because of...这句，说明J行星在外，t行星在内，那么t离太阳更近，A对。B对应第3段前两句，t最大的直径只有j最小直径的1/4，说明t比j小，B对。C对应第3段第2句，t的质量只有j的1/7，C对。D与第1段最后一句相反。选D

2. 词汇题

解析：前段首句说明两者最明显的差异，这段首句说另外一个markedly差异，说明不是最明显但是也是很大的一个差异，选D。Marked显著的

3. 事实信息题

解析：根据terrestrial, dense定位到第2句，但是这句没有说明为什么t的密度高，继续往后读。Variations in the composition...这句说明是构成的不同导致密度不同，而后文又说t主要是由岩石构成，选B

4. 事实信息否定题

解析：A对应2-3句，对。B没说。CD对应最后一句。

5. 词汇题

解析：这句与上句是对比关系，上句说J的大气层thick，这句说t的大气层meager，选B

6. 事实信息题

解析：第3句说行星保持大气的能力取决于它的温度和质量，J的质量更大、重力更高、逃逸速度更高（第7句）、温度更低（第9句），那么气体更难逃逸（第4句），则大气层越厚（第1句）。AD与原文相反，B没说，C对。

7. 事实信息题

解析：根据上题的分析，J质量更大、重力更高、逃逸速度更高、温度更低，那么气体更难逃逸，则大气层越厚，所以选B

8. 事实信息题

解析：根据第6题的分析，J质量更大、重力更高、逃逸速度更高、温度更低，那么气体更难逃逸，则大气层越厚。AC反，D错在only，B对

9. 词汇题

解析：primordial原始的、原生的，选C

10. 词汇题

解析：eventually最终，与A最近

11. 事实信息题

解析：根据lack of light gases and ices定位到倒数第2句(与void of同义表达)，但这句没有解释为何缺乏。继续往后看，下句说解释可能是t planets离太阳近，所以失去了这些物质，选A

12. 句子插入题

解析：插入句说这解释了它们的低密度，说明前句是对低密度的解释/为什么会出现低密度。A没有出现低密度，A、B前后联系紧密。D前说到J包含大量气体，那么相对前句的t密度更低，所以选D

13. 事实信息题

解析：A对应第3段第2句，属于t；c对应第4段最后一句，属于j；D对应第5段倒数第3句（t与j相反），属于t；F对应第5段的第6句，属于j；G对应第6段第2句，属于j。

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