机经真题 1 Passage 2

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Cryovolcanism on Titan

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It can be inferred from paragraph 1 that the existence of cryovolcanism on Titan would depend on which of the following?

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click one different oval.

  • A
    The flow of water from the surface to the interior
  • B
    The mixing of ammonia and methane
  • C
    Eruption of molten rock
  • D
    Heat produced by gravity and radioactivity
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正确答案: D

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  • Titan, the largest moon orbiting the planet Saturn, has long been thought to be a likely place for volcanic activity. Volcanic activity requires internal heat. Titan's large size and substantial density suggest that plenty of gravitational and radioactively generated energy is available for melting its interior. Titan has a substantial layer of water-ammonia liquid lying beneath its surface. Therefore, unlike volcanism on Earth, which is the eruption of molten (melted) rock, volcanism on Titan would be cryovolcanism, which is essentially the eruption of icy water, sometimes mixed with other materials-likely ammonia and methane in Titan's case



    Titan's thick atmosphere is about ninety-five percent nitrogen, with a few percent of methane. The methane in Titan's atmosphere is broken down by sunlight so that it recombines with other constituents of the atmosphere, forming organic compounds such as ethane.For this process to continue, the methane must somehow be replenished. One thought is that large liquid bodies on the surface (perhaps liquid methane or ethane) could re-supply the atmospheric methane; at Titan's temperatures (-176°C at the surface), methane behaves much like water on Earth. The Cassini spacecraft, which orbited Saturn and studied the planet and its moons, revealed large bodies of liquid on Titan's surface, perhaps enough to replenish the atmospheric methane, but another possibility is that cryovolcanism supplies methane and other gases to the atmosphere.



    Cassini results suggest that cryovolcanism has indeed been a significant geological process on Titan. The craft carries a radar instrument that can peer through the clouds and haze to the never-before seen surface. It showed that several large liquid flows were spread across Titan's frigid landscape. Some, particularly those that appear to come out of craters (surface depressions), are likely to be cryovolcanic, though some researchers argue that some of these flows could possibly be rivers. Titan's surface has fluvial (river) activity, as shown by plenty of branched channels, indicating that rivers of liquid methane run there. Cryovolcanism can also cause flows, so the challenge is to identify which process caused a particular flow deposit. Some of the flows seen in the radar images are more likely to be cryovolcanic than fluvial, particularly those that appear to come out of craters. The craters are elongated rather than circular, indicating origin by volcanic eruptions rather than by impact (collision with objects from space). The association of flows with non-impact craters is hard to explain by any process other than volcanism. Titan may still be actively cryovolcanic: Cassini observed period brightening of infrared light at two locations that could not be explained by changes in cloud cover. It is possible that active cryovolcanism, perhaps in the form of the release of gas, causes the brightness changes. When the radar instrument observed these locations, they showed flow features that could be due to cryovolcanism.



    Whether or not Titan is currently actively volcanic, it is likely that it was in the past. The Huygens spacecraft, which landed on Titan on January 14, 2005, obtained other evidence that cryovolcanism may have occurred on Titan. Although the amazing surface images did not show any features that could be unambiguously interpreted as cryovolcanic, Huygens did make a surprising finding. It detected a variant of the element argon in Titan's atmosphere. This variant is formed from the element potassium, and its presence in the quantities measured means that the atmosphere must be in communication with a reservoir of potassium. Titan is large enough to have differentiated, that is, it evolved into compositionally distinct layers, with the denser materials sinking to the center. Therefore, it is likely that most of the potassium-bearing material is in the rocks that form Titan's core (center). Cryovolcanism would be one means by which this material might be brought to the surface.


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    解析

    【题型】推理题

    【答案】D

    【解析】

    A. 这个选项是错误的,因为段落中并没有说水需要从表面流到内部才能导致火山活动。相反,段落强调内部液态水和氨的存在,但没有提到水需要从表面流动到内部。

    B. 段落确实提到了氨和甲烷的存在,但这不是火山活动的必要条件。段落主要关注的是热量产生和内层液体的存在,而不是化学成分的混合来推动火山活动。

    C. 这一选项是错误的,因为它指的是地球上的火山活动,而不是泰坦上的散发冷浆现象(cryvolcanism)。段落特别指出泰坦上的火山活动是与地球不同的,因此这不是适用的条件。

    D. 这是正确答案。段落强调了泰坦有足够的重力和放射性生成能量来融化它的内部提供火山活动所需的热量。这个热量是导致泰坦上火山活动的关键因素。

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