托福 新托福 54 - Elements of Life
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9.Look at the four squares [■] that indicate where the following sentence could be added to the passage. Where would the sentence best fit? Click on a square [■]to add the sentence to the passage.
To answer the question "Could life exist on other planets?" we must first look at the necessary preconditions for life..
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The creation of life requires a set of chemical elements for making the components of cells. Life on Earth uses about 25 of the 92 naturally occurring chemical elements, although just 4 of these elements—oxygen, carbon, hydrogen, and nitrogen—make up about 96 percent of the mass of living organisms. Thus, a first requirement for life might be the presence of most or all of the elements used by life.

Interestingly, this requirement can probably be met by almost any world. Scientists have determined that all chemical elements in the universe besides hydrogen and helium (and a trace amount of lithium) were produced by stars. These are known as heavy elements because they are heavier than hydrogen and helium. Although all of these heavy elements are quite rare compared to hydrogen and helium, they are found just about everywhere.

Heavy elements are continually being manufactured by stars and released into space by stellar deaths, so their amount compared to hydrogen and helium gradually rises with time. Heavy elements make up about 2 percent of the chemical content (by mass) of our solar system, the other 98 percent is hydrogen and helium. In some very old star systems, which formed before many heavy elements were produced, the heavy-element share may be less than 0.1 percent. Nevertheless, every star system studied has at least some amount of all the elements used by life.Moreover, when planetesimals—small, solid objects formed in the early solar system that may accumulate to become planets—condense within a forming star system, they are inevitably made from heavy elements because the more common hydrogen and helium remain gaseous.Thus, planetesimals everywhere should contain the elements needed for life, which means that objects built from planetesimals—planets, moons, asteroids, and comets-also contain these elements. The nature of solar-system formation explains why Earth contains all the elements needed for life, and it is why we expect these elements to be present on other worlds throughout our solar system, galaxy, and universe.

Note that this argument does not change, even if we allow for life very different from life on Earth. Life on Earth is carbon based, and most biologists believe that life elsewhere is likely to be carbon based as well. However, we cannot absolutely rule out the possibility of life with another chemical basis, such as silicon or nitrogen. The set of elements (or their relative proportions) used by life based on some other element might be somewhat different from that used by carbon-based life on Earth. But the elements are still products of stars and would still be present in planetesimals everywhere. No matter what kinds of life we are looking for, we are likely to find the necessary elements on almost every planet, moon, asteroid, and comet in the universe.

A somewhat stricter requirement is the presence of these elements in molecules that can be used as ready-made building blocks for life, just as early Earth probably had an organic soup of amino acids and other complex molecules. Earth's organic molecules likely came from some combination of three sources: chemical reactions in the atmosphere, chemical reactions near deep-sea vents in the oceans, and molecules carried to Earth by asteroids and comets. The first two sources can occur only on worlds with atmospheres or oceans, respectively. But the third source should have brought similar molecules to nearly all worlds in our solar system.

Studies of meteorites and comets suggest that organic molecules are widespread among both asteroids and comets. Because each body in the solar system was repeatedly struck by asteroids and comets during the period known as the heavy bombardment (about 4 billion years ago), each body should have received at least some organic molecules. However, these molecules tend to be destroyed by solar radiation on surfaces unprotected by atmospheres. Moreover, while these molecules might stay intact beneath the surface (as they evidently do on asteroids and comets), they probably cannot react with each other unless some kind of liquid or gas is available to move them about. Thus, if we limit our search to worlds on which organic molecules are likely to be involved in chemical reactions, we can probably rule out any world that lacks both an atmosphere and a surface or subsurface liquid medium, such as water.

生命的创造需要一套用于制造细胞成分的化学元素。“地球上的生命”使用了92种天然化学元素中的25种,尽管仅这些元素中的4种——氧,碳,氢和氮——构成了大约96%的活生物体。因此,生命的第一个要求可能是生命需要用到的大部分或全部元素的存在。 有趣的是,几乎所有的星球都可以满足这个要求。 科学家们已经确定,除了氢和氦(以及微量的锂)之外,宇宙中的所有化学元素也都是由恒星产生的。这些被称为重元素,因为它们比氢和氦重。尽管与氢和氦相比,所有这些重元素都很罕见,但它们几乎遍布各处。 重元素不断由恒星制造并通过恒星死亡释放到太空中,因此它们的量与氢和氦相比随着时间的推移逐渐增加。重元素占我们太阳系化学含量的百分之二(按质量计),另外百分之九十八是氢和氦。在一些非常古老的,形成于诸多种元素出现之前的恒星系统中,重元素份额可能低于0.1%。尽管如此,被研究过的所有恒星系统都至少有一定数量的生命使用的元素。而且,当星子——在早期的太阳系中形成的,可以积聚组成行星的小的固体物质——凝聚成一个稳定的行星系统,它们不可避免地由重元素制成,因为更常见的氢和氦气保持气态。因此,任何地方的星子都应该包含生命所需要的元素,这意味着由星子组成的物体——行星 、卫星、小行星和彗星等——也包含这些元素。太阳系形成的本质解释了为什么地球包含生命所需的所有元素,这就是为什么我们预期这些元素存在于整个太阳系,银行系乃至宇宙中的其他地方。 请注意,即使我们考虑到与地球上的生命截然不同的生命体,这个论点也不会改变。地球上的生命是以碳为基础的,大多数生物学家认为其他地方的生命也可能以碳为基础。但是,我们不能完全排除使用另一种化学基础如硅或氮的生命的可能性。基于某些其他元素的生命所使用的元素(或其相对比例)可能与地球上基于碳的生命所使用的元素有所不同。但这些元素仍然是恒星的产物,并且仍然会在各处的星子中出现。无论我们在寻找什么样的生物,我们都可能在宇宙中的几乎每个行星,月球,小行星和彗星上找到必要的元素。 一个更严格的要求是分子中存在这些元素,这些元素可以作为现成的生命基石,就像早期的地球可能有氨基酸和其他复杂分子的有机汤一样。地球的有机分子可能来自三种来源的某一种组合:大气中的化学反应,海洋深海通风口附近的化学反应以及小行星和彗星携带到地球的分子。 前两个来源分别只能出现在有大气或海洋的星球上。 但是第三个来源应该给我们的太阳系中的几乎所有星球带来类似的分子。 对陨石和彗星的研究表明,有机分子在小行星和彗星中都很普遍。由于太阳系中的每个个体在被称为重度轰炸的时期(大约40亿年前)一再遭到小行星和彗星的袭击,所以每个个体至少应该接受一些有机分子。然而,这些分子往往被不受大气保护的表面上的太阳辐射破坏。此外,尽管这些分子可能会在表面下保持完整(如它们明显在小行星和彗星表面下保持完整),但除非某种液体或气体可用于移动它们,否则它们可能无法相互反应。因此,如果我们将研究范围限制在仅有有机分子可能参与化学反应的星球上,我们可以排除任何缺乏大气和表面或地下液体介质(如水)的星球。

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最新提问
  • wx_6697
    觉得B C 意思一样,不知道选哪个
  • wx_5576
    这道题C为什么对,E为什么不对?
  • wx_5576
    B为什么不能选啊?
  • wx_6697
    TPO30 passage 2 Q5我选的 D,不明白为啥不对?
  • wx_6697
    鑫哥,TPO6passage3Q5 答案是给错了吗?好多人都选A
  • wx_6697
    这题也很容易选错选成了D
  • wx_6697
    这道题A为什么错了
  • 芊儿
    为什么这道题不选c??a中的variety不是应该对应文中的differentiating 吗??求解!
  • wx_1000
    这道题不选E是因为太细节了吗
  • 王金阁
    这个题为什么不选C啊。。。
  • 芊儿
    这道题的D选项不是和文中的better able to reproduce in open settings相对应么??
  • 风荨火
    有大佬解释一下这个为啥选D嘛?
  • 以沫
    请问这个D 在哪里提现?为什么D错?
  • 芊儿
    第六题 的C选择为什么不对,感觉A是明显驳斥啊...
  • wx_6697
    鑫哥,这道题D是从哪里看出来的
  • wx_6697
    这题选的A,根据是Joly’s calculations clearly supported those geologists who insisted on an age for Earth far in excess of a few million years.想问鑫哥为啥不选A
  • wx_6697
    这题我选的是C依据是into a new habitat outside of its natural range, it may adapt to the new environment and leave its enemies behind.C为啥错了呢?鑫哥
  • wx_8861
    F选项的weather-related destruction在哪里体现了呢?原文最后一段的开头Among the costs里的costs是不是打错了?应该是coast?
  • wx_6697
    求问这道题B为啥不选,原文依据:viable seeds of pioneer species can be found in large numbers on some forest floors.
  • 与托福的斗争史
    与托福的斗争史 去解答 去解答
    这题为什么选C?
  • 小雨淅沥哗啦的下
    小雨淅沥哗啦的下 去解答 去解答
    B哪里错了
  • 小雨淅沥哗啦的下
    小雨淅沥哗啦的下 去解答 去解答
    B为啥不对
  • 李浩然
    B选项错误,是因为残缺么?
  • wx_100
    请问在做题的时候如何排除c呢。看了答案,感觉是该选a的,但是当时做题脑子一热,就特别钟爱c,也没看其他选项。。求敲醒。。
  • wx xxxxx
    请问鑫哥,这段开头有写As one pesticide replaces another为什么不是对应a new pesticide is developed?
  • wx_7695
    鑫哥,从哪里看出来这个masks 不是use呀,原文说了wear呀
  • haiyuqiao
    @鑫哥,这题the damage will continue 不应该对应前面的 the target species evolves resistance to it,然后As one pesticide replaces another,不应该是结束了time cycle 吗
  • wx_2065
    鑫哥,想知道E错在哪里?
  • wx_7695
    鑫哥,B选项 cannot extended to earlier geological periods. 原文说的意思是后来的进化无法估计吧
  • wx_2163
    B为什么不选
  • wx_7780
    鑫哥,这个哪里看不use了。BD是修饰错,C是无中生有,怎么能直接选出A?
  • 100
    看到第一句话,以为是中心句就选了A... 为什么不能选A呢
  • 100
    为什么选b?
  • gu33
    请问下 这里选D的原因是 因为 evolutionary approach 对应着 原文的 Rates of evolution 嘛? 这里我选了C。。不是很懂 插入句和 D的关系 求解答
  • 我是啦啦啦
    我是啦啦啦 去解答 去解答
    这个题A哪里错了?是因为主语不对吗?这个C比A多一步推理啊
  • haiyuqiao
    鑫哥,D选项里的19世纪出现了很多假设,原文中并没有提到啊
  • wx_7060
    为什么选a 呢。我觉得a是细节。F哪错了?
  • wx_1105
    我想问一下,这道题为什么不能选A呢?
  • wx_8122
    D为什么不选
  • wx_1655
    f选项哪里说了
  • chaulaw
    鑫哥,原文是below经济损害水平,D是一触发经济损害就用,这也对吗?
  • chaulaw
    interclan婚姻是对的吗?不是只在自己的family结婚扩大家族吗?
  • wx_6697
    鑫哥,这道题答案是不是错了,好多人选D 我也选的D求解答
  • wx_6697
    这道题应该是一道易错题,每个选项的都有,然而我选的A错了,求解
  • wx_6697
    鑫哥,这题的C是怎么得到的?B也没有找到啊?难道不是应该选B
  • wx_6697
    鑫哥,还有这个,好多人选A答案是不是错了
  • wx_6697
    求问D是从哪里得出来的,我选的B呀哎呀
  • wx_4185
    it is difficult to say how far they were intended to be portraits rather than generalized images 这句话怎么理解呢
  • 此楠楠
    请问下这个插入题怎么选的呢?
  • 此楠楠
    求鑫哥讲解下A选项。。。 Even though in error, Joly’s calculations clearly supported those geologists who insisted on an age for Earth far in excess of a few million years.