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　　托福閱讀原文：

　　Elements of Life

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.

　　托福閱讀翻譯：

　　生命的元素

　　生命的創(chuàng)造需要一套用于制造細胞成分的化學元素。“地球上的生命”使用了92種天然化學元素中的25種，盡管這些元素中只有4種：氧，碳，氫和氮——構成了大約96%的活生物體。因此，生命的第一個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，幾乎所有的世界都可以滿足這個要求?？茖W家們已經確定，除了氫和氦(以及微量的鋰)之外，宇宙中的所有化學元素也都是由恒星產生的。這些被稱為重元素，因為它們比氫和氦重。盡管與氫和氦相比，所有這些重元素都很罕見，但它們幾乎遍布各處。

　　重元素不斷由恒星制造并通過恒星死亡釋放到太空中，因此它們的量與氫和氦相比隨著時間的推移逐漸增加。重元素占我們太陽系化學含量的百分之二(按質量計)，另外百分之九十八是氫和氦。在一些非常古老的恒星系統(tǒng)中，這些恒星系統(tǒng)在許多重元素出現(xiàn)之前形成，重元素份額可能低于0.1%。盡管如此，所研究的每個恒星系統(tǒng)都至少有一定數(shù)量的生命使用的元素。而且，當星子——在早期的太陽系中形成的，可以積聚組成行星的小的固體物質——凝聚成一個穩(wěn)定的行星系統(tǒng)，它們不可避免地由重元素制成，因為更常見的氫和氦氣保持氣態(tài)。因此，任何地方的星子都應該包含生命所需要的元素，這意味著由星子組成的——行星、衛(wèi)星、小行星和彗星等構成的物體——也包含這些元素。太陽系形成的本質解釋了為什么地球包含生命所需的所有元素，這就是為什么我們期望這些元素存在于整個太陽系，星系和宇宙中的其他世界。

　　請注意，即使我們允許生命與地球上的生命截然不同，這個論點也不會改變。地球上的生命是以碳為基礎的，大多數(shù)生物學家認為其他地方的生活也可能以碳為基礎。但是，我們不能完全排除使用另一種化學基礎如硅或氮的生命的可能性?；谀承┢渌氐纳褂玫脑?或其相對比例)可能與地球上基于碳的生命所使用的元素有所不同。但這些元素仍然是恒星的產物，并且仍然會在各處的星子中出現(xiàn)。無論我們在尋找什么樣的生物，我們都可能在宇宙中的幾乎每個行星，月球，小行星和彗星上找到必要的元素。

　　一個更嚴格的要求是分子中存在這些元素，這些元素可以作為現(xiàn)成的生命基石，就像早期的地球可能有氨基酸和其他復雜分子的有機湯一樣。地球的有機分子可能來自三種來源的某一種組合：大氣中的化學反應，海洋深海通風口附近的化學反應以及小行星和彗星攜帶到地球的分子。前兩個來源分別只能出現(xiàn)在有大氣或海洋的世界上。 但是第三個來源應該給我們的太陽系中的幾乎所有世界帶來類似的分子。

　　對隕石和彗星的研究表明，有機分子在小行星和彗星中都很普遍。 由于太陽系中的每個個體在被稱為重度轟炸的時期(大約40億年前)一再遭到小行星和彗星的襲擊，所以每個個體至少應該接受一些有機分子。然而，這些分子往往被不受大氣保護的表面上的太陽輻射破壞。此外，盡管這些分子可能會在表面下保持完整(因為它們明顯對小行星和彗星有影響)，但除非某種液體或氣體可用于移動它們，否則它們可能無法相互反應。因此，如果我們將研究范圍限制在僅有有機分子可能參與化學反應的世界上，我們可以排除任何缺乏大氣和表面或地下液體介質(如水)的世界。

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