本篇文章共分 6 段，針對科學界探討宇宙生命存在所需要的基本化學元素，以及這些元素存在的地區與如何從基本元素形成生命所需要的化合物做有系統的整理與說明。

本篇考題英文原文與對應之中文翻譯整理如下。練習作答解題時若有對語意不清楚之處，請仔細查閱對照，以提升閱讀理解能力。

Elements of Life 生命的要素

1. 構成生命的基本化學元素

   The mounting evidence for age genes that influence the aging process is by no means conclusive, but it is quite impressive, coming from a variety of independent research from aging in worms and fruit flies to antioxidants and gene repair mechanisms, and human mutations. Still, the connections are circumstantial.

   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.

   生命的創造需要一套化學元素來製造細胞的組成部分。地球上的生命使用 92 種自然發生的化學元素中的大約 25 種，儘管這些元素中光是 4 種—氧、碳、氫和氮—就佔了生物體質量的大約 96%。因此，生命的第一個要求可能就是生命所使用的大部分或全部元素的存在。

2. 生命元素無所不在

   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.

   有趣的是，這個要求可能幾乎可以由任何世界來滿足。科學家已經確定，宇宙中除了氫和氦（以及微量的鋰）之外的所有化學元素都是由恆星產生的。這些被稱為重元素，因為它們比氫和氦更重。儘管與氫和氦相比，所有這些重元素都相當罕見，但它們幾乎到處都能找到。

3. 重元素的形成過程與分布

   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.

   重元素不斷被恆星製造出來，並通過恆星的死亡釋放到太空中，因此它們與氫和氦相比，其數量隨著時間的推移逐漸增加。重元素在我們太陽系的化學成分（按質量）中約占 2%，其他 98% 是氫和氦。在一些非常古老的恆星系統中，它們在許多重元素產生之前就已經形成了，重元素的比例可能低於 0.1%。然而，每個被研究的恆星系統都至少有一定數量的生命所使用的所有元素。此外，當微星—在早期太陽系中形成的、可能積累成行星的小型固體物體—在一個正在形成的恆星系統中凝結時，它們不可避免地由重元素構成，因為更常見的氫和氦仍然是氣態的。因此，任何地方的微星應該都含有生命所需的元素，這意味著由微星建造的物體—行星、衛星、小行星和彗星—也含有這些元素。太陽系形成的性質解釋了為什麼地球包含生命所需的所有元素，這也是為什麼我們預計這些元素會存在於整個太陽系、銀河系和宇宙的其他世界。

4. 非地球生命類型所需之元素

   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.

   請注意，即使我們允許存在與地球上的生命非常不同的生命，這一論點也不會改變。地球上的生命是以碳為基礎的，大多數生物學家認為，其他地方的生命也可能是以碳為基礎的。然而，我們不能絕對排除有另一種化學基礎的生命的可能性，如矽或氮。基於其他一些元素的生命所使用的元素集（或其相對比例）可能與地球上的碳基生命所使用的元素集有些不同。但是這些元素仍然是恆星的產物，仍然會存在於各地的微星上。無論我們在尋找什麼樣的生命，我們都有可能在宇宙中幾乎每一顆行星、月球、小行星和彗星上找到必要的元素。

5. 生命元素的化學反應

   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.

   一個更嚴格的要求是，這些元素要存在於可以作為生命現成構件的分子中，就像早期地球可能有氨基酸和其他複雜分子的有機湯。地球上的有機分子可能來自三個來源的某種組合：大氣中的化學反應，海洋中深海噴口附近的化學反應，以及由小行星和彗星攜帶到地球的分子。前兩個來源只可能分別發生在有大氣層或海洋的世界上。但是第三種來源應該已經把類似的分子帶到我們太陽系的幾乎所有世界中。

6. 生命元素需要氣態或液態環境

   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.

   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。因為太陽系中的每個天體在被稱為重度轟擊的時期（大約 40 億年前）都被小行星和彗星反覆撞擊過，每個天體都應該至少收到一些有機分子。然而，這些分子往往在沒有大氣層保護的表面上被太陽輻射破壞。此外，雖然這些分子可能在表面下保持完整（正如它們在小行星和彗星上顯然呈現的那樣），但除非有某種液體或氣體可以移動它們，否則它們可能無法相互反應。因此，如果我們把搜索範圍限制在有機分子有可能參與化學反應的世界上，我們大概可以排除任何既沒有大氣層又沒有表面或地下比如水般液體介質的世界。