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.







  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世界上。 但是第三个来源应该给我们的太阳系中的几乎所有世界带来类似的分子。

  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(因为它们明显对小行星和彗星有影响),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世界上,我们可以排除任何缺乏大气和表面或地下液体介质(如水)的世界。