托福阅读真题第278篇ElementsofLife（答案文章最后）
Elements of Life阿胶的作用和功效
The creation of life requires a t of chemical elements for making the components of cells. Life on Earth us about 25 of the 92 naturally occurring chemical elements, although just 4 of the 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 prence of most or all of the elements ud by life.
Interestingly, this requirement can probably be met by almost any world. Scientists have determined that all chemical elements in the univer besides hydrogen and helium (and a trace amount of lithium) were produced by stars. The are known as heavy elements becau they are heavier than hydrogen and helium. lthough all of the 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 relead into space by st
ellar deaths, so their amount compared to hydrogen and helium gradually ris 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 ud by life. Moreover, when planetesimals small, solid objects formed in the early solar system that may accumulate to become planets conden within a forming star system, they are inevitably made from heavy elements becau the more common hydrogen and helium remain gaous. Thus, planetesimals everywhere should contain the elements needed for life, which means that objects built from planetesimals planets, moons, asteroids, and comets also contain the elements. The nature of solar-system formation explains why Earth contains all the elements needed for life, and it is why we expect the elements to be prent on other worlds throughout our solar system, galaxy, and univer.
Note that this argument does not change, even if we allow for life very different from life o
n Earth. Life on Earth is carbon bad, and most biologists believe that life elwhere is likely to be carbon bad as well. However, we cannot absolutely rule out the possibility of life with another chemical basis, such as silicon or nitrogen. The t of elements (or their relative proportions) ud by life bad on some other element might be somewhat different from that ud by carbon-bad life on Earth. ut the elements are still products of stars and would still be prent 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 univer.
somewhat stricter requirement is the prence of the elements in molecules that can be ud 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-a 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. ut the third source should have brought similar
molecules to nearly all worlds in our solar system.
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Studies of meteorites and comets suggest that organic molecules are widespread among both asteroids and comets. ecau 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, the molecules tend to be destroyed by solar radiation on surfaces unprotected by atmospheres. Moreover, while the 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 arch 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.
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隔代亲
►蹭课The creation of life requires a t of chemical elements for making the components of c
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夜景人像ells. Life on Earth us about 25 of the 92 naturally occurring chemical elements, although just 4 of the 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 prence of most or all of the elements ud by life.
►Interestingly, this requirement can probably be met by almost any world. Scientists have determined that all chemical elements in the univer besides hydrogen and helium (and a trace amount of lithium) were produced by stars. The are known as heavy elements becau they are heavier than hydrogen and helium. lthough all of the heavy elements are quite rare compared to hydrogen and helium, they are found just about everywhere.
