Текст "The search for extraterrestrial life."

1. Текст "The search for extraterrestrial life."

The discovery of life on another planet would be a monument to our age. Not only would it be an unparalleled technological achievement, but it would be striking scientific event that would enlarge our view of nature and ourselves and provide unique evidence bearing on the origin of life. In this article, I am going to throw light on coming search for life on Mars. Venus, our other close neighbor among the planets, has been excluded from consideration, for the time being at least, because its high surface temperature — in the neighborhood of 400° C — seems incompatible with life or, for that matter, with much organic chemistry of any kind. The planets of the solar system beyond Mars are out of reach for the present.

The Martian Environment

I have given the reason for thinking that if life ever existed on Venus, it does so no longer. What can be said about Mars? We can say that although the situation is not brimming with hope, neither is it hopeless. The Martian environment is a harsh one by terrestrial standards. The mean temperature is —55° C, compared to +15° C for the Earth. The atmosphere is thin and very dry; it contains carbon dioxide and a small amount of water vapor, but no detected oxygen. Owing to the low density of the atmosphere and the absence of a magnetic field, the surface of

Mars is bombarded by cosmic rays and solar radiation in an almost unbearable form. Mars is geologically a dead planet whose surface has been undisturbed by anything except meteorite impacts for a very long time which lacks the great variety of ecological habitatscharacterize the Earth.

Our knowledge of planetary environments is still fragmentary, and one's subjective estimate of the likelihood of finding life on Mars is liable to undergo violent fluctuations from time to time as new data accumulate. The fact is that nothing that we have learned about Mars — in contrast to Venus — excludes it as a possible abode of life. Martian temperatures are not very different from those of Antarctica, where a varied microbial life, and even a few flowering plants and invertebrate animals, have been found. Although the mean temperature on Mars is low, the seasonal and diurnal fluctuations are great, and temperatures as high as 25° C have been measured near the equator.

Scarcity of Water

The scarcity of water is probably the most serious limiting factor for any Martian biology. The atmosphere of Mars contains approximately 14 microns of precipitable water, or roughly 1/1000 the amount found in our atmosphere. In has been argued that the lack of water excludes the possibility of life as we know it on Mars. It is certainly true that no terrestrial species could survive under average Martian conditions as we know them, except in a dormantstate. But if we admit the possibility that Mars once had a more favorable climate which was gradually transformed to the severe one we find there today, and if we accept the possibility that life arose on the planet during this earlier epoch, then we cannot exclude the possibility that Martian life succeeded in adapting itself to the changing conditions and survives there still.

The phenomenon that first led astronomers to suggest that Mars is an inhabited planet—the seasonal change of color in the maria, or dark regions—is still unexplained. This effect is described as a wave of darkening that starts at the edge of the melting polar ice cap in the spring and progresses toward the equator as the season advances. The color of the maria transforms from grayish to violet, although some observers have reported vivid greens and blues. By midsummer, the wave reaches the equator; then, with the approach of winter, the color fades. It is generally agreed that the phenomenon is associated with the seasonal translocationof water vapor from one pole to the other. It could thus reflect the growth of vegetation, stimulated by the availability of water, or it could result from an inorganic process such as the uptake of water by hygroscopic salts. The biological explanation readily accounts for one striking fact; namely, that the maria continue to reappear despite the great dust storms that sometimes obscure the entire disc of the planet. This regenerative capacity suggests that something in the maria is capable of growing up through the dust layer.

Microbial life could conceivably be the only form of life on Mars, but it is hard to imagine there being life on Mars without microbes. For the purposes of fundamental biology, it would be just as striking to find microbial life on Mars as higher forms. Any form of Martian life would be intensely interesting to science. From a fundamental viewpoint, there is only one form of life on the Earth. All species are constructed out of the same few building blocks; despite appearances, the differences between species are relatively superficial. The question we ask is whether another form of life exists on Mars.

If there is life on Mars, then it is a reasonable assumption— indeed, I believe it is a necessary consequence—that its carbon cycles through this atmosphere. We would expect to find on Mars, as we find on Earth, a continual exchange of carbon between the atmosphere and the biosphere.

One of the strongest statements that can be made about Martian biology is that if there is life on the planet there must be at least one photosynthetic species. This is so because the sun is the only inexhaustible source of energy in the solar system. All life on the Earth depends ultimately on those species which are capable of utilizing solar energy. Since photosynthetic organisms must receive light from the sun, this argument leads to the corollary that, if there is life on Mars, some of it must live on the surface. There is no use imagining that if there is no life at the surface it may be found under rocks or in caves. The presence of little or no oxygen in the atmosphere of Mars does not, of course, rule out the possibility that photosynthesis is occurring there.

Summary and Conclusions

In summary, current theories about the history of the solar system and the origin of life suggest that conditions on primitive Mars may have been sufficiently like those of the primitive Earth to have made possible an independent origin of life. The present environment of Mars is extremely harsh, but our knowledge of it does not permit the conclusion that, if life ever existed there, it is now extinct.

It is not optimism about the outcome that gives impetus to the search for extraterrestrial life; rather, it is the immense importance that a positive result would have. One has to multiply the first of these somewhat subjective quantities by the second to find the scientific worth of the Mars undertaking. The argument of this article is that the value so obtained is high.

Whatever the final answer to the question of life on Mars may turn out to be, the search will be one of the great scientific and engineering enterprises of the 20th century. And I'm personally convinced that some day we'll be crowned by success.