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  3. When the first of the two Viking landers touched down on Mars on July 20, 1976, and began to send camera images back to earth, t

When the first of the two Viking landers touched down on Mars on July 20, 1976, and began to send camera images back to earth, t

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问题     When the first of the two Viking landers touched down on Mars on July 20, 1976, and began to send camera images back to earth, the scientists at the Jet Propulsion Laboratory could not suppress a certain nervous anticipation, like people holding a lottery ticket that they have a one-in-a-million chance of winning. The first photographs that arrived, however, did not contain any evidence of life. What revealed itself to them was merely a barren landscape littered with rocks and boulders. The view resembled nothing so much as a flat section of desert.
    The scientists were soon ready to turn their attention from visible life to microorganisms. The twin Viking landers carried three experiments designed to detect current biological activity and one to detect organic compounds, because researchers thought it possible that life had developed on early Mars just as it is thought to have developed on earth, through the gradual chemical evolution of complex organic molecules. To detect biological activity, Martian soil samples were treated with various nutrients that would produce characteristic by-products if life forms were active in the soil. The results from all three experiments were inconclusive. The fourth experiment heated a soil sample to look for signs of organic material, but found none, an unexpected result because at least organic compounds from the bombardment of the Martian surface by meteorites were thought to have been present.
    The absence of organic materials, some scientists speculated, was the result of intense ultraviolet radiation penetrating the atmosphere of Mars and destroying organic compounds in the soil. Although Mars’ atmosphere was, at one time, rich in carbon dioxide and thus thick enough to protect its surface from the harmful rays of the sun, the carbon dioxide had gradually left the atmosphere and been converted into rocks. This means that even if life had gotten a start on early Mars, it could not have survived the exposure to ultraviolet radiation when the atmosphere thinned.
    Despite the disappointing Viking results, there are those who still keep the possibility of life on Mars open. They point out that the Viking data cannot be considered the final word on Martian life because the two landers only sampled two limited—and uninteresting—sites. The Viking landing sites were not chosen for what they might tell of the planet’s biology. They were chosen primarily because they appeared to be safe for landing a spacecraft. The landing sites were on parts of the Martian plains that appeared relatively featureless from orbital photographs.
    The type of Martian terrain that these researchers suggest may be a possible hiding place because active life has an earthly parallel: the ice-free region of southern Victoria Land, Antarctica, where the temperatures in some dry valleys average below zero. Organisms known as endoliths, a form of blue-green algae that has adapted to this harsh environment, were found living inside certain rocks in these Antarctic valleys. The argument based on this discovery is that if life did exist on early Mars, it is possible that it escaped worsening conditions by similarly seeking refuge in rocks. Skeptics object, however, that Mars in its present state is simply too dry, even compared with Antarctic valleys, to sustain any life whatsoever.
    Should Mars eventually prove to be completely barren of life, as some suspect, then this would have a significant impact on the current view of the chemical origin of life. It could be much more difficult to get life started on a planet than scientists thought before the Viking landings.
The author concludes the passage by stating that________.
选项 A、human beings could eventually discover life on Mars even if there are many difficulties
B、it is a tougher job for scientists to spot life on Mars than they expected
C、the detection of life on Mars was not a primary objective of the scientists who sent the Viking landers
D、Viking landing influences scientific theory regarding the chemical origin of life on the earth
答案B
解析 文章结尾句表示，在一个星球上发展出生命可能比很多科学家想象的要困难得多，因此B项“科学家在火星上找寻生命，是一项比他们预期要困难的工作”正确。A项“即便困难重重，人类最终能在火星上发现生命”，文中并没有提及，可排除；C项“探测火星上的生命迹象不是科学家们送维京号上火星的最初目的”，与文意刚好相反，因此也排除；D项“维京号登陆火星影响了有关地球上生命化学起源的科学理论”，根据最后一段可知，影响这一理论的是“如果火星最终被证明完全没有生命迹象”，因此D项也排除。
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