When the first of the two Viking landers touched down on Martian soil 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 who hold a ticket to a lottery 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. The view resembled nothing so much as a flat section of desert —in fact, the winning entry in a contest at J. P. L. for the photograph most accurately predicting what Mars would look like as a snapshot taken in a particularly arid section of the Mojave 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（分子）.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 sign of organic material but found none, an unexpected result because at least organic compounds from the steady bombardment of the Martian surface by meteorites （限星）were thought to have been present.
The absence of organic material, some scientists speculated, was the result of intense ultraviolet radiation penetrating the atmosphere which was at one time rich in carbon dioxide and thus thick enough to protect its surface from the harmful ray 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. Mars never developed a protective layer of ozone as Earth did.
Despite the disappointed Viking results, there are those who still keep open the possibility of life on Mars. 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 of landing a spacecraft. The landing sites were on part of the Martian plains that appeared relatively featureless from orbital photography.
The type of Martian terrain that these researchers suggest may be a possible hiding place for active life has an earthly parallel: the ice-free region of southern Victoria land. Antarctica, where the temperatures in some dry valley average below zero. Organisms known as endoliths, a form of blue-green algae (藻类)that has adopted to this harsh environment, were found living inside certain porous 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 condition by similarly seeking 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.
1. The major purpose of the passage is to（ ）.
2. The author used the evidence from the four Viking experiments (paragraph 2) to establish that（ ）.
3. The author suggests that important difference between Mars and Earth is that unlike Earth, Mars（ ）.
4. In paragraph 5, the researchers' argument that life may exist in Martian rocks rests on the idea that（ ）.
