《英语流行话题阅读:语境识词4500》88 Is There Life on Mars?

　　Is There Life on Mars?

For most of the 20th century, there has been life on Mars. Or, at least in the minds of the Earthlings, who inhabit its closest planetary neighbor. When at the end of the 19th century, the American astronomer 1 Percival Lowell thought he saw canals on the Red Planet, he was convinced that they were part of a planetwide irrigation system, built by an advanced Martian civilization.

Then, in 1965, the Mariner 2 4 spacecraft flew past the Red Planet. It revealed a desolate 3 world; the prospects 4 for life were fading fast. All hope died in 1976, when the Viking Lander of Nasa failed to find a single organic molecule 5.

Twenty years after the last mission to Mars failed to detect any signs of life, Nasa is preparing another series of missions to the Red Planet to find evidence of life. The new unmanned Nasa missions to Mars are a result of the growing optimism among scientists that life exists, or did exist, on Mars. The strongest evidence in support of life on Mars comes from the study of microbes. Scientists are finding microbes in more and more inhospitable conditions -- in rocks hundreds of meter below the Earth's surface and in volcanic 6 springs well above the boiling point of water. And these conditions are remarkably 7 similar to conditions on Mars. These organisms can live and multiply without oxygen or light in extreme temperatures, using only rocks and water to sustain themselves.

Scientists believe that conditions on Mars around 3.8 billion years ago were similar to those of the early Earth, when primitive 8 organisms were spreading through our oceans. Under these conditions, it is highly probable that life may have arisen on Mars as well.

Even if life did not arisen naturally on Mars, it does not mean that it could not have existed there. Life forms could have been transferred between the Earth and Mars in debris 9 created by the impact of comets and asteroids 10 on the surface of the two planets. Even today, about 500 tons of material from Mars lands on Earth every year. It is mainly in the form of dust but occasionally a larger chunk 11 strikes the Earth. It is in these chunks 12 of rock, which were much larger and more frequent in the past, that life forms could have been transported from planet to planet. But how could these life forms have survived their journey through space full of deadly cosmic radiation? A rock ten meters across would shield life inside it from a lot of radiation and the temperature might only be minus 10 or 20 degrees (Celsius), the sort of thing we have on Earth. A further complication of this theory is that life could have originally arisen on Mars and then be transferred to Earth. In that case, we would be Martians, who colonized 13 the Earth.

But could life exist on Mars, given that it is a very different world from what it was 3.8 billion years ago? Bathed in ultraviolet radiation from the Sun, it has virtually no atmosphere and no liquid water on the surface. Mars had moved from a warm, wet place to a dead world of dust storms, volcanoes and vast canyons 14. It happened because its carbon dioxide atmosphere could not remain stable in a wet environment. Carbon dioxide reacts with rocks, and it rapidly absorbed by water, where it becomes solid carbonates and sinks to the bottom of seas. It is a greenhouse gas, which traps heat from the Sun. As it disappeared from the Martian atmosphere, the planet began to cool. Its atmosphere grew thinner and all the water on the surface froze. This is a fate that the Earth escaped because of water vapor 15 and other greenhouse gases, constantly present in its atmosphere.

Despite the inhospitable surface of today's Mars, scientists believe that Martian microbes would only fed on rock and water, and they are likely to be kilometers below the surface. There could be water deep underground, kept liquid by inner heat from radioactivity.