高考英语《流行话题 语境识词4500》Unit 88 Is There Life on Mars素材.docVIP

　Unit 88 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 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 4 spacecraft flew past the Red Planet. It revealed a desolate world; the prospects for life were fading fast. All hope died in 1976, when the Viking Lander of Nasa failed to find a single organic molecule.　　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 Earths surface and in volcanic springs well above the boiling point of water. And these conditions are remarkably 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 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 created by the impact of comets and asteroids on the surface of the two planets. Even today, about 500 tons of material f