大学英语六级考试拓展阅读-背诵美文76

　　76 MARS

　　Mars According to the best evidence gathered by space probes and astronomers, Mars is an inhospitable planet, more similar to Earths Moon than to Earth itself - a dry, stark, seemingly lifeless world. Mars air pressure is equal to Earths at an altitude of 100,000 feet. The air there is 95% carbon dioxide. Mars has no ozone layer to screen out the Suns lethal radiation. Daytime temperatures may reach above freezing, but because the planet is blanketed by the mere wisp of an atmosphere, the heat radiates back into space. Even at the equator, the temperature drops to 50 ℃ at night. Today there is no liquid water, although valleys and channels on the surface show evidence of having been carved by running water. The polar ice caps are made of frozen water and carbon dioxide, and water may be frozen in the ground as permafrost. Despite these difficult conditions, certain scientists believe that there is a possibility of transforming Mars into a more Earth like planet. Nuclear reactors might be used to melt frozen gases and eventually build up the atmosphere. This in turn could create a greenhouse effect that would stop heat from radiating back into space. Liquid water could be thawed to form a polar ocean. Once enough ice has melted, suitable plants could be introduced to build up the level of oxygen in the atmosphere so that, in time, the planet would support animal life from Earth and even permanent human colonies. This was once thought to be so far in the future as to be irrelevant, said Christopher McKay, a research scientist at the National Aeronautics and Space Administration. But now its starting to look practical. We could begin work in four or five decades. The idea of terra forming Mars, as enthusiasts call it, has its roots in science fiction. But as researchers develop a more profound understanding of how Earths ecology supports life, they have begun to see how it may be possible to create similar conditions on Mars. Dont plan on homesteading on Mars any time soon, though. The process could take hundreds or even thousands of years to complete, and the cost would be staggering.