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　　Mars According to the best evidence gathered by space probes and astronomers, Mars is an inhospitable planet, more similar to Earth's Moon than to Earth itself - a dry, stark, seemingly lifeless world. Mars' air pressure is equal to Earth's at an altitude of 100,000 feet. The air there is 95% carbon dioxide. Mars has no ozone layer to screen out the Sun's 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℃ （ 60 ） 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 it's 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 Earth's ecology supports life, they have begun to see how it may be possible to create similar conditions on Mars. Don't 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.

　　据宇宙探测器和天文学家收集的有力证据，火星是一个人类不能居住的星球。它不像地球，而更像月球——一个干涸、荒芜，看上去没有任何生命的世界。火星的气压相当于地球上十万英尺高空处的气压。火星大气的构成中有95%是二氧化碳，而且，火星上没有能屏蔽太阳致命射线的臭氧层。白天，那里的温度可以达到零上，但因为包裹火星的大气层极为稀薄，热量又会辐射回宇宙中。就算是在火星赤道，夜里的温度也在 50℃。尽管火星上的山谷沟渠说明它曾经被流水蚀刻过，但如今那里已没有液态水了。极地冰帽是由固态水和二氧化碳组成的，水也有可能存在于永久冻土之中。尽管困难重重，某些科学家依然认为有可能把火星改造成类似地球的星球。核反应可以用来融化冰冻的气体最终形成火星大气层。由此就可以产生温室效应，阻止热量散射回宇宙中。液化的水可以融化成极地海洋。足够量的冰融化后，可将地球上的植物移植上去。植物又可以向大气层提供氧气。久而久之火星就可以维持从地球过去的动物生命甚至成为人类的永久居留地。"这一切在过去看起来遥远得近乎无稽"，国家航天局的一位研究人员，克里斯托弗·麦克凯说，" 但是现在已经开始展现出可能性。四五十年后，我们就可以着手于这项工作。"这种支持者们称为"移居火星"的想法最早出现在科幻小说中。但随着研究者对地球上生态如何支持生命的理解越来越深入，在火星上创造类似地球环境也显得越来越现实。但千万别计划很快在火星上建造家园。这个历程需要几百乃至上千年，而且耗费将是惊人的。