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1、120142014 届最新高考英语一轮单元复习届最新高考英语一轮单元复习 精品阅读理解提升文章精选一精品阅读理解提升文章精选一百篇（百篇（8888）Unit 88Is There Life on Mars?For most of the 20th century,there has been life on Mars.Or,at least in theminds of the Earthlings,who inhabit its closest planetary neighbor.When at theend of the 19th century,the American astrono
2、mer Percival Lowell thought he sawcanals on the Red Planet,he was convinced that they were part of a planetwideirrigation system,built by an advanced Martian civilization.Then,in 1965,the Mariner 4 spacecraft flew past the Red Planet.It revealeda desolate world;the prospects for life were fading fas
3、t.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 evidenceof life.The new unmanned Nasa missions to Mars
4、 are a result of the growing optimismamong scientists that life exists,or did exist,on Mars.The strongest evidencein support of life on Mars comes from the study of microbes.Scientists are findingmicrobes in more and more inhospitable conditions-in rocks hundreds of meter belowthe Earths surface and
5、 in volcanic springs well above the boiling point of water.And these conditions are remarkably similar to conditions on Mars.These organismscan live and multiply without oxygen or light in extreme temperatures,using onlyrocks and water to sustain themselves.Scientists believe that conditions on Mars
6、 around 3.8 billion years ago weresimilar to those of the early Earth,when primitive organisms were spreading throughour oceans.Under these conditions,it is highly probable that life may have arisenon Mars as well.Even if life did not arisen naturally on Mars,it does not mean that it couldnot have e
7、xisted there.Life forms could have been transferred between the Earthand Mars in debris created by the impact of comets and asteroids on the surface ofthe two planets.Even today,about 500 tons of material from Mars lands on Earthevery year.It is mainly in the form of dust but occasionally a larger c
8、hunk strikesthe Earth.It is in these chunks of rock,which were much larger and more frequentin the past,that life forms could have been transported from planet to planet.Buthow could these life forms have survived their jo urney through space full of deadlycosmic radiation?A rock ten meters across w
9、ould shield life inside it from a lotof radiation and the temperature might only be minus 10 or 20 d egrees(Celsius),the sort of thing we have on Earth.A further complication of this theory is thatlife could have originally arisen on Mars and then be transferred to Earth.In thatcase,we would be Mart
10、ians,who colonized the Earth.2But could life exist on Mars,given that it is a very different world from whatit was 3.8 billion years ago?Bathed in ultraviolet radiation from the Sun,it hasvirtually no atmosphere and no liquid water on the surface.Mars had moved from awarm,wet place to a dead world o
11、f dust storms,volcanoes and vast canyons.Ithappened because its carbon dioxide atmosphere could not remain stable in a wetenvironment.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 greenhousegas,which traps
12、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 thesurface froze.This is a fate that the Earth escaped because of water vapor and othergreenhouse gases,constantly present in its atmosphere.Despite the inhospitable surface of todays Mars,scientists believe thatMart ian microbes would only fed on rock and water,and they are likely to bekilometers below the surface.There could be water deep underground,kept liquidby inner heat from radioactivity.