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For hundreds of years humans have dreamed of finding life on distant planets．Now the hope of discovering real evidence of life has greatly increased．
NASA’s Mars detector(探测器), Opportunity, succeeded in finding signs that water once existed on the planet．Opportunity landed on Mars in January．Scientists now believe that the planet could once have supported life．
This discovery was chosen by Science, one of the world’s leading magazines, as the most important scientific achievement of 2004 last Friday．
“This little, wheeled, one-armed box went around another planet and has done something no human has ever managed,”according to Science．“It has discovered another place in the universe where life could once have existed．
“Although we still can’t say that life could have existed in this environment, it is now certain that there was water on Mars,”said Steve Squyres, one of the scientists working on the Mars mission(任务)．The evidence comes from pictures and chemical readings taken by Opportunity．It includes marks on rocks like those caused by following water on Earth and salty chemicals like those found in dried-out sea-beds．Scientists said the new evidence proved beyond doubt that water has existed on Mars．But it is still unknown whether the water was like an ocean or just ice．
While Opportunity has not found any signs of life, the presence of water means life is possible．“In everything we know about life on Earth, there is no example without liquid water,”Squyres said．“So water is important for the search for life on Mars．”
Researchers agree that a future mission(任务)should bring back physical samples(样品)．But some scientists worry about the risk that this could introduce dangerous foreign creatures to Earth．“The problem here is how to get the samples back,”Squyres replied．“I think it is our responsibility to limit any risk．”
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