英语听力：自然百科 行星旅行指南：火星 Mars—5
时间：2014-05-29 06:55:01
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（单词翻译）
In the four decades since our robots first arrived, the once fuzzy ball at the end of our telescopes has steadily1 focused into a red planet we can understand, and it is not a welcoming place. The problem is the atmosphere is so thin and cold that water exists only as solid ice in the ground or vapor2 in the air, not as liquid on the surface. You might see some fine wispy3 clouds, high in the sky, but don’t bother bringing an umbrella.
The whole planet is
dryer4 than the dustiest desert on earth, and there hasn’t been a drop of rain here for millions, perhaps billions of years. The thing that fascinated me was that we could see valleys,
sneaking5 across the surface that had clearly been carved by flowing water, so this is telling us that in the past it was different, not only that it was different in a way that made more suitable for life than it is today, and that I found truly compelling.
We are very convinced that one time it was very
hospitable6 planet, with liquid water and enough atmosphere to sustain the climate, and so now we are trying to understand how did it change, why did it change and what still might be on Mars.
These are deep Martian mysteries. If Mars and earth started as sister planets, did life once festoon the Martian surface? Might it still be there? And where did all the atmosphere and water go?
Solving these puzzles has challenged our planetary explorers from the moment the first Martian postcards were sent back to earth. It’s the fact that it is so much like earth that kind of makes Mars such a special place.
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