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Unit 4
第四單元
Astronomy: The Science of the Stars
天文學 星星的科學
Reading How Life Began on the Earth
閱讀 地球上生命的起源
No one knows exactly how the earth began, as it happened so long ago.
沒有人確切知道地球是怎樣開始形成的，因為在很早很早以前它就形成了。
However, according to a widely accepted theory, the universe began with a "Big Bang" that threw matter in all directions.
然而，一種普通為人們所接受的理論是：宇宙起源于一次大爆炸，這次大爆炸將物質(zhì)投射到四面八方。
After that, atoms began to form and combine to create stars and other bodies.
然后，原子開始形成并結(jié)合成恒星和其他天體。
For several billion years after the "Big Bang", the earth was still just a cloud of dust.
大爆炸后好幾十億年過去了，但地球還只是一團塵埃。
What it was to become was uncertain until between 4.5 and 3.8 billion years ago when the dust settled into a solid globe.
隨后它會變成什么沒人能知道，直到38-45億年前，這團塵埃才慢慢地形成一個固體的球狀物。
The earth became so violent that it was not clear whether the shape would last or not.
地球（開始）變得激烈動蕩，不知道這個固體形狀是否會繼續(xù)存在下去。
It exploded loudly with fire and rock.
它（地球）猛烈的爆炸噴出了烈火與巖石。
They were in time to produce carbon, nitrogen, water vapour and other gases, which were to make the earth's atmosphere.
最終產(chǎn)生了碳、氮氣、水蒸氣和其他多種氣體，從而形成了地球的大氣層。
What is even more important is that as the earth cooled down, water began to appear on its surface.
更為重要的是，地球冷卻了下來，地球的表面就開始出現(xiàn)了水。
Water had also appeared on other planets like Mars but unlike the earth, it had disappeared later.
在火星這些其他行星上都出現(xiàn)了水，但和地球不同的是，這些水后來都消失了。
It was not immediately obvious that water was to be fundamental to the development of life.
水會對生命的發(fā)展起關(guān)鍵作用，這一點在當時并不明顯。
What many scientists believe is that the continued presence of water allowed the earth to dissolve harmful gases and acids into the oceans and seas.
很多科學家相信，由于地球表面上長期有水存在，它使得地球得以把有害氣體和酸性物質(zhì)溶解在海洋里。
This produced a chain reaction, which made it possible for life to develop.
這就產(chǎn)生了一系列的反應，使得生命就有可能開始發(fā)展了。
