地球每2700万年遭受一次大灭绝

杭州泰弗尔英语 · 发表于 2011-3-28 12:05:17

科学家称，地球上的生命每隔2700万年就会被灭绝一次，我们到下一次大灭绝之前还有约1600万年时间。
　　科学家研究了过去5亿年间地球上发生的所谓的“灭绝事件”，时间跨度是先前任何一项研究的两倍。此次研究证实大灭绝的出现是有时间规律的。
　　来自堪萨斯大学和华盛顿史密森研究所的科学家们确信每2700万年就会发生一次大灭绝事件，认为可信度达99%。
　　20世纪80年代时科学家们认为地球周期性的生物灭绝可能是由遥远太空的涅墨西斯星球造成的，该星球和太阳很相似。
　　该理论认为涅墨西斯星球每隔2700万年会穿过奥尔特云向地球发射一大批彗星。
　　奥尔特云是一个由粉尘和冰组成的庞大云带，据信它距离太阳约一光年，许多穿越我们太阳系的彗星都来自那里。
　　然而现在科学家称大灭绝的周期性实际上证明涅墨西斯星球理论是错误的，因为随着时间的推移，在和其他星球的相互作用下，星球的运行轨迹会发生改变。
　　研究者称：“引发涅墨西斯星球理论的化石资料现在却成了对该理论不利的证据。”
　　上一次大灭绝事件是在1100万年前发生的，当时地球上10%的居住者遭到灭绝。
　　这意味着到下一次毁灭性事件发生时还有约1600万年的时间，尽管据图表显示这种大灭绝事件有时会提前1000万年发生。
　　闯入地球的小行星一般被认为是白垩－第三纪灭绝，即恐龙遭受大灭绝的主要原因之一。
　　那次毁灭性事件灭绝了地球上超过一半的物种，为哺乳动物成为地球上的首要物种扫清了道路。
　　那次大灭绝是由闯入地球的一颗巨型小行星造成的，形成了墨西哥希克苏鲁伯陨石坑。
　　这颗小行星直径约15公里，据认为它撞击地球的力量比轰炸日本广岛的原子弹力量要大10亿倍。
Life on Earth is wiped out every 27 million years – and we have about 16 million years left until the next extinction, according to scientists.
　　Research into so-called ‘extinction events’ for our planet over the past 500 million years - twice as long as any previous studies - has proved that they crop up with metronomic regularity.
　　Scientists from the University of Kansas and the Smithsonian Institute in Washington DC are 99 per cent confident that there are extinctions every 27 million years.
　　In the 1980s scientists believed that Earth’s regular extinctions could be the result of a distant dark twin of the Sun, called Nemesis.
　　The theory was that Nemesis crashed through the Oort cloud every 27 million years and sent a shower of comets in our direction.
　　The Oort cloud is a vast belt of dust and ice that is believed to lie around one light year from the Sun and is the origin of many of the comets that pass through our solar system.
　　But now scientists claim that the regularity of the mass extinctions actually disproves the Nemesis theory because its orbit would have changed over time as it interacted with other stars.
　　‘Fossil data, which motivated the idea of Nemesis, now militate against it,’ say the researchers.
　　The last extinction event, 11 million years ago, saw 10 per cent of the Earth’s inhabitants wiped out.
　　This means there is around 16 million years until the next event takes place, although the graph shows that it occasionally the event takes place up to 10 million years early.
　　Asteroids crashing into the Earth are commonly believed to be one of the main reasons behind mass extinctions like that suffered by the dinosaurs - the Cretaceous-Tertiary (KT) extinction.
　　The extinction wiped out more than half of all species on the planet clearing the way for mammals to become the dominant species on Earth.
　　The extinction was caused by a massive asteroid slamming into Earth at Chicxulub in Mexico.
　　The asteroid, which was around 15 kilometres wide, is believed to have hit Earth with a force one billion times more powerful than the atomic bomb at Hiroshima.