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地球上的水究竟从哪儿来的?

Where Did Earth's Water Come From?

与太阳系的其它行星不同,地球表面70%由液态水覆盖。这
Unlike every other planet in our solar system, Earth’s surface is 70% liquid water, which
对生命正合适,但也很奇怪,因为一切我们所知的地球形成的原因及时间
while useful for life, is also kind of weird, because everything we know about how and when
都表明它的表面应该极度干燥。
our planet formed says Earth’s surface should be bone dry.
事情是这样的:太阳系是从由尘埃和气体所组成的巨大云团的崩塌中形成的
The story goes like this: our solar system formed from the collapse of a large cloud
处于中心浓度极强的气体云团被引燃而形成了太阳,这
of dust and gas. The dense blob of gas at the center ignited to form the sun, which
这就像处于初期,不稳定的恒星释放猛烈的太阳风一样,随着时间的推移,带电粒子的流动
as a young, unstable star unleashed a fierce solar wind. Over time this stream of charged
将残余气体越推越远,只留下固体粒子
particles pushed the remaining gas cloud farther and farther out, leaving only solid particles
在它身后凝结成块形成岩石,微行星,最终形成我们如今所知的内太阳系
behind to clump together into rocks, planetesimals, and finally, the rocky planets of the inner
的一些岩态行星。
solar system that we know today.
问题是:水,以冰的形式,不可能是游荡的固态粒子中的一员,
And here’s the problem: water, in the form of ice, couldn’t have been one of the solid
因为最初期的太阳系内部温度极高,不可能与冰冻水共存
particles that stuck around, because the early inner solar system was far too hot for frozen
以及任何的水蒸气,它们应该早就被太阳风所吹散。
water, and any water vapor would have been blasted away by the solar wind.
因此,如果地球开始就没有水,最终又如何形成如此壮阔的大海?我们
So if Earth didn’t start off with water, how did we end up with such splendid oceans? We
知道水不是在被制好后而一直放在这,因为自然的演变就像燃烧,
know H2O wasn’t manufactured here over the eons, because natural processes like combustion,
呼吸以及光合作用,产出的水量几乎等于它所毁灭的量—以及,不管以哪种方式
breathing and photosynthesis create and destroy roughly equal amounts of water – and either
这些分量的基数如此之小,不可能把它计算在如此丰富的
way, the amounts in question are so miniscule that they can’t account for the abundance
水量之内。
of water on the planet.
既然地表水既不是本就存在的也不是被人工制成放在这的,它
Since Earth’s water was neither part of the original package nor manufactured here, it
一定是从遥远的地方流动过来的,如流星体,彗星,或者是起源于其它的主体,它们在远到
must have flown in from far away, on meteoroids or comets or other bodies originating in the
足够能使太阳与冰冻水共存的太阳系之外的地方。
outer solar system where they were far enough from the Sun for frozen water to survive.
彗星,一些脏冰球,极有可能是地球水的来源,但是,
Comets, being dirty iceballs, are a logical candidate for the source of our water, but
当我们发现它的重氢含量丰富的多时,已经排除了这种可能,(这种氢
were ruled out when we discovered that they are far richer in heavy hydrogen (that’s hydrogen
的原子内有一个中子和一个质子)它比地球水丰富得多,
with a neutron as well as a proton in its nucleus) than Earth water. For every million
地球水中的氢原子, 每一百万个氢原子中就有150个是重氢 ,而典型的彗星水
hydrogen atoms in Earth water, about 150 are heavy ones, while typical comet water has
重氢含量比地球水的两倍还多。这些不相称的化学特征告诉我们地球水
twice that many. These mismatched chemical signatures suggest that Earth’s water could
不可能来源于彗星。
not have arrived on comets.
被提出的最大的可能性是地球水的来源是一种被称为
It turns out that the most likely source for Earth’s water is a type of meteorite called
碳质球粒陨石的陨星。碳质球粒陨石;它名字的来源于
a carbonaceous chondrite. "Chondrite" is just the name given to the class of stony meteoroids
那种最常撞击地球的石陨石类别物质。这种碳质球粒陨石不仅含水也有
that most commonly strike the Earth. But only the carbonaceous chondrites contain water
大量碳成分,如果你不能仅依据名字来辨别的话。他们都含水
– as well as lots of carbon, if you couldn’t tell from their name. They have water in them
因为它们的形成在太阳霜冻线之外;更重要的是,它们的水
because they formed out beyond the sun’s "frost line", and what’s more, their water has levels
与地球水的重氢含量程度相似。 强有力的证明了,这些地球的不速之客
of heavy hydrogen similar to that of earth water, strongly suggesting that these earth-crashers
是地球上冰盖、云层、 河流和海洋的来源。
are the source of our ice caps, clouds, rivers, and oceans.
因此,才使我们星球变的像蓝色大理石一样。毫不夸张,
And thus the water that turned our planet into a blue marble came, quite literally,
也出人
out of
意料。
the blue.

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