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与行星约会

How To Date A Planet

月亮年代久远,至少,它看上去很年迈,因为它整个表面都堆满了
The moon is old – at least, it looks old, since its entire surface is strewn with the
碎石堆和密密麻麻的伤痕,这些是由于很久以前小行星冲撞的影响。
rubble of broken down rock and pockmarked with scars from ancient asteroid impacts.
我们知道地球上也有岩石,陨石造成了相当大的
We know that rocks weather here on Earth too, and that meteorites have made some considerable
凹坑,这么多年来,但是现在的地球看起来相当的清新没有遍地
dents over the years, but today Earth looks reasonably fresh and new without widespread
的损坏。
craters.
地球拥有年轻的外表得益于它的最外层是不断
Earth owes its youthful appearance to the fact that its outermost layer is constantly
自我更新的,从而隐藏了它的年龄。新的岩石会铺满地球表面通过
renewing itself – hiding its age. New rock spills onto the planet’s surface from the
几个月的火山喷发。岩浆会在海洋板块衔接处释放并冷却成为
mouths of volcanoes; magma is released at the seams of ocean floors and cools to become
那些地板的一部分。岩石会随着大陆的碰撞而变形。
part of those floors; rocks metamorphose (and mountains form) when continents collide; and
海床就会下降,大陆就会上升在它之上。
the sea floor is recycled as continents ride over it.
重塑岩石的过程不仅仅是使它看起来年轻,而是使它年轻,
The process of remaking rock doesn’t just make it look young, it makes it young – at
至少,对于我们而言,因为我们最好的工具去测岩石的年代就是测量它的
least, to us, because our best tool to date rocks is to measure the amount of decay of
确定的放射性元素的衰减量。当岩石融化形成新的,所有的
certain radioactive elements in the rocks. And when rocks melt and reform anew, all evidence
衰减证据就被损坏了。
of that decay is destroyed.
正常来说这是很棒的,因为它意味着岩石携带着它们自己的年龄,它们
Normally this is great, because it means rocks carry around a record of their age – the time
成型的时间。例如,我们可以分辨海洋板块正在扩展,因为
since they formed. For example, we can tell that ocean floors are spreading because their
在靠近海洋中脊的地方是最年轻的,当你朝向陆地时它们的年代
rocks are youngest near mid-ocean ridges and get progressively older as you head towards
久远一点,它们通过沉入大陆底部被回收。
land, where they’re often recycled by sinking under continents.
然而,因为地球重新制造这么多岩石,因为重新制作岩石就会重置
However, because the earth remakes its rock so much, and because remaking rocks resets
它们的计时器,所以很难追溯我们地球很远很远的历史
their built-in clocks, it’s hard to look really really far back into our planet’s history
大多数第一层固体岩石很久以前在地球炙热的地表形成当
– most of the first solid rocks that formed ages ago on Earth’s fiery surface have probably
再次重造时可能会隔很长时间。
long since been remade.
这就是有弹性的锆石来源。锆石和石英很接近,
That’s where a resilient little mineral called zircon comes in. Zircon is similar to quartz,
在化学结构和耐用性方面,所以它才能存在很长时间
both in its chemical structure and in its durability, so it sticks around a long time
当其他矿石随着气候变化而消失。但是锆石有两种东西是石英没有的:锆,
after other minerals weather away. But zircon also has two things quartz doesn’t: zirconium,
赋予了它的名字,有小的放射性铀的痕迹。铀原子
which gives it its name, and small traces of radioactive uranium. Uranium atoms are
比锆小很多它们可以偶然的钻进矿石
similar enough to zirconium that they can occasionally slip into the mineral’s crystal
锆的晶体中,和锆不同,不稳定的铀原子最终会衰变
lattice in zirconium’s place, but unlike zirconium, the unstable uranium atoms eventually radioactively
成为铅原子。
decay into lead atoms.
奇怪的是,由于铅和锆并不接近,所以它才永远不会
Which is weird, because lead isn’t similar to zirconium, and so would never have ended
在锆石的晶体中。如果你在锆石晶体中发现越多的铅,
up in a zircon crystal on its own – so the more lead you find in a zircon crystal, the
它的年代越久远。
older it is.
曾经陆地上最古老的矿石被发现了,可能是
And one of the oldest terrestrial minerals ever found, quite possibly the oldest piece
我们所知的地球上最古老的一片,恰巧是一个铅芯锆石嵌入
of the earth we know of, happens to be a very lead-filled grain of zircon found embedded
在西部澳大利亚的砂岩里。在某种程度上,这个晶体,设法逃离
in sandstone in Western Australia. Somehow, this crystal has managed to escape destruction
侵蚀和火山喷发以及行星的影响超过一千年,然而我们并不知道
by erosion and volcanic eruptions and asteroid impacts over the millennia, and while we don’t
它是否是地球上形成的第一个岩石,如果不是那么从定义来讲
know whether it formed in the very first rock on Earth, if it didn’t then by definition
第一个岩石可能更久远。
the first rock must be even older.
所以尽管地球尽最大的力对我们来掩藏它的年龄,少量铀铅的结合
So despite earth’s best efforts to hide its age from us, uranium-lead dating of little
在锆石内,告诉我们我们的星球至少在44亿年前形成。这儿
bits of zircon tells us our planet formed at least 4.4 billion years ago. There’s also
还有其他证明来自陨石,表明地球可能更古老,但是在最低程度上,
other evidence from meteorites that suggests the earth is even older, but at bare minimum,
锆石让我们更有信心的说地球确实要比灰尘更古老。
this zircon lets us say with confidence that the earth is literally older than dirt.

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