百科知識視頻之煎蛋小學(xué)堂43 如何測定行星年齡？

The moon is old-at least,it looks old since its entire surface is strewn with the rubble of broken down rock.and pockmarked with scars from ancient asteroid impacts.We know that rocks weather here on Earth too.and that meteorites have made some considerable dents over the years.but today Earth looks reasonably fresh and new without widespread craters.

月球很古老---至少看上去很古老 因?yàn)樵虑虻谋砻娌紳M了巖石碎塊 和遠(yuǎn)古時代小行星撞擊的凹痕和傷疤。與地球一樣，月球上的巖石和也會被侵蝕 日積月累，小行星也在月球表面留下來巨大的凹陷 但是今天的地球看上去卻像新生的星球，隕石坑也并非隨處可見。

Earth owes its youthful appearance to the fact that its outermost layer is constantly renewing itself-hiding its age.New rock spills onto the planet's surface from the mouths of volcanoes,magma is released at the seams of ocean floors and cools to become part of those floors.rocks metamorphose (and mountains form)when continents collide,and the sea floor is recycled as continents ride over it.

地球的外觀如此年輕是有原因的 因?yàn)樗淖钔鈱硬粩喔?--隱瞞了真實(shí)年齡。新生巖石從火山口噴濺到地球表面 從海底涌出的巖漿冷卻后又成為海底的一部分 大陸板塊碰撞時巖石結(jié)構(gòu)會改變(造山過程) 大陸將海底覆蓋的同時也在回收海底巖石

The process of remarking rock doesn't just make it look young,it makes it young-at least,to us because our best tool to date rocks is to measure the amount of decay of certain radioactive elements in the rocks.And when rocks melt and reform anew,all evidence of that decay is destroyed.

這種重新生成巖石的過程不只是讓巖石看著更年輕而是真正讓巖石更年輕---至少在我們看來如此 因?yàn)槿祟愖罴舛说膸r石壽命分析工具的原理是測量巖石中某種特定放射性元素的衰變量。一旦巖石熔化并生成新的巖石，所有能證明衰變的證據(jù)都會消失。

Normally this is great,because it means rocks carry around a record of their age-the time since they formed.For example,we can tell that ocean floors are spreading because their rocks are youngest near mid-ocean ridges.and get progressively older as you head towards land.where they're often recycled by sinking under continents.

一般來說這是好事，因?yàn)檫@意味著巖石能夠記錄自身的年齡———它們生成的年代。例如，我們可以借此斷定海底正在擴(kuò)張，因?yàn)榭拷笱笾屑沟膸r石更年輕 而靠近陸地的巖石更古老 這塊區(qū)域的巖石常常會沉入大陸之下，再次生成新巖石。

However,because the earth remakes its rock so much and because remaking rocks resets their built-in clocks,it's hard to look really far back into our planet's history.most of the first solid rocks that formed ages ago on Earth's fiery surface have probably long since been remade.That's where a resilient little mineral called zircon comes in.Zircon is similar to quariz,both in its chemical structure and in its durabillity,so it sticks around a long time after other minerals weather away.

不過，因?yàn)榈厍蛞恢痹诖罅窟M(jìn)行著巖石回收工作 而且再造巖石的年齡記錄會清零 要查看很早以前的地球歷史是一件很有難度的工作 在地球形成階段，熾熱的地球表面形成的固體巖石很早以前可能就被回收。這時一種名為鋯石的小型彈性礦物就派上了用處。鋯石的化學(xué)結(jié)構(gòu)和持久性都很像石英 即使其他礦物都已經(jīng)風(fēng)化，鋯石依然能堅(jiān)挺很長一段時間。

But zircon also has two things quartz doesn't zirconium,which gives it its name,and small traces of radioactive uranium.Uranium atoms are similar enough to zirconium that they can occasionally slip into the mineral's crystal lattice in zirconium's place,when the mineral is forming.but unlike zirconium the unstable uranium atoms eventually radioactively decay into lead atoms.Which is weird,because lead isn't similar to zirconium and so would never have ended up in a zircon crystal on its own.so the more lead you find in a zircon crystal,the older you know it is.

但是鋯石有石英石不具備的兩種物質(zhì) 鋯原子和微量放射性鈾元素。鈾原子和鋯很類似，會在鋯石生成過程中偷偷溜進(jìn)鋯石晶格，替代鋯原子的位置 但是和鋯原子不同，不穩(wěn)定的鈾原子最終會衰變成鉛原子。奇怪的事情來了，鉛原子與鋯原子并不類似 鉛原子不可能獨(dú)自進(jìn)入鋯的晶格 所以鋯石晶體中的鉛原子越多，就表示越古老。

And one of the oldest terrestrial minerals ever found quite possibly the oldest piece of the earth we know of happens to be a very lead-filled grain of zircon found embedded in sandstone in Western Australia,Somehow,this crystal has managed to escape destruction by erosion and volcanic eruptions and asterold impacts over the millennia.and while we don't know whether it formed in the very first rock on Earth.if it didn't then by definition the first rock must be even older.

目前發(fā)現(xiàn)的最古老的陸地礦石 可能也是已知的最古老的地球碎片 碰巧就是一塊在澳洲西部砂巖中發(fā)現(xiàn)的大量填充有鉛原子的鋯石碎片。這塊晶體幸運(yùn)地逃過了侵蝕，火山爆發(fā)和小行星撞擊的毀滅命運(yùn) 雖然我們不知道它是否形成于地球最初階段的巖石 如果不是，第一塊巖石的歷史必定更加古老。

So despite earth's best efforts to hide its age from us,uranium-lead dating of little bits of zircon tells us our planet formed at least 4.4 billion years ago.There's also other evidence from meteorites that suggests the earth is even older but at bare minimum,this zircon lets us say with confidence that the earth is literally older than dirt.

所以就算地球?qū)⒛挲g隱藏得再好 通過鈾鉛測年法測量小塊鋯石我們知道地球至少形成于44億年前。來自隕石的證據(jù)也顯示地球的年齡應(yīng)該更加老 至少這塊鋯石讓我們有信心相信地球的確非常古老。