百科知识视频之煎蛋小学堂43 如何测定行星年龄？

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 因为月球的表面布满了岩石碎块 和远古时代小行星撞击的凹痕和伤疤。与地球一样，月球上的岩石和也会被侵蚀 日积月累，小行星也在月球表面留下来巨大的凹陷 但是今天的地球看上去却像新生的星球，陨石坑也并非随处可见。

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.

地球的外观如此年轻是有原因的 因为它的最外层不断更新---隐瞒了真实年龄。新生岩石从火山口喷溅到地球表面 从海底涌出的岩浆冷却后又成为海底的一部分 大陆板块碰撞时岩石结构会改变(造山过程) 大陆将海底覆盖的同时也在回收海底岩石

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.

这种重新生成岩石的过程不只是让岩石看着更年轻而是真正让岩石更年轻---至少在我们看来如此 因为人类最尖端的岩石寿命分析工具的原理是测量岩石中某种特定放射性元素的衰变量。一旦岩石熔化并生成新的岩石，所有能证明衰变的证据都会消失。

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.

一般来说这是好事，因为这意味着岩石能够记录自身的年龄———它们生成的年代。例如，我们可以借此断定海底正在扩张，因为靠近大洋中脊的岩石更年轻 而靠近陆地的岩石更古老 这块区域的岩石常常会沉入大陆之下，再次生成新岩石。

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.

不过，因为地球一直在大量进行着岩石回收工作 而且再造岩石的年龄记录会清零 要查看很早以前的地球历史是一件很有难度的工作 在地球形成阶段，炽热的地球表面形成的固体岩石很早以前可能就被回收。这时一种名为锆石的小型弹性矿物就派上了用处。锆石的化学结构和持久性都很像石英 即使其他矿物都已经风化，锆石依然能坚挺很长一段时间。

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.

但是锆石有石英石不具备的两种物质 锆原子和微量放射性铀元素。铀原子和锆很类似，会在锆石生成过程中偷偷溜进锆石晶格，替代锆原子的位置 但是和锆原子不同，不稳定的铀原子最终会衰变成铅原子。奇怪的事情来了，铅原子与锆原子并不类似 铅原子不可能独自进入锆的晶格 所以锆石晶体中的铅原子越多，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.

目前发现的最古老的陆地矿石 可能也是已知的最古老的地球碎片 碰巧就是一块在澳洲西部砂岩中发现的大量填充有铅原子的锆石碎片。这块晶体幸运地逃过了侵蚀，火山爆发和小行星撞击的毁灭命运 虽然我们不知道它是否形成于地球最初阶段的岩石 如果不是，第一块岩石的历史必定更加古老。

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.

所以就算地球将年龄隐藏得再好 通过铀铅测年法测量小块锆石我们知道地球至少形成于44亿年前。来自陨石的证据也显示地球的年龄应该更加老 至少这块锆石让我们有信心相信地球的确非常古老。