万物简史 第222期:把铅撵出去(11)

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　　有7年时间，先是在芝加哥大学，后在加州理工学院（他于1952年迁往那里），他在无菌实验室里埋头苦干，仔细选择古老岩石的样品，精密测定里面铅/铀的比例。

　　The problem with measuring the age of the Earth was that you needed rocks that were extremely ancient, containing lead- and uranium-bearing crystals that were about as old as the planet itself—anything much younger would obviously give you misleadingly youthful dates—but really ancient rocks are only rarely found on Earth. In the late 1940s no one altogether understood why this should be. Indeed, and rather extraordinarily², we would be well into the space age before anyone could plausibly³ account for where all the Earth's old rocks went. (The answer was plate tectonics, which we shall of course get to.) Patterson, meantime, was left to try to make sense of things with very limited materials. Eventually, and ingeniously, it occurred to him that he could circumvent⁴ the rock shortage by using rocks from beyond Earth. He turned to meteorites⁵.

　　测定地球年龄的问题在于，你需要有极其古老的岩石，内有含铅和铀的晶体，其古老程度几乎与这颗行星一样──要是岩石年轻得多，测出的年代显然会比较年轻，从而得出错误的结论，而真正古老的岩石在地球上是很难找得着的。到20世纪40年代末，谁也不知道这是什么原因。实际上，要等到太空时代，才可能有人貌似有理地说明地球上古老岩石的去向，这真是不可思议的。（答案在于板块构造，我们当然将谈到这个问题。）与此同时，彼得森只能在材料非常有限的情况下把这一切搞清楚。最后，他突然聪明地想到，他可以利用地球之外的岩石，从而绕开缺少岩石的问题。他把注意力转向陨石。

　　地球爷爷

　　The assumption he made—rather a large one, but correct as it turned out—was that many meteorites are essentially⁶ leftover⁷ building materials from the early days of the solar system, and thus have managed to preserve a more or less pristine⁸ interior chemistry. Measure the age of these wandering rocks and you would have the age also (near enough) of the Earth.

　　他提出了一个假设──一个很有远见的假设，结果证明非常正确，即，许多陨石实际上是太阳系早期留下来的建筑材料，因此多少保留着原始的内部化学结构。测定了这些四处游荡的岩石的年代，你也就（接近于）测定了地球的年龄。

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