行星科学家们可以通过计算一个特定元素有多少“母亲”放射性同位素衰变为“女儿”同位素来确定岩石的年龄。
Planetary scientists can determine a rock's age by calculating how many radioactive "parent" isotopes of a particular element have decayed into "daughter" isotopes.
更妙的是，显然人们可以用放射性作为时钟，用衰变系列的半衰期计算出岩石的年龄。
Better still, it became apparent to him that one could use radioactivity as a clock, with the half-life of a decay series enabling us to calculate the age of a rock.
最为有名的就是铀- 238衰变为铅- 206的半衰期为45亿年，当然放射性铷和钐也同样对测算几十亿年前的物体的年龄很有用。
Most famously, uranium-238 decays into lead-206 with a half life of 4.5 billion years, though radioactive rubidium and samarium are also useful for dating things billions of years old.
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