argon-40

After a few hundreds of millions of years have elapsed, a scientist comes upon the same enclosed space and measures the relative proportions of potassium-40 and argon-40.

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Notice that we must know the ratio of parent (potassium-40) to daughter (argon-40) isotopes.

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So, imagine that you start with some quantity of potassium-40 in an enclosed space with no argon-40.

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But, for the reason I have just explained, this quantity is meaningful only if expressed as the ratio of potassium-40 to argon-40.

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So geologists, by measuring the ratio between potassium-40 and argon-40 in a piece of igneous rock that they pick up today, can tell how long ago the rock first crystallized out of its molten state.

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If you start with some quantity of potassium-40, after 1.26 billion years half of the potassium-40 will have decayed to argon-40.

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The accumulating quantity of argon-40 is a measure of the time that has elapsed since the rock was formed.

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In 2005, the Huygens probe also discovered the presence of argon-40 on Titan, the largest moon of Saturn.

Argon 2009

After another 1.26 billion years, half of the remaining potassium-40 will have been converted to argon-40, and so on.

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It is often called the potassium argon clock, because argon-40 (one lower in the periodic table) is one of the elements to which potassium-40 decays (the other, resulting from a different kind of radioactive decay, is calcium-40, one higher in the periodic table).

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