Half-life and dating meteorites

Radioactive isotopes decay at a steady, predictable rate. Measuring how much has decayed in a meteorite tells us how long ago it formed.

Random but predictable

Some isotopes have unstable nuclei that decay by giving out radiation, changing into a different element. When one particular nucleus will decay is random. But in a sample of billions of nuclei, the proportion that decays in a given time is constant.

The half-life is the time taken for half the radioactive nuclei in a sample to decay, or for the activity to halve. After one half-life, 1/2 remains; after two, 1/4; after three, 1/8; and so on. The original isotope is called the parent and the product is the daughter.

Clocks in rocks

When a mineral crystallises, it traps some parent isotopes. From then on, daughter isotopes build up inside it. By measuring the amounts of parent and daughter, and knowing the half-life, scientists can calculate how long ago the mineral formed.

The right clock must suit the age. Potassium-40 decays partly to argon-40, a gas that builds up inside the mineral, with a half-life of about 1.25 billion years. Uranium-238 decays, through a chain of steps, to lead-206 with a half-life of about 4.5 billion years. Carbon-14, with a half-life of about 5,730 years, is used for much younger things, including how long a meteorite has been lying on Earth.

The age of everything

In 1956 Clair Patterson used lead isotopes from meteorites, including iron sulfide from the meteorite that made Meteor Crater, to calculate that Earth and the meteorites formed about 4.55 billion years ago. Modern lead isotope measurements on the oldest grains in chondrites give 4,567 million years, with an uncertainty of well under a million years. No rock on Earth's surface is that old, because Earth has recycled its earliest crust.

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