Dating meteorites

A meteorite has several ages: when it formed, when it was broken off its parent body, and how long it has been on Earth. Each is measured with a different isotope clock.

Radioactive clocks

Some isotopes are radioactive: their nuclei decay at a steady rate into other isotopes. The half-life is the time for half of the original, the parent, to decay. Measuring how much parent remains and how much daughter has built up in a mineral, and knowing the half-life, gives the time since the mineral formed. Uranium–lead, rubidium–strontium, samarium–neodymium and argon–argon methods all work this way, and agreement between them gives confidence.

Three ages

The formation age dates crystallisation or a heating event: about 4,560 million years for most asteroid meteorites, much younger for many from Mars. The cosmic-ray exposure age measures the time spent as a small body in space, usually millions to tens of millions of years for stones. The terrestrial age measures time on Earth since the fall, using isotopes made by cosmic rays in space, such as carbon-14 and chlorine-36, which decay once the meteorite is shielded by the atmosphere. Desert meteorites are typically thousands to tens of thousands of years old on Earth; some Antarctic meteorites have lain there for hundreds of thousands to a few million years.

The age of the Earth

In 1956 Clair Patterson used lead isotopes from meteorites, including troilite from the iron that made Meteor Crater, to calculate the age of Earth and the meteorites as about 4.55 billion years, a figure that still stands. Earth's own oldest rocks are younger because the planet has recycled its early crust; the meteorites preserved the original lead.

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