Carbon-14 terrestrial dating
Measuring cosmic-ray-made carbon-14 left in a meteorite to estimate how many thousands of years ago it fell.
In space, cosmic rays strike oxygen and other atoms in a meteoroid and create radioactive carbon-14. After a few tens of thousands of years its amount reaches a steady level, where production balances decay. Once the meteorite falls, Earth's atmosphere blocks most cosmic rays, production stops and the carbon-14 decays with a half-life of about 5,730 years. Comparing what remains with the level expected in space gives the terrestrial age. Unlike archaeological radiocarbon dating, this carbon-14 is made inside the rock itself, not taken in from the air.
Scientists heat the sample to release carbon, carefully removing carbon from Earth such as weathering products and contamination, then count carbon-14 atoms with accelerator mass spectrometry. The method works up to about 40,000 to 50,000 years; older samples need isotopes with longer half-lives, such as chlorine-36.
It has been applied widely to finds from hot deserts, including the Sahara, Oman and the Nullarbor Plain of Australia. Many have terrestrial ages of less than about 50,000 years, revealing how fast meteorites weather in each climate. Results come with sizeable uncertainties, partly because the amount produced in space depends on the meteoroid's size and the sample's depth within it.
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