Terrestrial age
The length of time a meteorite has been on Earth since it fell.
While a meteoroid travels through space, cosmic rays keep topping up radioactive isotopes inside it. Once it lands, the atmosphere and ground shield it, production stops, and those isotopes start to decay without being replaced. Measuring how much remains, compared with the amount expected in a meteoroid in space, gives the terrestrial age. For a witnessed fall it is zero; for finds it can be anything from years to many millennia.
Different isotopes cover different ranges. Carbon-14, with a half-life of about 5,730 years, suits ages up to a few tens of thousands of years. Chlorine-36, beryllium-10 and krypton-81 reach back hundreds of thousands to millions of years. Terrestrial ages show how quickly meteorites weather in different climates and how long they accumulate in places such as deserts. Some Antarctic meteorites have been on Earth for over a million years.
The extreme cases are fossil meteorites. In limestone quarries at Kinnekulle in southern Sweden, dozens of fossil meteorites have been found in rocks about 470 million years old; their original minerals have mostly been replaced, but chromite grains survive and identify them as L chondrites. Terrestrial ages are given in years before present, often with a large uncertainty.
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