How do scientists know how long a meteorite has been on Earth?
They measure special atoms made by cosmic rays in space. These atoms slowly disappear once a meteorite lands, so measuring what is left shows how long ago it fell.
Many meteorites are found long after they fell. Some have been lying on Earth for thousands or even a million years. Scientists can work out how long, using a clever clock.
Atoms made in space. While a space rock travels through space, it is hit by cosmic rays, tiny fast particles from the Sun and distant stars. These rays change some of the atoms in the rock into special radioactive atoms, such as carbon-14 and chlorine-36.
The clock starts. When the meteorite lands on Earth, the air above protects it from cosmic rays, so no new special atoms are made. The ones already inside slowly change into other atoms at a steady, known rate.
Reading the clock. By measuring how many special atoms are left, scientists can work out how long the meteorite has been on the ground. Carbon-14 works for meteorites up to about 40,000 years old. Chlorine-36 works for much older ones, hundreds of thousands of years.
Results. Many meteorites from hot deserts have been on Earth for thousands to tens of thousands of years. Some from Antarctica have been there for more than a million years, frozen in ice.
Other clues. How rusty a meteorite is also gives a rough hint, but weather matters too: a meteorite rusts much faster in rainy Britain than in a dry desert.