What is a cosmic-ray exposure age?
A cosmic-ray exposure age is the length of time a meteorite spent as a small body in space, exposed to cosmic rays, between breaking off its parent body and landing on Earth. It is measured from the rare isotopes those cosmic rays create.
Space is filled with cosmic rays, extremely fast-moving atomic nuclei, mostly from outside the Solar System. When they strike a rock, they smash atoms apart and create new isotopes that would otherwise be very rare, such as helium-3, neon-21 and argon-38, along with radioactive ones such as beryllium-10 and aluminium-26. These are called cosmogenic nuclides.
Cosmic rays only penetrate about a metre or two into rock. While a fragment is buried deep inside its parent asteroid, it is shielded. Once a collision breaks it free as a small body, its whole volume is exposed and cosmogenic nuclides begin to build up at a predictable rate. When it lands on Earth, our atmosphere and magnetic field largely stop the process. By measuring how much of these isotopes a meteorite contains, and knowing roughly how fast they form, scientists can calculate how long it spent travelling. That is its cosmic-ray exposure age.
The results are revealing. Most stony meteorites have exposure ages of a few million to a few tens of million years; many H chondrites cluster around 7 to 8 million years, suggesting that a single collision released lots of them. Iron meteorites, being tougher, often have much longer exposure ages, from hundreds of millions of years to more than a billion. Lunar meteorites often travelled for well under a million years, while groups of Martian meteorites share exposure ages that show they were launched by a handful of separate impacts on Mars.
Because clusters of similar ages point to particular collision events, exposure ages help connect meteorites to the break-ups that produced them, and they tell us that most of the rocks in our collections are recent arrivals by cosmic standards.