Cosmic-ray exposure age
An estimate of how long a meteoroid travelled through space as a small body, based on the effects of cosmic rays on its atoms.
Space is full of cosmic rays, extremely fast atomic nuclei, mostly protons. They penetrate only about a metre or two into rock, so while a meteorite is buried deep inside its parent asteroid it is shielded. Once an impact frees a metre-sized fragment, cosmic rays begin smashing nuclei throughout it and producing new cosmogenic isotopes, such as helium-3, neon-21 and argon-38. They build up at a roughly known rate, so measuring how much has accumulated gives the cosmic-ray exposure age: the time between the fragment's release and its arrival on Earth.
These ages are a record of collisions. Many H chondrites have exposure ages clustering at about 7 to 8 million years, suggesting a large break-up event on their parent body at that time. Most stony meteorites have exposure ages of under about 100 million years, while iron meteorites, which are tougher, can have ages of hundreds of millions of years. The martian nakhlites, including Nakhla, share an exposure age of around 11 million years, suggesting they were blasted off Mars by a single impact.
Exposure ages are usually given in millions of years and need corrections for the depth of the sample within the meteoroid.
Related terms
Level: Enthusiast Teens and adults