What is the cosmic-ray exposure age of most meteorites?
Most stony meteorites travelled as small bodies for about 1 to 100 million years; iron meteorites typically for hundreds of millions of years.
A cosmic-ray exposure age measures how long a meteoroid travelled through space as a small body, roughly less than a few metres across, after being broken from its parent. During that time, cosmic rays created new isotopes inside it, which scientists measure to calculate the age.
Typical values: - Ordinary chondrites: mostly between about 2 and 60 million years. Many H chondrites have exposure ages around 7 to 8 million years, and many L chondrites around 40 million years, peaks that suggest specific collision events produced large numbers of fragments. - Carbonaceous chondrites: often shorter, with many CM chondrites below about 2 million years, perhaps because their fragile material does not survive long in space. - HED meteorites: clusters around about 6, 12, 22 and 36 to 39 million years, suggesting several impact events on Vesta or its family members. - Lunar meteorites: often very short, from less than a million years down to a few thousand years, because the Moon is so close to Earth. - Martian meteorites: mostly within the last 20 million years, in clusters linked to a small number of ejection events. - Iron meteorites: commonly hundreds of millions of years, up to over a billion, because metal is strong and survives collisions in space.
What this tells us. Peaks in exposure ages reveal collision events in the asteroid belt and on other bodies. Short exposure ages mean that delivery to Earth can be fast. The fact that most meteorites have exposure ages much shorter than the age of the Solar System shows that they were broken off their parent bodies relatively recently.