cosmic-ray exposure age
The cosmic-ray exposure age, also known simply as the exposure age, is the interval for which a meteoroid was an independent body in space. In other words, the time between when a meteoroid was broken off its parent body (such as an asteroid) and its arrival on Earth as a meteorite. The exposure age can be estimated from the observed effects on a meteorite of bombardment by cosmic rays from the Sun and the rest of the Galaxy. As these cosmic rays struck the meteoroid while in space, they produced characteristic isotopes, both radioactive, such as helium-3, neon-21, and argon-38, and stable. The longer a meteoroid was exposed to cosmic rays, the more of these new isotopes are found to be present. Further dating information comes from an analysis of the fission tracks (thin trails left in a substance by a fast-moving atomic nucleus) that cosmic rays cause. Exposure ages range typically from a few million to a few hundred million years. To obtain the total age of a meteorite, including its time on Earth, the exposure age must be added to the terrestrial age. The period over which a meteoroid has been exposed to radiation in space, or that a rock has been exposed on the surface of a body.