The time it takes for a meteoroid to reach Earth from the asteroid belt is an important constraint when trying to identify the mechanism or mechanisms responsible for delivering meteoroids to Earth. The time cannot be measured directly, but an indication of it can be found from cosmic-ray exposure ages of meteorites. This age measures how long a meteorite existed as a small meteoroid (less than a few metres across) in space or near the surface (within a few metres) within a larger body.
High-energy galactic cosmic rays— primarily protons—have a range of penetration on the order of a few metres in meteoroidal material. Any meteoroid of smaller dimensions will be irradiated throughout by this proton bombardment. The high-energy protons knock protons and neutrons out of the atomic nuclei of various elements present in the meteor-oid. As a consequence, a large number of otherwise rare isotopic species, both stable and radioactive, are produced. They include the stable noble gas isotopes helium-3, neon-21, argon-38, and kryp-ton-83 and various short- and moderately long-lived radioactive isotopes, including beryllium-10 (half-life 1.6 * 106 years), alu-minium-26 (7.3 * 105 years), chlorine-36 (3 * 105 years), calcium-41 (105 years), man-ganese-53 (3.7 * 106 years), and krypton-81 (2.1 * 105 years). The concentration of the radioactive isotopes can be used to monitor the cosmic-ray bombardment rate, and the accumulation of the stable species (e.g., neon-21) measures the total time since this bombardment began—i.e., the time since the meteoroid was excavated by collisions from an object that was large enough to shield it from cosmic rays.
The vast majority of meteorites have exposure ages that are greater than one million years. For chondritic meteorites, the number of meteorites with a given cosmic-ray exposure age drops off quite quickly as the age increases. Most ordinary chondrites have exposure ages of less than 50 million years, and most carbonaceous chondrites less than 20 million years. Achondrites have ages that cluster between 20 and 30 million years. Iron meteorites have a much broader range of exposure ages, which extend up to about two billion years. There are often peaks in the exposure age distributions of meteorite groups; these probably reflect major impact events that disrupted larger bodies.
The ranges of exposure ages relate both to the dynamic evolution of meteor-oid orbits and to the collisional lifetime of the meteoroids. The almost total absence of meteorites with exposure ages of less than a million years suggests that mete-oroid orbits cannot become Earth-crossing in much less than a million years. Numerical simulations on computers are consistent with this, but they also predict that orbital lifetimes should fall off much faster than do the cosmic-ray exposure ages. This has prompted the suggestion that meteorites spend a significant fraction of their time as small meteoroids migrating within the asteroid belt until their orbits intersect a resonance—i.e., a region in the belt where they experience strong gravitational perturbations by the planets, particularly Jupiter—that puts the meteoroids in Earth-crossing orbits. The general drop-off in the frequency of meteorites with older exposure ages and the upper limit for most stony meteorites of 50 million years are consistent with estimates that half of any given meteor-oid population is eliminated by collisions in 5-10 million years. The longer exposure ages of iron meteorites suggest that their greater strength allows them to survive longer in space.
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