Fossil tracks
Fossil tracks are microscopic trails of damage left in crystals by heavy cosmic-ray nuclei or fission fragments.
When a heavy, fast-moving nucleus, such as an iron nucleus in a cosmic ray, passes through a crystal, it leaves a narrow trail of damage. These trails are too small to see directly, but chemical etching widens them into visible channels under a microscope. They are called fossil tracks or nuclear tracks.
In meteorites, track densities in olivine and pyroxene fall off rapidly with depth, because heavy cosmic-ray nuclei are stopped within a few tens of centimetres. Measuring track densities across a meteorite shows how deeply each sample was buried in the original meteoroid and how much material was lost in the atmosphere.
Tracks can also come from the spontaneous fission of plutonium-244, an extinct radionuclide, or uranium-238. Very high track densities in grains near the surface of regolith breccias show those grains were exposed to solar flare particles at an asteroid's surface before being buried. Fossil tracks are therefore both a clock and a depth gauge.
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