Spallation
Spallation is the splitting off of fragments, either of rock near an impact or of atomic nuclei struck by cosmic rays.
Spallation has two meanings in meteoritics. In impact physics, it describes how a shock wave reflecting from a free surface can fling off slabs of rock at high speed while subjecting them to relatively low pressure. Near the surface of a struck planet, a zone forms where the incoming and reflected waves partly cancel out.
This helps explain how Martian and lunar meteorites escaped. Rocks in the spall zone could be launched faster than escape velocity, about 5 kilometres per second for Mars, while staying solid and only moderately shocked.
In nuclear physics, spallation is what happens when a high-energy cosmic ray hits an atomic nucleus and knocks out protons and neutrons, leaving a lighter nucleus. This creates cosmogenic nuclides such as neon-21 and beryllium-10 in meteoroids. Measuring these products gives cosmic-ray exposure ages. The same word, from the English 'spall' meaning a chip of stone, covers both processes.
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