Short-lived radionuclide
A short-lived radionuclide has a half-life of millions of years or less and was present only in the early Solar System.
A short-lived radionuclide is a radioactive isotope with a half-life short compared with the age of the Solar System, usually less than about 100 million years. Examples include aluminium-26, with a half-life of about 0.72 million years, manganese-53 at about 3.7 million years, iron-60 at about 2.6 million years, hafnium-182 at about 9 million years and iodine-129 at about 16 million years.
All of the original supply has long since decayed. We know they were present because meteorite minerals contain excess daughter isotopes, such as magnesium-26 from aluminium-26, in proportion to the parent element.
Short-lived radionuclides matter in two ways. They provide high-precision clocks for the first few million years of history, dating chondrules, CAIs and core formation relative to one another. And their decay produced heat: aluminium-26 in particular is thought to have melted many early asteroids, giving rise to iron cores, basaltic crusts and the achondrites.
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