Aluminium-26
A short-lived radioactive isotope of aluminium, now extinct in early solar-system materials, that dated and heated the first planetesimals.
Aluminium-26 decays to magnesium-26 with a half-life of about 0.72 million years. It was present when the Solar System formed, probably delivered from nearby massive stars, and its former presence is detected through excess ²⁶Mg that correlates with the ²⁷Al/²⁴Mg ratio of minerals. Such correlations, first reported in 1976 in calcium-aluminium-rich inclusions from the Allende meteorite, give an initial ²⁶Al/²⁷Al of about 5 × 10⁻⁵ in most CAIs, the so-called canonical value.
It matters in two ways. As a chronometer, the decline of ²⁶Al/²⁷Al allows relative ages with resolution of around a hundred thousand years: many chondrules show initial ratios implying formation about one to three million years after CAIs. As a heat source, its decay energy was enough to melt planetesimals that accreted within roughly the first two million years, driving the differentiation that produced iron meteorites and basaltic achondrites. Bodies that formed later melted less and became chondrite parent bodies, with varying degrees of thermal metamorphism.
Cosmic rays also make ²⁶Al in meteoroids today. Its gamma-ray activity, measurable non-destructively, helps identify recent falls and estimate exposure and terrestrial histories. Relative ages assume ²⁶Al was evenly distributed, an assumption still debated.
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