Extinct radionuclides and the first few million years

Some radioactive isotopes present when the Solar System formed have long since decayed away. Their daughters date events in the first few million years with remarkable precision, and one of them melted the early asteroids.

Isotopes that have vanished

Aluminium-26 decays to magnesium-26 with a half-life of about 0.72 million years. It was present in the early Solar System but has decayed completely. Its former presence is shown by excess ²⁶Mg in aluminium-rich minerals, in proportion to their aluminium content.

Because the initial ²⁶Al/²⁷Al ratio declined with time, the ratio frozen into a mineral when it formed tells us when it formed relative to the CAIs, which have the highest ratio. Chondrules, for example, record ratios that indicate formation about 1 to 3 million years after CAIs. Other extinct isotopes, such as hafnium-182, which decays to tungsten-182 with a half-life of about 9 million years, date the formation of metal cores.

A heat source

Each ²⁶Al decay releases about 4 MeV. In the first million years, enough aluminium-26 was present in rock to release far more heat than the decay of all the long-lived isotopes combined. Bodies that assembled within roughly the first 1.5 million years, while ²⁶Al was still plentiful, heated up enough to melt and differentiate, producing iron cores and basaltic crusts. Bodies that formed later had too little ²⁶Al left to melt, and became the chondrite parent bodies, heated only enough for metamorphism.

Anchoring the timescale

Short-lived isotopes give only relative ages. To convert them to absolute ages, the same samples are dated by lead–lead, which gives an absolute age of about 4,567 million years for CAIs. Anchored in this way, the chronology shows that iron meteorite cores formed within a million or two years of CAIs, earlier than many chondrules, so melted planetesimals existed while chondrules were still forming.

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