Early Solar System chronology

Meteorites let us build a timeline of the first few million years of the Solar System with a precision that seems almost impossible. Here is how it is done.

Absolute ages: time zero

Lead-lead dating exploits the decay of uranium-238 and uranium-235 to lead-206 and lead-207, with half-lives of about 4.47 billion and 704 million years. Because the two decay at different rates, the ratio of radiogenic lead-207 to lead-206 gives an age without needing to measure uranium directly, provided the uranium isotope ratio is known.

Earlier work assumed a constant uranium-238 to uranium-235 ratio. Studies around 2010 showed that this ratio varies slightly between samples, so modern ages correct for measured uranium isotopes.

Using this approach, CAIs from CV chondrites give an age of about 4,567.3 million years, with an uncertainty of only a fraction of a million years. This is taken as time zero. Chondrules from the same meteorites give ages ranging from close to time zero to about three or four million years later.

Short-lived radionuclides

Several radioactive isotopes present at the start of the Solar System have since decayed completely. They include aluminium-26, with a half-life of about 0.72 million years; iron-60, about 2.6 million years; manganese-53, about 3.7 million years; and hafnium-182, about 8.9 million years.

Their former presence is detected through excesses of their daughter isotopes that correlate with the parent element. The slope of such an isochron gives the initial abundance of the radionuclide when the sample closed, and comparison with the initial abundance in CAIs gives a relative age.

These systems give relative ages, not absolute ones. To convert them, scientists use anchors: samples dated precisely by both lead-lead and a short-lived system, such as quickly cooled angrites like D'Orbigny. Assumptions of homogeneous distribution, especially for aluminium-26, are actively tested and debated.

Accretion and differentiation

Hafnium-tungsten dating is ideal for core formation, because hafnium stays in silicate while tungsten goes into metal. Iron meteorites from magmatic groups show tungsten isotope compositions that imply metal segregation within roughly the first one to three million years after CAIs, after correction for cosmic-ray effects.

Thermal models link this to aluminium-26 heating: a body that accreted within about the first one and a half million years would have contained enough aluminium-26 to melt. Bodies that accreted later could only be warmed enough for metamorphism or aqueous alteration.

The surprising consequence is that the parent bodies of iron meteorites are older than those of most chondrites, which accreted roughly two to four million years after CAIs. Chondrites are primitive in composition, but their parent bodies were latecomers. Basaltic achondrites such as angrites and some eucrites crystallised within a few million years.

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