Age of the Solar System

Evidence of Solar System Originsfrom Radiometric Dating

Radiometric dating uses the radioactive decay of heavy elements to tell time

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• Radioactive decay is mostly independent of external influences

• Decay times span a large range:< 1 s – > billions of yrs

• Heavy elements are found in a wide range of materials

Radioactive decay is a natural result of dynamic interactions in large nuclei

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• Protons in the nucleus strongly repel each other

• Protons and neutrons in the nucleus attract each other by the strong nuclear force

• Stability of the nucleus is determined by the interplay between these two effects

Larger nuclei require more neutrons to counteract proton repulsion

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• Protons in the nucleus strongly repel each other

• Protons and neutrons in the nucleus attract each other by the strong nuclear force

• Stability of the nucleus is determined by the interplay between these two effects

Larger nuclei require more neutrons to counteract proton repulsion

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Uranium isotopes are good examples of long-lived nuclei, decaying over billions of years PA STEM

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There are two main ways that heavy nuclei decay

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• Alpha decaynucleus gives off a 4He nucleus(2 protons, 2 neutrons)

• Beta decayneutron changes to protonand gives off an electron

n → p + e-

Heavy elements continue alpha and beta decay until a stable nucleus is reached

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Time of decay is measured by the half-life, which is the time for half the radioactive nuclei to change into stable nuclei PA STEM

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# of half-lives parent daughter

1 1/2 1/2

2 1/4 3/4

3 1/8 7/8

4 1/16 15/16

A number of radioactive nuclei have half-lives of use in dating the solar system

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Parent Half-life (years) Daughter

87Rb37 48.8 billion 87Sr38

232Th90 14.0 billion 208Pb82

238U92 4.47 billion 206Pb82

40K19 1.25 billion 40Ar18

235U92 0.704 billion 207Pb82

Solar system should be the same age as the oldest meteorites (4.567 billion years)

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Both CAIs and chondrules in meteorites can be dated to this earliest period

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The average age of the earliest material is 4.56730 ± 0.00016 billion years1

1. James N. Connelly, et al., Science 338, 651 (2012)

CAIs formed quickly but chondrules continued forming for 3 million years

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The CAIs may have formed within 50,000 years of the start of the solar system1

1. James N. Connelly, et al., Science 338, 651 (2012)

Inferred relative abundances of short-lived isotopes 182Hf and 129I reveal the pre-history of the solar system PA STEM

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1. Maria Lugaro, et al., Science 345, 650 (2014)

Earth must be younger than the solar system but older than the oldest rocks

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The famous Jack Hills zircons (Australia) are dated to 4.404 billion years ago

Other surface rocks support this datingPA STEMScience

Ages of rocks from Mars and the Moon also support it