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How old is the Earth? Today you can look up 4.5 billion years in a second, but someone had to measure it, and the measurement turned out to be a fight against invisible contamination. Clair Patterson, an American geochemist, got the answer from lead in meteorites. Along the way he discovered how far industrial lead, from petrol, paint and solder, had spread through the environment. This post covers his life, how the dating worked, a worked half-life example, common mistakes, and the working habit that made him different.
Who was Clair Patterson?
His National Academy of Sciences biographical memoir by George R. Tilton, who worked alongside him, gives his dates as 2 June 1922 to 5 December 1995. He grew up in Mitchellville, Iowa, where a chemistry set from his mother started his interest. He studied chemistry at Grinnell College and the University of Iowa, worked on the Manhattan Project, including at Oak Ridge where he met mass spectrometers, and then did a PhD at the University of Chicago with Harrison Brown.
Brown's idea was that iron meteorites contain almost no uranium, so the lead in them should still have the isotope mix it had when the solar system formed. Patterson's job was to measure tiny amounts of lead precisely.
How did Patterson date the Earth?
The memoir describes the steps. Patterson followed Brown to the California Institute of Technology, where he built facilities that "set new standards for low-level lead work". By 1953 he measured the primordial lead in the troilite (iron sulphide) of the Canyon Diablo iron meteorite. Chemical separation was done at Caltech, and the mass spectrometer runs at the University of Chicago. The first answer was 4.5 billion years.
He then measured lead in two chondrites and an achondrite. Combined with the iron meteorite lead, these gave a 207Pb/206Pb age of 4.55 ± 0.07 billion years, published as Age of meteorites and the earth in Geochimica et Cosmochimica Acta in 1956. The memoir explains that the achondrite mattered most, because its lead ratios were unlike any found in rocks on Earth, which ruled out contamination as the explanation.
Worked example: why two uranium clocks help
The IAEA's nuclear data service lists the half-life of uranium-238 as 4.468 × 10^9 years and of uranium-235 as 7.04 × 10^8 years. The fraction of a radioactive isotope left after time t is (1/2)^(t/T), where T is the half-life.
Take t = 4.55 × 10^9 years.
Uranium-238: t/T = 4.55 ÷ 4.468 ≈ 1.018 half-lives. Fraction left = (1/2)^1.018 ≈ 0.494. Just under half remains.
Uranium-235: t/T = 4.55 ÷ 0.704 ≈ 6.46 half-lives. Fraction left = (1/2)^6.46 ≈ 0.0113, about 1.1%.
Compare. Over the same time, uranium-235 has decayed far more completely than uranium-238. The ratio of fractions left is about 0.494 ÷ 0.0113 ≈ 44.
Because the two parent isotopes decay at such different speeds, the amounts of lead-207 and lead-206 they produce grow at different rates. Their ratio therefore changes steadily with time. Measuring it, and correcting for the lead that was present at the start (the role of the Canyon Diablo troilite), gives an age without needing to know how much uranium was there originally.
For the basic half-life maths, see our post on radioactive decay and half-life.
How did dating the Earth lead to a fight over lead?
To get clean numbers, Patterson had to remove every trace of outside lead from his samples, so he kept finding it in places it should not be. The memoir describes how measuring lead in ocean water and sediments convinced him that human activity was adding lead far faster than nature removed it. In his 1965 paper, Contaminated and Natural Lead Environments of Man, he estimated that lead in the blood of many Americans was over 100 times the natural level.
Later work by his group, according to the memoir, found that modern Greenland snow contained over 100 times the lead of preindustrial snow, and that lead in canned tuna was far higher than in fresh fish because of lead solder. He pressed governments to act, and the memoir records the United States EPA announcing a phased reduction of lead in petrol in December 1973.
What mistakes do students make with radioactive dating?
Thinking half the sample is gone after one half-life of the whole rock. Half of that particular isotope has decayed, not half the rock.
Using t/T as the fraction. The fraction left is (1/2)^(t/T), not t/T.
Forgetting initial daughter atoms. Some lead was there from the start; Patterson's meteorite lead was the correction for that.
Assuming any contamination is small. For trace measurements, a speck of outside lead can swamp the signal.
If exponents like (1/2)^6.46 slow you down, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer.
What can a student learn from how Patterson worked?
Patterson did not trust a number until he trusted the blank, the measurement of how much contamination his own process added. That habit gave him the age of the Earth and then showed him a public health problem that earlier, contaminated measurements had underestimated. The memoir notes that when asked whether other labs agreed with him, he replied that scientific matters are not decided by majority vote. In any practical, measure your blank or zero error first.
Use (1/2)^(t/T) to find how much uranium-238 would be left after 9.1 billion years, and check that your answer is close to one quarter.
