How can I explain the age of the Earth using meteorites?
In 1956 the American scientist Clair Patterson used lead in meteorites to calculate an age of about 4.55 billion years for Earth and the Solar System.
The story of how scientists found the age of the Earth is a superb case study in using evidence.
The problem. Earth's oldest rocks have been melted, weathered and recycled, so no surviving rock records the planet's birth. How could anyone measure its age?
The idea. If Earth formed at the same time as meteorites, from the same cloud of material, then dating meteorites would date Earth. Uranium slowly decays into lead at known rates, so measuring lead isotopes in meteorites can reveal their age.
Patterson's work. The American geochemist Clair Patterson measured lead isotopes in several meteorites, including troilite from the Canyon Diablo iron meteorite, which contains almost no uranium and so preserves the original lead. In 1956 he showed that meteorites and Earth's lead fit a single age of about 4.55 billion years. To make accurate measurements, he had to build an ultra-clean laboratory, because lead from petrol and paint contaminated everything. This led him to campaign against leaded petrol, a major public health achievement.
Since then. More precise measurements on the oldest meteorite inclusions give about 4,567 million years for the start of the Solar System. Earth formed over the following tens of millions of years.
In class. Pupils can make a timeline of Earth's history on a long roll of paper, at one metre per billion years, marking meteorite formation at 4.57 metres and the dinosaurs' extinction just 6.6 centimetres from the end.
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