How do scientists date a meteorite?
Scientists measure the products of natural radioactive decay in a meteorite's minerals, using several 'clocks' such as uranium-lead and rubidium-strontium. Other methods measure how long it travelled in space and how long it has been on Earth.
Rocks contain small amounts of radioactive elements that decay into other elements at steady, known rates. When minerals crystallise, they trap a certain amount of a parent element. Over time some of it turns into a daughter element. By measuring how much of each is present with an extremely sensitive instrument called a mass spectrometer, scientists can calculate how long the clock has been running. This is radiometric dating.
Several clocks are used, each suited to different jobs. Uranium-lead dating, based on uranium decaying into lead, is the most precise for very old material and has fixed the age of the oldest Solar System solids at about 4.567 billion years. Others include rubidium-strontium, samarium-neodymium and argon-argon dating. Argon is a gas that escapes when rocks are strongly heated, so argon ages often record later impacts rather than formation. A famous early success came in 1956, when the American geochemist Clair Patterson used lead from the Canyon Diablo iron meteorite to calculate the age of Earth and the Solar System at about 4.55 billion years, a figure that has barely changed since.
Scientists also use isotopes that were alive only in the early Solar System, such as aluminium-26 and hafnium-182, which have long since decayed. Their leftover products act as fine-scale timers, showing which objects formed first, sometimes to within a few hundred thousand years.
Two further ages describe the journey. Cosmic rays striking a small body in space create new isotopes, revealing how long it spent travelling. After landing, those isotopes, including radioactive carbon-14 and chlorine-36, slowly decay, letting researchers estimate how long a meteorite has lain on Earth.