How meteorites are dated

Meteorites carry several clocks at once. Read correctly, they tell us when a rock formed, when it was knocked off its parent body and when it landed on Earth.

Radioactive clocks

Some atoms are radioactive: they change, or decay, into other atoms at a fixed rate. The time it takes for half of a radioactive isotope to decay is its half-life. Isotopes are forms of the same element with different numbers of neutrons.

If a mineral traps a radioactive parent isotope when it forms, the daughter isotope builds up over time. By measuring the ratio of parent to daughter, and knowing the half-life, scientists can calculate how long ago the mineral formed or last cooled enough to hold on to the daughter.

Different systems suit different questions. Uranium-lead and lead-lead dating, using uranium's long half-lives, give the most precise ages for ancient materials. CAIs dated this way give about 4,567 million years, the oldest dated solids from the Solar System.

Formation and events

Many clocks have been applied to meteorites. Rubidium-strontium and samarium-neodymium are used for igneous rocks such as eucrites and Martian meteorites. Potassium-argon and its refined form, argon-argon dating, are sensitive to heating and are often used to date impact events.

Short-lived radioactive isotopes, which have now completely decayed, also leave traces. Aluminium-26, with a half-life of about 717,000 years, decayed into magnesium-26. Excess magnesium-26 in a mineral reveals that it formed while aluminium-26 was still alive, and the amount gives a relative age. Such methods show that chondrules mostly formed within about the first 1 to 3 million years after CAIs.

Results from different methods must be checked against each other, because each can be affected by later heating, shock or weathering.

Time in space

While a meteoroid travels through space, it is exposed to cosmic rays, very high-energy particles mainly from beyond the Solar System. They penetrate about a metre into rock and break atoms apart, producing new isotopes called cosmogenic nuclides, such as helium-3, neon-21 and argon-38.

Inside a large asteroid, rock is shielded. Once a fragment of metre size or smaller is broken off, cosmogenic nuclides start to build up. Measuring them gives the cosmic-ray exposure age, roughly the time since the meteoroid became a separate small body.

Most stony meteorites have exposure ages of less than about 100 million years, and many H chondrites cluster around 7 to 8 million years, suggesting a single break-up event. Iron meteorites, being tougher, often have ages of hundreds of millions of years.

Time on Earth

Once a meteorite lands, Earth's atmosphere shields it from most cosmic rays. Radioactive cosmogenic nuclides formed in space then begin to decay without being replaced.

By measuring how much remains, scientists calculate the terrestrial age, the time since the meteorite fell. Carbon-14, with a half-life of about 5,730 years, suits meteorites that fell within the last few tens of thousands of years. Longer-lived isotopes such as chlorine-36 and krypton-81 can reach back hundreds of thousands of years or more.

Terrestrial ages show that many hot desert meteorites have been on Earth for thousands to tens of thousands of years, while some Antarctic meteorites have been there for more than a million years. They also help identify paired meteorites and measure how quickly weathering proceeds in different climates.

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