A clock from the dawn of time
In the early morning of February 8, 1969, a brilliant fireball exploded over northern Mexico, scattering thousands of fragments of a rare meteorite across a 50 by 8-kilometer patch of the Chihuahuan Desert. This event delivered over two metric tons of the Allende meteorite to Earth, making it the largest and most-studied carbonaceous chondrite in history. Inside this meteorite are some of the oldest solid objects in our solar system: light-colored specks called Calcium-Aluminum-rich Inclusions (CAIs). Radiometric dating of CAIs from meteorites like Allende places their formation at 4.567 billion years ago, an age that effectively marks the birth of the Solar System. These inclusions are cosmic time capsules, recording chemical and isotopic clues about the conditions in the solar nebula before the planets formed.
The Allende meteorite and its CAIs hold evidence of elements that no longer exist in nature. One of these is Hafnium-182 (¹⁸²Hf), a radioactive isotope with a half-life of about 8.9 million years. Because of this relatively short half-life, all the ¹⁸²Hf that was present at the beginning of the Solar System has long since decayed away, primarily into a stable isotope of tungsten, Tungsten-182 (¹⁸²W). While ¹⁸²Hf is now extinct, its daughter product, ¹⁸²W, remains locked in ancient materials, recording its parent element. By measuring subtle variations in the amount of ¹⁸²W in meteorites, scientists can reconstruct events that happened within the first few tens of millions of years of the Solar System's life.