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.
The Hafnium-Tungsten chronometer
The decay of ¹⁸²Hf to ¹⁸²W is a tool for dating the formation of planets and their cores, a system known as the Hafnium-Tungsten (Hf-W) chronometer. The method works due to the different chemical behaviors of the two elements. Hafnium is "lithophilic," meaning it prefers to bond with silicate minerals and stays in a planet's rocky mantle. Tungsten, on the other hand, is "siderophilic," or iron-loving, and preferentially sinks into a planet's metallic core during its formation.
This chemical separation is the basis of the clock. If a planetesimal melts and differentiates into a core and mantle very early—while ¹⁸²Hf is still abundant—the iron core will form with very little hafnium and thus will not gain any new ¹⁸²W. The mantle, however, retains all the ¹⁸²Hf, which continues to decay and produces an excess of ¹⁸²W in the silicate portion of the body compared to undifferentiated meteorites.
By measuring the ¹⁸²W excess in planetary mantle materials (like Earth rocks or Martian meteorites) and comparing it to the baseline tungsten isotopic ratios found in the CAIs of the Allende meteorite, scientists can calculate how quickly that body's core formed. Measurements show that Mars formed its core within about 10-20 million years, while Earth's core took longer, finishing its formation within the first 30 to 50 million years of the Solar System's existence.