Hidden inside meteorites, scientists have found the fossils of isotopes that once proliferated around the solar system, but have since sunk into nothingness. Here’s how we study the “bones” of extinct radioactive elements.

Astronomers can’t wait to peer inside meteors. They don’t know exactly what they’ll find, but they know it will probably include magnesium-26, potassium-41, and niobium-93. These themselves aren’t what the researchers are looking for—any more than the minerals that make up a fossil are what paleontologists are looking for. They’re what’s left behind after the interesting isotopes have gone extinct. Magnesium-26 used to be aluminum-26, and potassium-41 used to be calcium-41. The original elemental crew of the meteor had half-lives much too short to make it to the present, and have long since died out. Collectively they’ve earned the name “extinct radionuclides.”

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Some of these isotopes can still be found out in space. Aluminum-26, for example, is produced when cosmic rays hit argon deposits on meteors. When the meteors fall to Earth, the cosmic rays get cut off and the aluminum decays, with a 700,000-year half-life, to magnesium-26.

Other extinct radionuclides are nowhere to be found in our solar system. Iron-60, with four more neutrons than the most common isotope of iron, is created in supernovas and nowhere else. Two of its neutrons spit out electrons and turn themselves into protons, the entire isotope become nickel-60, and it stays that way. We deduced that it existed, because of what it left behind.

Think dinosaur fossils are old? Deposits of nickel-60, according to some astronomers, are the remains of a particle that was formed in a supernova and that clumped into rocks orbiting when our star was still an infant. That’s old.

Image: USGS/D. Roddy