Planetary Science

The 'Oddball' Meteorite That Killed the Dinosaurs Was a Rare Type of Space Rock

For decades, scientists have known that a massive asteroid impact 66 million years ago ended the age of dinosaurs. But what kind of asteroid was it? A new study using advanced nickel isotope analysis has finally identified the impactor as a rare class of meteorite called a CO chondrite — an "oddball" space rock that makes up less than 0.1 percent of all known meteorites.

• Researchers from the University of British Columbia, Vrije Universiteit Brussel, and Université de Paris analysed nickel isotopes preserved in the thin global layer of clay left by the Chicxulub impact. The isotopic signature matched only one type of meteorite: carbonaceous chondrites of the Ornans class (CO chondrites), which are among the most primitive and pristine materials in the solar system.
• CO chondrites contain far less sulphur, carbon, and water than ordinary meteorites. This finding challenges the prevailing theory that sulphur from the impactor itself was the main driver of the global winter that followed the strike. Instead, the team concluded that sulphur from the target rock — the lime-rich crust of the Yucatán Peninsula — was the primary culprit behind the climate catastrophe.
• The impactor was roughly 10 to 15 kilometres wide and struck at an estimated 64,000 kilometres per hour, forming the 180-kilometre-wide Chicxulub crater beneath modern-day Mexico. The study's lead author noted that being hit by such a rare projectile "really underscores how unlucky the dinosaurs were."

Sixty-six million years ago, a space rock more than 10 kilometres wide slammed into what is now the Yucatán Peninsula in Mexico. The impact released energy equivalent to billions of atomic bombs, triggering a chain of events that wiped out roughly 75 percent of all species on Earth, including every non-avian dinosaur. The resulting crater, Chicxulub, was discovered in the 1970s, and since then scientists have debated the exact composition of the object that caused it.

Now, a team of researchers from UBC, Paris, Brussels, and Vienna has cracked the case. Using high-precision nickel isotope measurements of samples taken from the global layer of clay deposited by the impact — known as the K-Pg boundary layer — they found a chemical fingerprint that matches only one type of meteorite: a CO chondrite. Carbonaceous chondrites make up only about five percent of meteorites found on Earth, and the Ornans subtype is an even rarer subset. These are some of the most primitive objects in the solar system, unchanged since the planets first formed.

The finding has important implications for understanding the extinction event. CO chondrites contain relatively little sulphur compared to other meteorite types. This means that the sulphur that caused the global cooling and acid rain after the impact likely came not from the asteroid itself, but from the sedimentary rock at the impact site. The Yucatán crust is rich in evaporite minerals, including gypsum and anhydrite, which contain vast quantities of sulphur. When the asteroid hit, it vaporised these rocks, injecting sulphur dioxide into the stratosphere, where it reflected sunlight and caused a prolonged "impact winter."

The origin of the impactor remains an open question. Potential sources include the outer asteroid belt near Jupiter or the debris-rich outer reaches of the solar system. Further isotopic studies of other impact events may help trace the orbits of these rare objects back to their source regions.