The asteroid that struck Earth 66 million years ago, wiping out three-quarters of species including the non-avian dinosaurs, was most likely a rare CO chondrite from the outer solar system. That conclusion rests on careful nickel isotope analysis of samples from the global K-Pg boundary clay layer, the thin deposit that marks the Cretaceous-Palaeogene extinction horizon.
Researchers from the University of British Columbia together with collaborators in Paris, Brussels and Vienna examined these isotopes, which act as a distinctive fingerprint for extraterrestrial material. Their work, published in Science Advances, indicates the impactor belonged to the Ornans-type carbonaceous chondrite family. Such meteorites make up only a small fraction of the roughly five per cent of all meteorites that are carbonaceous chondrites.
This incremental advance in isotopic geochemistry illustrates the strength of sustained empirical inquiry. Rather than reaching for dramatic speculation, the team worked within the limits of preserved evidence to clarify one of the pivotal events in Earth history.
The identified meteorite type contains low levels of volatile elements, including sulphur. As a result, sulphur derived from the asteroid itself appears less likely to have been the main driver of the extinction. Instead the primary cause is attributed to the vast quantities of fine debris ejected into the atmosphere by the impact.
The researchers concluded that the fine debris thrown into the atmosphere would have been the primary factor in the extinction event.
The object itself is thought to have measured between 10 and 15 kilometres across and to have been travelling at roughly 64,000 kilometres per hour when it carved out the Chicxulub crater beneath what is now the Yucatán Peninsula. The new data refine earlier ruthenium isotope studies that had already pointed toward a carbonaceous chondrite but left the precise subtype unresolved.
Dr Philippe Claeys stated that carbonaceous chondrites of the Ornans class are definitely not like the typical meteors found in museum collections and that being impacted by such a rare distant projectile really underscores how unlucky the dinosaurs were.
