About 66 million years ago, an asteroid roughly ten kilometres across struck what is now the Yucatán Peninsula in Mexico, and the age of the dinosaurs ended not long after. That much has been broadly settled for a while. What was missing until recently was an answer to a simpler-sounding question: where did the thing come from?
In 2024 a group of geochemists finally offered a strong answer, and it points to the outer solar system. I want to walk through what we actually know, what is well established, and what is still a reasonable argument rather than a settled fact.
I am a writer with a long interest in this material, not a planetary scientist, so treat this as a careful reading of the work rather than expert testimony.
What actually hit
The impactor is usually estimated at about ten to fifteen kilometres wide, travelling somewhere around twenty kilometres per second when it arrived. It left the Chicxulub crater, a buried scar roughly 180 kilometres across, partly under the seabed off the Yucatán and partly beneath the land.
The immediate effects were local and violent. Rock vaporised at the point of impact, and debris thrown into the sky reheated as it fell, heating the atmosphere and igniting fires over a wide area. The longer and deadlier problem was global. Dust and soot lofted into the upper atmosphere blocked sunlight for months to years, cooling the planet and collapsing the food chains that depend on photosynthesis.
Roughly three-quarters of species died out, including every non-avian dinosaur. The birds are the branch of the dinosaur family that made it through, which is worth remembering the next time one lands on your windowsill.
How we know an asteroid did it
The case was not obvious at first. It began in 1980, when the physicist Luis Alvarez and his geologist son Walter Alvarez noticed something odd in the thin clay layer that marks the end of the Cretaceous in rocks around the world. The layer was unusually rich in iridium, a metal rare in the Earth’s crust but more common in asteroids.
Their proposal, that a large impact had scattered this material worldwide, was contested for years. It gained ground as the supporting evidence accumulated: shocked quartz and glassy droplets formed under enormous pressure, and eventually the crater itself, identified at Chicxulub around 1990. The pieces fit a single event.
This is a good example of a hypothesis that started as a bold claim and became consensus because independent lines of evidence kept agreeing.
Where it came from
Knowing an asteroid hit is not the same as knowing which kind, or where it formed. That is the gap the 2024 work closed.
A team led by Mario Fischer-Gödde at the University of Cologne, writing in the journal Science, measured ruthenium isotopes in the impact layer at sites in Denmark, Italy and Spain. Ruthenium is delivered mainly by the impactor rather than the Earth’s crust, so its isotopic fingerprint effectively identifies the culprit. As reported in the coverage of the study, the signature matched carbonaceous chondrites, a primitive class of asteroid that formed in the cold outer reaches of the solar system, beyond the orbit of Jupiter.
That result also ruled out some alternatives. It did not match a comet, and it did not match the silicate, inner-system asteroids that the same team found were responsible for several other, smaller impact craters they tested. The dinosaur-killer, on this evidence, was a visitor from far out, not one of the usual near neighbours.
It is one study, built on a specific and well-understood tracer, but a single line of geochemistry, so it is fair to call it a strong finding rather than the last word.
Could it happen again
This is the question everyone reaches for, and the honest answer needs some proportion.
Objects the size of the Chicxulub impactor are rare, arriving on average tens of millions of years apart, and the largest near-Earth asteroids have mostly been found and catalogued. None of the known large ones are on a collision course. The genuine work now is finding the smaller objects that are harder to spot, which is why asteroid surveys and sample-return missions matter.
Scale is the thing to hold onto. I have written about asteroid Apophis, which will pass very close to Earth in 2029 without hitting it. Apophis is a few hundred metres across. Chicxulub was tens of times larger in diameter and vastly larger in energy. The two belong in different categories of event, and it is easy to blur them by using the same word for both.
What continues to be studied is less the fact of the impact than its mechanism and its aftermath: exactly how much of the killing was done by dust, how much by soot, and how much by sulphur released from the rocks at the impact site. Those proportions are still being worked out.
For now, the clearest new sentence we can add to the old story is a short one. The rock that ended the Cretaceous did not come from nearby. It formed beyond Jupiter, drifted inward over a very long time, and arrived on the worst possible day.
