The dinosaur-killing impactor was probably a rare asteroid from beyond Jupiter

The minerals left behind when the Chicxulub crater formed, ending the Cretaceous period and the reign of the dinosaurs, suggest it was an asteroid, not a comet. However, the same analysis suggests it came from the outer solar system, not the main asteroid belt. This knowledge is useful, if disheartening, when trying to prevent the next such event.

When evidence first emerged that the extinction of the dinosaurs was caused by an impact from space, no one knew what the object was. Many people called it a comet, perhaps because they were more familiar with it. This suspicion was confirmed by observations of the comet Shoemaker-Levy 9, which hit Jupiter, and the films that Deep impact And Don't look up.

On the other hand, comet impacts are difficult to predict more than a few months in advance. The impact of an asteroid swirling around the inner solar system is much easier to prepare for. So NASA took the first step by testing our ability to move Dimorphos – with great success. Knowing which category the dino-killer falls into could help us at least a little in preparing for future dangers.

To achieve this goal, Dr. Mario Fischer-Gödde of the University of Cologne and his colleagues studied the isotope ratios of ruthenium samples deposited by the object. Ruthenium is one of the metals that is rare in the Earth's crust and mostly trapped in the Earth's core, but relatively common in space rocks. The concentration of these metals at the boundary between rocks deposited in the Cretaceous and Paleogene periods alerted scientists to the possibility of an impact before the Chicxulub crater was discovered.

Ruthenium has an unusually high proportion of seven stable isotopes, which provides ample scope for variations in their relative abundance. Fischer-Gödde and co-authors report that the ratios of five of these isotopes are consistent within measurement uncertainties at the Cretaceous-Paleogene boundary at five sites across Europe.

For comparison, the authors looked at the ratios of meteorites and ruthenium released when five other large craters formed over the past 541 million years. The isotope ratios for ruthenium released by volcanoes and for impacts 3.2 to 3.5 billion years old were also compared.

Based on this, the authors conclude that the dino-killer was a type C asteroid. This type produces carbonaceous chondrite meteorites, a rare type known to contain the molecules necessary for the emergence of life.

C-type asteroids form in the outer solar system, beyond the orbit of Jupiter. Although comets also form at such distances, the meteorites they produce, called CI chondrites, have very different ruthenium ratios.

In the Archean, it was considered particularly cautious to warn about C-type asteroids, provided that there were other living beings besides single-celled organisms that could make these observations. The other five impacts studied from the Age of Animals, however, all appear to have been S-type asteroids, so these probably pose the greatest threat.

A C-type asteroid could be thrown into an orbit around the inner solar system where it passes Earth often, allowing us to detect and deflect it. But there is also a great risk that it could come from too far away to notice and hit us with only a few months or years' warning.

S-type asteroids, on the other hand, are more likely to give us plenty of opportunity to take evasive action, so it's encouraging that most of the big impacts after the solar system settled down came from them.

A recent study suggested a comet as the most likely impactor, but this was based on model calculations rather than hard evidence. Even with comets, the probability of a direct impact is higher than one that precedes an impact thousands of orbits in the inner solar system. On the other hand, at least the gases released when a comet approaches the sun could help us detect it a little earlier.

The study was published in Science.