The minerals left behind when the Chicxulub Crater was created, ending the Cretaceous Era and the dinosaurs’ reign, indicate it was an asteroid, not a comet. However, the same analysis indicates it came from the outer Solar System, rather than the main asteroid belt, knowledge that is useful, if disheartening, in trying to prevent the next such event.

When evidence first emerged that the demise of the non-avian dinosaurs was triggered by an impact from space, no one knew what type of object was responsible. Many people referred to it as a comet, perhaps because they’re more familiar. This suspicion was reinforced by observations of comet Shoemaker-Levy 9 smashing into Jupiter, and the films Deep Impact and Don’t Look Up. 

On the other hand, cometary impacts are hard to predict more than a few months ahead of time. It’s much easier to prepare for a hit from an asteroid rattling around the inner Solar System, so NASA took the first step by testing our capacity to move Dimorphos, with great success. Knowing which category the dino-killer fell into could help us prepare for future dangers, at least a little.

To assist that goal, Dr Mario Fischer-Gödde of the University of Cologne and colleagues investigated the isotope ratios of samples of ruthenium deposited by the object. Ruthenium is one of the metals that are rare in the Earth’s crust, having been mostly captured by the core, but are relatively abundant in space rocks. The concentration of these metals at the boundary between rocks deposited in the Cretaceous and Paleogene periods is what alerted scientists to the possibility of an impact before the Chicxulub Crater was found.

Ruthenium has an unusually high seven stable isotopes, providing plenty of opportunity for their relative abundance to vary. Fischer-Gödde and co-authors report ratios of five of these isotopes are consistent at the Cretaceous-Paleogene boundary at five sites across Europe within measuring uncertainties.

For comparison, the authors looked at ratios from meteorites and ruthenium released when five other large craters were produced over the last 541 million years. The isotope ratios for ruthenium released by volcanoes, and for 3.2-3.5 billion year-old impacts, were also compared.

Based on this, the authors conclude the dino-killer was a C-type asteroid, the sort that produce carbonaceous chondrite meteorites, a rare type known to contain the molecules needed to build life. 

C-type asteroids formed in the outer Solar System, beyond the orbit of Jupiter. Although comets also originate at such distances, the meteorites they produce, known as CI chondrites, have very different ruthenium ratios.

During the Archaean Age, C-type asteroids were the ones to watch out for, had there been anything beyond single-celled organisms to do the watching. On the other hand, the other five impacts investigated from the age of animals all appear to have been S-type asteroids, so these are probably the most likely threat.

A C-type asteroid could be slung into an orbit around the inner Solar System that makes many passes by Earth, giving us opportunities to detect, and subsequently deflect, it. However, there is also a great danger that it will come in from a distance where we are unlikely to spot it and hit us with only months or a few years’ warning.

S-type asteroids, on the other hand, are much more likely to give us plenty of opportunity to take evasive action. Consequently, it’s encouraging that most of the big impacts, once the Solar System settled down, came from these.

One recent study proposed a comet was the most likely impactor, but this was based more on modeling than hard evidence. Comets would also have a high likelihood of a direct strike, rather than one preceded by thousands of loops around the inner Solar System. On the other hand, at least the gasses released as one approaches the Sun might help us spot it a little earlier.

The study is published in Science.