From the Dark Depths Beyond Jupiter: The Origin of the Dinosaurs’ Killer Asteroid Revealed

Ever since the discovery of the ancient crater left over from the massive impact that wiped out the dinosaurs 66 million years ago, researchers have sought to uncover the origin of the meteor that heralded the extinction of more than three-fifths of life on Earth: was it an asteroid that originated from the depths of space, or one that had an orbit closer to home? An international team of researchers may have answered that question, using the chemical makeup of the geology directly affected by that fateful impact.

Estimated to be a city-sized object roughly 10 kilometers (6.2 miles) in diameter and travelling at 20 kilometers per second (12.4 miles/second), the object struck a point off of the coast of the modern-day Yucatán Peninsula in Mexico with enough force to release an amount of energy nearly two million times greater than the largest nuclear bomb ever detonated, the Soviet Union’s Czar Bomba. The resulting cloud of debris altered the planet’s climate dramatically, leading to an extinction event that wiped out more than 60 percent of Earth’s species, ending the age of the dinosaurs.

Using samples of Cretaceous/Palaeogene-era (K/Pg) rocks, the researchers compared the samples’ composition with ones taken from other impact sites dating back as far as 3.5 billion years. During the study, the team focused on the presence of a metal called ruthenium (Ru): although it is rare on Earth, it is relatively abundant in asteroids, particularly in those that orbit in the space beyond that of Jupiter.

During the formation of the early Solar System, the center of the protoplanetary disk of dust and debris that the Sun and planets would eventually form from was too hot to allow substances that are easily vaporized, such as water, to condense into either their liquid or solid forms; conversely, temperatures further out in the cloud were cooler, allowing these chemicals to condense.

This resulted in the asteroids that formed in the inner Solar System having fewer of these volatile compounds, instead being rich in silicate minerals, much like the crusts of Mercury, Venus, Earth and Mars. Further out, where conditions were cooler, the asteroids forming there wound up being richer in these volatile substances, including many carbon-based compounds, leading to such asteroids being referred to as ‘carbonaceous’, or C-type asteroids.

This brings us back to ruthenium: the researchers were able to map the distinctive mix of isotopes of Ru found in the samples, and matched those signatures to what was most common across the different types of asteroids, including C-types, and the silicate-rich S-type that formed closer to the Sun.

“Isotopic signatures that we measure can be regarded as some sort of fingerprint,” explained lead study author Mario Fischer-Gödde, an isotope geochemist from the Institute of Geology and Mineralogy at the University of Cologne. “So if there’s a huge impact, we vapourize rocks and the asteroid itself, but this fingerprint remains preserved.”

“All results clearly shows that no matter what site we are looking at… they all gave consistently the same isotopic signature of a C-type asteroid material,” Fischer-Gödde continued, with these results indicating that the massive Chicxulub meteor was a C-type that came from the region of space beyond Jupiter’s orbit. “So that’s why we can be quite confident about this.”

Although it was determined some time ago that the Chicxulub object was not a comet, Fischer-Gödde pointed out that we have yet to obtain a sample from the core of a comet: while the cores of meteors that enter the Earth’s atmosphere are hardy enough to survive their plunge to the surface, comets, being made largely of water, vaporize under the same conditions, so we are unable to say with absolute certainty that any traces of ruthenium they might contain don’t match those of C-type asteroids.

“I’m a scientist. I consider all possible outcomes, complexities and so on,” Fischer-Gödde said