About 66 million years ago, an asteroid about 10 kilometres wide slammed into what is now the Yucatan Peninsula in Mexico. The debris hurled into Earth's sky would have plunged our planet into a freezing darkness, deadly for the photosynthetic life that formed the base of the food chain.
Scientists still debate whether this event was Solely responsible for the mass extinction of at least 60% of the Earth's species (including dinosaurs)or whether it was associated with a climatic catastrophe caused by volcanic eruptions that flooded India at the same time, or whether the volcanic calamity had already sealed the fate of the dinosaurs before the asteroid hit.
Illustration by Mark Garlick
But the Chicxulub impact certainly left its mark. In addition to a 200 km wide scar along the Mexican coast, the event It left a thin layer of clay in the global sedimentary recordPrevious studies of this layer have found rare elements that are best explained as asteroid debris, In particular, a type of asteroid called a carbonaceous chondrite..
Carbonaceous chondrites are dark, primitive objects, some of the most primitive remaining in the solar system. They are quite fragile and formed in the outer solar system, but today they are found throughout the asteroid belt between Mars and Jupiter.
Studies of other ancient impacts from the past few hundred million years have identified rocky asteroids, harder objects that formed in the inner solar system and now dominate the inner asteroid belt. That would make Chicxulub a rarity.
Mario Fischer-Gödde (University of Cologne, Germany) and an international team of scientists have reanalyzed it and confirmed that Chicxulub really was a carbonaceous chondrite.
The team used ruthenium, one of the platinum group elements found in meteoritic material but rare on Earth. The bodies will have different ratios of ruthenium isotopes depending on where they formed in the solar system, with objects that formed closer to the Sun containing more Ru-102 and Ru-100 than those that coalesced farther away. Each group of meteorites has its own “genetic fingerprint,” the authors explain in the Aug. 16 issue. ScienceBy measuring the isotopic ratios of the impacting objects, scientists can determine where the body was formed.
The researchers analyzed ruthenium levels in three samples from the Chicxulub sediment layer in Europe, as well as samples from five other impact structures between 36 and 470 million years old from around the world. They also analyzed samples from a much older layer of impact debris from South Africa. This layer of Small spheres of molten material. It formed between 3.5 and 3.2 billion years ago – the oldest evidence we have of large asteroids impacting our planet.
All of the samples had levels of ruthenium distinct from those on Earth and matched different types of meteorites (and therefore asteroids). Chicxulub stood out as the only carbonaceous chondrite. Importantly, it does not match a specific type of carbonaceous chondrite known as CI. CI meteorites are thought to have a composition similar to comet nuclei. Some scientists have suggested that Chicxulub could have been a comet, although That theory was on shaky groundIf CI chondrites are indeed indicators of comets, then the comet theory is ruled out, the authors write.
The other five impactors are indeed rocky asteroids. The ancient spherules are more complicated: ruthenium levels favor rocky asteroids, but previous results with chromium exclude it. The mysterious impactors that created the spherules could have been carbonaceous. They could be remnants of the late stages of Earth's growth, the authors note.
The study is excellent and the first to use ruthenium to identify the origin of the Chicxulub impactor, says Honami Sato (Kyushu University, Japan), who has worked extensively at Chicxulub but was not involved in the current study. Scientists have previously used chromium for this purpose, but ruthenium is more resistant to terrestrial contamination or modification by the impact process, both Sato and Fischer-Gödde explain.
About 80% of known meteorites are rocky, while carbonaceous fragments are rare. “I think our data on terrestrial impact structures reflect this evidence from the meteorite record quite well,” says Fischer-Gödde. However, that does not mean that the same has been true throughout Earth’s history, she cautions.
In fact, a 2021 study looked at the dynamics of what types of large asteroids would hit Earth. The results suggested that About half of the huge objects that hit us may have been dark and primitive like Chicxulub..
For now, scientists have no evidence of other large carbonaceous impactors like Chicxulub and whatever formed the ancient spherules, Fischer-Gödde says. “But we will keep looking.”
Given this result, researchers can also explore how different compositions might change an impactor's effect on the global environment and ecosystem, Sato notes.
Reference:
M. Fischer-Gödde and others.Ruthenium isotopes show that the Chicxulub impactor was a carbonaceous asteroid.” ScienceAugust 16, 2014.
Asteroid impacts