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For nearly 180 million years, dinosaurs ruled planet Earth until their sudden demise some 66 million years ago during what scientists call the Cretaceous–Paleogene extinction event. This mass extinction event wiped out all non-avian dinosaurs, and, in all, led to the demise of three-quarters of Earth’s plant and animal species.
In 1980, the father-son scientist duo of Luis and Walter Alvarez posited that the mass extinction event was triggered by a giant asteroid, roughly 10-15 km wide. This hypothesis gained weight after the discovery in the early 1990s of the 180 km wide Chicxulub crater in the Gulf of Mexico and the Yucatán Peninsula.
A new study, published on Thursday (August 15) in the journal Science, looked at geochemical evidence from the impact site to find that the destructive asteroid was formed beyond the orbit of Jupiter. This discovery suggests that the mass extinction was the result of a chain of events that began during the very birth of the Solar System.
A team of geochemists, led by Mario Fischer-Gödde from the University of Cologne in Germany, obtained samples from three sites at the Chicxulub crater, and compared them with rocks from eight other asteroid impact sites from the past 3.5 billion years.
Specifically, the scientists looked for signatures of the metal ruthenium. Ruthenium is extremely rare in Earth rocks, Fischer-Gödde told nature.com, so samples from an impact site offer “pure signature” of the celestial body that has collided with Earth. Moreover, the specific ruthenium isotope can help scientists pinpoint exactly where in the Solar System an asteroid came from.
The Solar System began forming some 4.6 billion years ago, starting as a dense cloud of gas and dust which eventually collapsed to give shape to the celestial bodies that today are Earth’s neighbors. During the formation, the temperature in the inner regions of the molecular cloud were too high for volatile chemicals (which can be in the vapour form) to condense.
Consequently, asteroids produced there had low levels of volatiles, and became rich in silicate minerals (which make 90 per cent of Earth’s crust). On the other hand, asteroids formed further out became ‘carbonaceous’, containing lots of carbon and volatile chemicals. Fischer-Gödde’s team found that the ruthenium isotopes in the Chicxulub impactor were a good match for a carbonaceous asteroid from the outer Solar System, rather than a siliceous asteroid from the inner Solar System.
The findings also debunked the theory that a comet, rather than an asteroid, had triggered the Cretaceous–Paleogene extinction event.
Exactly what sent the asteroid Earth’s way is not clear. The dino-killing asteroid was a rather exceptional event — most other asteroids studied by Fischer-Gödde and his team were formed in the inner solar system.
Computer simulations have shown a number of situations in which an object from the outer Solar System can be pulled out of its orbit to head towards Earth. A number of possible reasons, such as bumping into other asteroids or gravitational pulls of planets and the Sun, can be behind this.
However, even if an asteroid’s normal orbit is disrupted, a collision with Earth is highly unlikely. A 2021 study by researchers from the Texas-based Southwest Research Institute found that Earth is likely to collide with large asteroids (of the size that wiped out dinosaurs) only once in 250 million years. While such a collision is unlikely to take place in humanity’s lifetime, if it does occur, it will be deadly.
And the impact will be the least of its dangers. Very few dinos died because of the impact itself. What led to the mass extinction of species were the after effects of the impact. The massive volumes of sulfur, dust and soot that the asteroid-impact threw in the air, partially blocked the Sun, and caused temperatures on Earth to plummet and impaired photosynthesis in plants and phytoplankton. This caused massive disruptions in the ecosystem that, in the end, very few creatures could survive.
