An object that killed the dinosaurs roughly 66 million years ago may have been a comet, rather than an asteroid, say researchers Prof Jason Moore and Prof Mukul Sharma at Dartmouth College, New Hampshire.
An image shows a comet striking coastal Yucatan, forming the giant Chicxulub impact crater and causing the extinction of the non-avian dinosaurs (Don Davis / NASA)
Something killed off almost all the dinosaurs and some 70 percent of all other species living on Earth around 66 million years ago. Only those dinosaurs related to birds appear to have survived. Most researchers agree that the culprit in this extinction was extraterrestrial, and the prevailing opinion has been that the party crasher was an asteroid.
Dartmouth scientists favor another explanation, asserting that a high-velocity comet led to the demise of the dinosaurs. They presented their findings at the 44th Lunar and Planetary Conference on March 22, 2013.
The asteroid impact theory originated in the 1980s, when Nobel Laureate Luis Alvarez and his son, the geologist Prof Walter Alvarez of the University of California Berkeley, identified extremely high concentrations of the element iridium in a layer of rock known as the K-Pg boundary. The layer marks the end of the Cretaceous period (abbreviated ‘K’), the epoch of the dinosaurs, and the beginning of the Paleogene period (‘Pg’), with its notable absence of the large lizards.
While iridium is rare in the Earth’s crust, it is a common trace element in rocky space debris such as asteroids. Based on the elevated levels of iridium found worldwide in the boundary layer, the Alvarezes suggested that this signaled a major asteroid strike around the time of the K-Pg boundary. Debate surrounded their theory until 2010, when a panel of 41 scientists published a report in support of the Alvarezes’ theory. The panel confirmed that a major asteroid impact had occurred at the K-Pg boundary and was responsible for mass extinctions.
Researchers today look to the deeply buried and partially submerged, 110-mile wide Chicxulub crater in Mexico’s Yucatán as the place where the death-dealing asteroid landed. The 66-million-year age of Chicxulub coincides with the K-Pg boundary, leading to the conclusion that what caused the crater also wiped out the dinosaurs.
The Dartmouth researchers dispute the characterization of the object from space as an asteroid.
Prof Moore noted that in the past geochemists toiled away, isolated from their geophysicist colleagues, each focused on his or her particular area of expertise. “There hadn’t been a concerted synthesis of all the data from these two camps,” he said. “That’s what we’ve tried to do.”
The duo compiled all the published data on iridium from the K-Pg boundary. They also included the K-Pg data on osmium – another element common in space rock. In sifting through all this they found a wide range of variability, so consequently kept only the figures they demonstrated to be most reliable.
For example, they deleted data drawn from deep ocean cores where there were very high amounts of iridium.
“We discovered that even then there was a huge variation. It was much worse in the oceans than on the continents,” Prof Sharma said. “We figured out that the oceanic variations are likely caused by preferential concentration of iridium bearing minerals in marine sediments.”
In the final analysis, the overall trace element levels were much lower than those that scientists had been using for decades and being this low weakened the argument for an asteroid impact explanation. However, a comet explanation reconciles the conflicting evidence of a huge impact crater with the revised, lower iridium/osmium levels at the K-Pg boundary.
“We are proposing a comet because that conclusion hits a ‘sweet spot.’ Comets have a lower percentage of iridium and osmium than asteroids, relative to their mass, yet a high-velocity comet would have sufficient energy to create a 110-mile-wide crater,” Prof Moore said. “Comets travel much faster than asteroids, so they have more energy on impact, which in combination with their being partially ice means they are not contributing as much iridium or osmium.”
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Bibliographic information: J. R. Moore and M. Sharma. 2013. The K-Pg Impactor was Likely a High Velocity Comet. 44th Lunar and Planetary Conference; paper # 2431
