demise of the dinosaurs.
A visitor looks at the head of the Apatosaurio dinosaur fossil "Einstein", on display in Monterrey, Mexico.
The asteroid impact theory of extinction began with discoveries by the late physicist and Nobel Laureate Luis Alvarez and his son, the geologist Walter Alvarez, a professor at the University of California, Berkeley. In 1980 they identified extremely high concentrations of the element iridium in a layer of rock known as the K-Pg (formerly called K-T) boundary. The layer marks the end of the Cretaceous period (abbreviated K), the epoch of the dinosaurs, and the beginning of the Paleogene period, 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?about 66 million years ago. 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.
The scientific community today looks to the deeply buried and partially submerged, 110-mile wide Chicxulub crater in Mexico's Yucatan as the place where the death-dealing asteroid landed. The 66-million-year age of Chicxulub, discovered in 1990, coincides with the KT boundary, leading to the conclusion that what caused the crater also wiped out the dinosaurs.
Professors Jason Moore and Mukul Sharma of the Department of Earth Sciences do agree with fellow scientists that Chicxulub was the impact zone, but dispute the characterization of the object from space as an asteroid.
The Dartmouth 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.
"Because we are bringing a fresh set of eyes into this field, we feel our decisions are objective and unbiased," said Sharma.
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," Sharma said.
"We figured out that the oceanic variations are likely caused by preferential concentration of iridium bearing minerals in marine sediments," he added.
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," said Moore.
"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," he added.
They described their somewhat controversial findings in a paper presented to the 44th Lunar and Planetary Conference last month.