Dino-killing meteorite may have been half the size predicted

Washington, April 11 : A new technique developed to estimate the size of ancient impactors has determined that the meteorite that wiped out the dinosaurs might have been less than half the size of what previous models predicted.

The study team was lead by Gregory Ravizza of the University of Hawaii in Honolulu.

According to a report in National Geographic News, the new technique involves scientists using chemical signatures in seawater and ocean sediments to study the dino-killing impact that occurred at the end of the Cretaceous period, about 65 million years ago.

The scientists also looked at two impact events at the end of the Eocene epoch, roughly 33.9 million years ago.

Though the most recent computer models had predicted a size of 9 to 12 miles (15 to 19 kilometers) across, the new size estimate for the dino-killing meteorite came across to be a mere 2.5 to 3.7 miles (4 to 6 kilometers) across.

The team has thus determined that their findings could also mean that the makeup of the impactor is different from what scientists commonly assume. The most common types of meteorites to hit Earth are chondrites, stony objects that originate in the asteroid belt. They contain two different versions, or isotopes, of the naturally occurring element osmium: osmium 187 and osmium 188.

Seawater and sediments also contain the two osmium isotopes, but the ratio of osmium 187 to osmium 188 is usually much larger in the ocean than it is in chondrites.

When a small- to medium-size meteorite enters Earth's atmosphere, much of the object is vaporized and the osmium ratio in seawater around the world is temporarily decreased.

Over time, this osmium imprint is transferred to sediments at the ocean bottom, creating a more enduring record of the impact.

The new technique therefore looks for osmium spikes in ocean sediments and analyzes the isotope ratio. Scientists can then predict when an impact event occurred and the size of the projectile.

In addition to the smaller Cretaceous impact, the team estimates that two known meteorites from the late Eocene were smaller than previously believed.

Boris Ivanov, an impact modeler at the Russian Academy of Sciences, said that if the new size estimates prove correct, they would create a "dramatic controversy" within the impact physics community.

"Most numerical modeling specialists believe the current modeling gives us fidelity of a factor of a few times the mass of a projectile with assumed average impact velocity," Ivanov said.

ANI