Scientists: Huge asteroid killed off dinosaurs

Huge fragments showered the Earth after a collision of two asteroids.

SAN FRANCISCO CHRONICLE

Planetary scientists say they’ve pinned down the specific space object that crashed into Earth some 65 million years ago, driving dinosaurs to extinction and killing off half of all the other species at that time.

It was, say the scientists, one huge asteroid that broke up in a violent collision 160 million years ago, sending a massive fragment careering out of the asteroid belt and eventually into Earth’s crust. The impact kicked up a storm of dust, cold and darkness that shrouded the world like a nuclear winter — and goodbye, dinosaurs.

The dinosaurs, both huge and tiny, had ruled Earth for at least 120 million years until the end of what’s called the Cretaceous geologic period. Then they were gone within a very few thousand years, according to the fossil evidence, when the period known as the Tertiary began about 65 million years ago.

That abrupt departure in geological terms during the so-called K-T time boundary was a mystery until about 30 years ago, when Walter Alvarez, a University of California-Berkeley geologist, and his father, Luis, a Nobel Prize-winning physicist, found the cause: An unknown object from space that smashed a great crater into Earth’s crust off the coast of Yucatan, Mexico, and spread debris in the form of a rare element called iridium that Walter Alvarez detected in clay formations all around the world.

Their theory was highly controversial but over time gained support. The discovery of a huge crater called Chicxulub off the Yucatan peninsula, plus the iridium and glassy debris scattered for thousands of miles around the impact crater, nailed down the idea.

Asteroid named Baptistina

On Thursday, in the journal Nature, a group headed by William F. Bottke, an asteroid expert at the Southwest Research Institute in Boulder, Colo., traces that impact back to a giant asteroid named Baptistina nearly 100 miles in diameter. Baptistina, the scientists say, was rammed by another, unnamed asteroid at least 35 miles in diameter in a violent collision about 160 million years ago — give or take 20 million years.

The collision showered nearby space with at least 300 fragments bigger than 20 miles in diameter and more than 140,000 smaller asteroids, each one more than three miles around, Bottke contends.

The smaller asteroids are now known as the Baptistina family, and according to Bottke and his colleagues — David Vokrouhlicky of Chares University in Prague, Czech Republic, and David Nesvorny of Bottke’s institute — it was one of those “refugees” from Baptistina that created the 110-mile-wide Chicxulub crater.

Not only that, they say, it was another earlier Baptistina offshoot asteroid that crashed into the moon about 110 million years ago.