Scientists say they know where dinosaur-killing aste... http://sfgate.com/cgi-bin/article.cgi?f=/c/a/2007/0...
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Scientists say they know where dinosaur-killing asteroid came from
David Perlman, Chronicle Science Editor
Thursday, September 6, 2007
(45)
Ever since two Berkeley scientists named Alvarez discovered evidence that
some monster object from the sky crashed into the Earth some 65
million years ago and drove all the dinosaurs to extinction, the story has
taken on a life of its own.
Why not? Dinosaurs are fascinating, and extinction is a grim fate for lots of creatures - maybe even
us Homo sapiens, too, some day. Another thing, anything extraterrestrial is out of this world.
Now comes a team of planetary scientists who say they've pinned down the specific space object that
did in all those dinosaurs and killed off half of all the other species on Earth 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 the Earth's crust.
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The impact kicked up a storm of dust, cold and darkness that shrouded the world like a nuclear
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winter - and goodbye dinosaurs.
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The dinosaurs, both huge and tiny, had ruled the Earth for at least 120 million years until the end of
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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 UC 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 the 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.
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 team of 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 3 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 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 and gouged out the well-known lunar crater called Tycho, whose debris
the Apollo 17 astronauts encountered on America's last manned flight to the moon 35 years ago.
Bottke and his team used a chain of reasoning for their conclusion: One was the increased rate of
impact formation of craters on the Earth and moon, which their calculations indicate most probably
doubled in the past 100 million to 150 million years. The other is the fact that the sediments and
debris inside the Chicxulub crater indicate the asteroid held a lot of carbon and was one of a
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Scientists say they know where dinosaur-killing aste... http://sfgate.com/cgi-bin/article.cgi?f=/c/a/2007/0...
relatively rare class called carbonaceous chondrites - most probably the Baptistina type.
What caused the dinosaur-killer to fly out of the asteroid belt and smash into Earth? The asteroid
belt lies between the orbits of Mars and Jupiter, and Bottke says a kind of gravity "resonance"
between the orbits of Jupiter and the Baptistina objects created a "dynamical superhighway" that sent
those crater-forming asteroids into the Earth-moon system.
They're so sure of it, in fact, that they claim there's a 90 percent probability it was a wandering
fragment from Baptistina that sealed the fate of the dinosaurs, and a 70 percent probability that
another fragment from the same asteroid was responsible for the moon's Tycho crater.
But Richard A. Muller, a physicist at UC Berkeley who has followed the Alvarez work since it began,
is not so sure.
His group's analysis of cratering rates on the moon, published in Science seven years ago, indicate
that over the past 3.5 billion years, the rate of crater formation had dropped by two to three times
until about 600 million years ago, and then had increased three- or fourfold about 400 million years
ago.
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Muller called the Bottke team's work "a nice piece of solid modeling work," but suggested another
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candidate for dinosaur-killer quite apart from asteroids.
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"They haven't considered a comet shower," Muller said. A vast body of long-frozen comets known as
the Oort Cloud lies far out on the distant edges of the solar system, and Muller argues that a random
passing star could well have disrupted the Oort Cloud many millions of years ago and sent showers
of comets - as many as a billion of them - hurtling into the inner solar system to blast craters into
both the moon and the Earth. "That's just as likely as asteroid impacts," Muller said.
In a commentary also published in Nature on Thursday, geologists Philippe Claeys and Steven
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Goderis of the Free University in Brussels, Belgium, say the Bottke team's "hypothesis is nothing if not
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provocative," and warn that errant asteroids continue to threaten the Earth.
"The terrestrial impact record needs to be scrutinized more closely," they write, "to identify and
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understand these periods of more intense bombardment, and to link them to the huge and dangerous
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game of billiards continuously being played out between the orbits of Mars and Jupiter."
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E-mail David Perlman at [email protected].
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