"Warm, comforting, fireball"

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You'll read about it in SCIENCE (AAAS); here's what one popular press
article reported about the "warm fireball". Peter
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URL: http://www.sfgate.com/chronicle/old/aug-18-95/MN7439.html
Size: 117 lines

The San Francisco Chronicle Friday, August 18, 1995 : Page A7 )1995
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                   DINOSAUR-EXTINCTION THEORY EVOLVES
          TEAM RECONSTRUCTS EVENTS FOLLOWING ASTEROID'S IMPACT

Charles Petit, Chronicle Science Writer 

The once-outlandish idea that the last dinosaurs and myriad other
creatures died 65 million years ago after an asteroid smote the Earth
is now accepted by most scientists, yet odd patterns in the debris
have mystified experts.

Now, a team of scientists, including the Berkeley geologist who more
than 15 years ago found the first stunning sign that a calamity from
space rained death across the entire planet, thinks it can
reconstruct, moment by moment, the complex events that immediately
followed a blast in Mexico, which was mightier than 100 million
nuclear bombs.

The detailed scenario, laid out in today's issue of the journal
Science, explains why layers of once- vaporized rock, shattered
quartz, melted glass and other debris still found in ancient sediments
worldwide seemed to have landed back on Earth in the wrong order. The
work appears to close a chapter on the evolution of an idea once
rejected out of hand by most experts, but now put in textbooks.

The scale of the destruction, as it is reconstructed, is no surprise.

``I think that if you have an event in which most of the species on
Earth perished, it was safe to say it was pretty awful,'' said Walter
Alvarez, chairman of the department of geology and geophysics at the
University of California at Berkeley. ``Now we are in a position to
start filling in the details.''

Alvarez's co-authors are post- doctoral fellow Philippe Claeys of
Belgium and cratering expert Susan W. Kieffer of the University of
British Columbia in Vancouver.

Their key conclusion is that the asteroid's impact produced at least
two distinct fireballs that were propelled into the atmosphere. One
was exceedingly hot, perhaps 10,000 degrees or more, and lofted the
vaporized rock of the 16-trillion-ton asteroid plus a good chunk of
the Earth's crust back into space.

A second, cooler fireball filled with unmelted bits of rock from near
the rim of the crater rose within seconds of the first, propelled to
high speed by expanding steam and carbon dioxide from boiled ocean
waters and shattered limestone. While the two fireballs rose through
the atmosphere, a curtain of melted rock splashed from the edge of the
crater and spread at a low angle over much of the Caribbean and North
America.

The material from both fireballs raining back down heated the upper
atmosphere enough to almost instantly incinerate all plants across
much of the world's dry land.

In 1980, Alvarez and his father, the Nobel Prize-winning physicist
Luis Alvarez, first published a paper that said the rare element
iridium, found in a thin clay layer almost worldwide, indicated that a
comet or asteroid hit Earth right when the dinosaurs disappeared.
Iridium is an element rare in the Earth's crust but common in meteors
and asteroids.

The younger Alvarez found the first iridium layer in Italy. They
estimated it came from a 1.7 trillion- ton rock, six miles wide and
traveling at least 55,000 miles per hour. In recent years, its
110-mile-wide crater was recognized in the Chicxulub region of
Yucatan, Mexico.

But as the broad picture came together, mysteries emerged.

Chief among them was that in North America and much of the Pacific,
the debris was found in distinct layers. On the bottom is a layer of
clay, presumably the remains of melted rock and glassy globules. On
top of that were bits of quartz that had not melted, but were filled
with peculiar patterns showing they had been shocked by high
pressures. And, the upper layer often held the iridium.

``We couldn't figure out why these layers were so clearly separated,''
Alvarez said. ``If they all came out of the same explosion, how could
they follow different paths?''

Another mystery was why more debris can be found west of the impact
site, across North America and into the Pacific, than to the east in
Europe. Still another question was why grains of shocked quartz were
often found many thousands of miles away, so far away that according
to standard theory they would have had to have been blasted so hard
that they would have melted.

The critical insight came from Kieffer, who calculated that limestone
and shallow ocean waters where the asteroid hit would have been
transformed into a cloud of steam and carbon dioxide rising as a
second, ``warm fireball.'' These expanding gases would have
accelerated the quartz to high speed without melting it. The skewed
distribution of the debris, favoring the west, is simply due to
Earth's rotation, Alvarez said, a factor overlooked in earlier
analyses.

``It all makes sense now,'' Alvarez said.
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