[meteorite-list] Analyses Pinpoint Origin of Asteroid's Fatal Fireball 65 Million Years Ago

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 14 20:24:50 2005
Message-ID: <200504150024.j3F0OMu03769_at_zagami.jpl.nasa.gov>

http://chronicle.uchicago.edu/050414/fireball.shtml
    
Analyses pinpoint origin of asteroid's fatal fireball
Steve Koppes
The University of Chicago Chronicle
April 14, 2005
Vol. 24 No. 13
  
Scientists at the University and the American Museum of Natural
History recently released a study explaining how a
globe-encircling residue formed in the aftermath of the asteroid
impact that triggered the extinction of the dinosaurs. The study,
published in the April issue of the journal Geology, draws the
most detailed picture yet of the complicated chemistry of the
fireball produced in the impact.

The residue consisted of sand-sized droplets of hot liquid that
condensed from the vapor cloud produced by an impacting asteroid
65 million years ago.

Scientists have proposed three different origins for these
droplets, which they call "spherules." Some researchers have
theorized that atmospheric friction melted the droplets off the
asteroid as it approached Earth's surface. Still others suggested
the droplets splashed out of the Chicxulub impact crater off the
coast of Mexico's Yucatan Peninsula following the asteroid's
collision with Earth.

But analyses conducted by Lawrence Grossman, Professor in
Geophysical Sciences and the College, and Denton Ebel, assistant
curator of meteorites at the American Museum of Natural History,
provide new evidence for a third theory. According to their
research, the droplets must have condensed from the cooling vapor
cloud that girdled the Earth following the impact.

Ebel and Grossman base their conclusions on a study of spinel, a
mineral rich in magnesium, iron and nickel contained within the
droplets.

"Their paper is an important advance in understanding how these
impact spherules form," said Frank Kyte, adjunct associate
professor of geochemistry at the University of California, Los
Angeles. "It shows that the spinels can form within the impact
plume, which some researchers argued was not possible."

When the asteroid struck approximately 65 million years ago, it
rapidly released an enormous amount of energy, creating a fireball
that rose far into the stratosphere. "This giant impact not only
crushes the rock and melts the rock, but a lot of the rock
vaporizes," Grossman said. "That vapor is very hot and expands
outward from the point of impact, cooling and expanding as it
goes. As it cools the vapor condenses as little droplets and rains
out over the whole Earth."

This rain of molten droplets then settled to the ground, where
water and time altered the glassy spherules into the clay layer
that marks the boundary between the Cretaceous and Tertiary (now
officially called the Paleogene) periods. This boundary marks the
extinction of the dinosaurs and many other species.

Grossman's laboratory, where Ebel formerly worked, specializes in
analyzing meteorites that have accumulated minerals condensed from
the gas cloud that formed the sun 4.5 billion years ago. Together
the two scientists decided to apply their experience in performing
computer simulations of the condensation of minerals from the gas
cloud, which formed the solar system, to the problem of the
Cretaceous-Paleogene spinels.

UCLA's Kyte, who himself favored a fireball origin for the
spinels, has measured the chemical composition of hundreds of
spinel samples from around the world.

Ebel and Grossman built on Kyte's work and on previous
calculations Arizona researchers did that show how the asteroid's
angle of impact would have affected the chemical composition of
the fireball. Vertical impacts contribute more of the asteroid and
deeper rocks to the vapor, while impacts at lower angles vaporize
shallower rocks at the impact site.

Ebel and Grossman also drew upon the work of Mark Ghiorso,
Professor in Geophysical Sciences and the College, and the
University of Washington's Richard Sack, who have developed
computer simulations that describe the stability of melted rocks.

Ebel and Grossman's resulting computer simulations show how rock
that vaporized in the impact would condense as the fireball,
cooling from temperatures tens of thousands of degrees. The
simulations paint a picture of global skies filled with a bizarre
rain of a calcium-rich, silicate liquid, reflecting the chemical
content of the rocks around the Chicxulub impact crater.

Their calculations told them what the composition of the spinels
should be, based on the composition of both the asteroid and the
bedrock at the impact site in Mexico. The results closely matched
the composition of spinels that geologists found at the
Cretaceous-Paleogene boundary around the world and which UCLA's
Kyte and his associates have measured.

Scientists already had known the spinels found at the boundary
layer in the Atlantic Ocean distinctly differed in composition
from those found in the Pacific Ocean. "The spinels that are found
at the Cretaceous-Paleogene boundary in the Atlantic formed at a
hotter, earlier stage than the ones in the Pacific, which formed
at a later, cooler stage in this big cloud of material that
circled the Earth," Ebel said.

The event would have dwarfed the enormous volcanic eruptions of
Krakatoa and Mount St. Helens, Ebel said. "These kinds of things
are just very difficult to imagine," he said.
Received on Thu 14 Apr 2005 08:24:21 PM PDT