Asteroid Impact Fueled Global Rain of BBs

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A spherule that has been sliced and polished. The dark areas are clay minerals and the bright, reflective grains are spinels.
Credit: Frank Kyte

The asteroid that struck
the Yucatan Peninsula 65 million years ago presumably initiated the extinction
of the dinosaurs. The huge collision also unleashed a worldwide downpour
of tiny BB-sized mineral droplets, called spherules.

The hard rain did not pelt
the dinosaurs to death.

But the planet-covering
residue left behind may tell us something about the direction of the incoming
asteroid, as well as possible extinction scenarios, according to new research.
The falling spherules might have heated the atmosphere enough to start a global fire, as one example.

How the spherules formed in
the first place, though, has been a bit of a mystery. One theory is that these
half-millimeter-wide globules precipitated out of a giant cloud of vaporized
rock that circled the planet after the collision.

"That vapor is very
hot and expands outward from the point of impact, cooling and expanding as it
goes," said Lawrence Grossman of the University of Chicago. "As it cools
the vapor condenses as little droplets and rains out over the whole
Earth."

Grossman and Denton Ebel,
from the American Museum of Natural History, have shown that this vapor
condensation model is consistent with data taken from spherules around the world.

The scientists also found
that chemical differences in spherules from the Atlantic and Pacific Ocean
imply that the vapor plume initially moved east, which might pinpoint the
arrival direction of the asteroid.

Apocalyptic fireball

The spherules populate a
three-millimeter layer, called the K-T boundary, which separates the Cretaceous
from the Paleogene (formerly called the Tertiary) geologic periods. The abrupt
disappearance of large dinosaur fossils – as well as many marine life fossils –
above this boundary implies a major extinction event occurred 65 million years ago.

Around this same time, a
city-sized asteroid landed near the present-day town of Chicxulub, Mexico,
where traces of a 100-mile-wide crater can still be found.

There is evidence for the
asteroid in the unique mineral content of the KT boundary – specifically a high
concentration of iridium. This heavy element is very rare on the Earth’s
surface but is found in high quantities in meteorites.

The implication is that the
energy released in the collision fueled a fireball of vaporized rock that rose
above the clouds. In this way the asteroid’s contents – as well as the material
at the crash site – were dispersed across the globe.

"The [KT] layer is
thought to be the fallout from the fireball," Grossman told Space.com
in a telephone interview.

Included in this fallout
were glassy spherules, which have been largely transformed due to weathering,
but still contain 100s of spinels – tiny mineral deposits with magnesium, iron
and nickel.

"One reason the
spinels are important is that most of the original minerals in the spherules
are all gone – turned into clay," said Frank Kyte from UCLA, who was not
involved in this work. "The spinels appear to not have been altered."

Oxidizing environment

Because of their pristine
state, scientists have studied the spinels in hopes of learning about the
cataclysm that created them. One such study concluded that – because the
spinels contain metal oxides – they could not have precipitated out of the
vapor plume up above the atmosphere.

"The argument is
silly," Grossman said.

He knows this because the
conclusion drew on his own work describing how spinels are created in
meteorites. These spinels formed long ago in the pre-planet gas that surrounded
the Sun. This gas was mostly hydrogen, so it makes sense that very little
oxides – metals bonded with oxygen atoms – are found in meteorites.

But the vapor plume was a
different story.

"When you vaporize
rock, there is very little hydrogen – whereas 50 percent of the atoms in a rock
are oxygen," Grossman said.

As described in the
upcoming April issue of the journal Geology, Grossman and Ebel performed
computer simulations that showed that the spinels in the KT boundary – with
their high concentration of oxides – are consistent with formation inside a
liquid droplet condensing out of the impact plume.

These droplets solidified
into the spherules, which – according to Kyte – all fell to the ground in a
matter of hours or days.

Plume’s weather front

A more detailed analysis of
the spherules shows that some of them formed earlier than others.

"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," Ebel said.

This implies that the cloud
of vaporized rock moved east following the collision – a fact that may provide
hints as to how the asteroid hit the Earth. This is important because different
sorts of debris get thrown into the sky, depending on whether the asteroid came
in at an angle or straight down.

A complete picture of the
impact’s geometry and its immediate consequences should help answer questions
concerning the eventual effects on the planet’s living creatures.

This article is part of
SPACE.com’s weekly Mystery Monday series.

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