[meteorite-list] Tiny 'spherules' reveal details about Earth's asteroid impacts

From: Jeff Kuyken <info_at_meteoritecentral.com>
Date: Sun, 29 Apr 2012 14:21:55 +1000
Message-ID: <001501cd25bf$9f809dc0$de81d940$_at_meteorites.com.au>

http://www.spacedaily.com/reports/Tiny_spherules_reveal_details_about_Earths
_asteroid_impacts_999.html

Tiny 'spherules' reveal details about Earth's asteroid impacts
by Staff Writers
West Lafayette, IN (SPX) Apr 27, 2012

Researchers are learning details about asteroid impacts going back to the
Earth's early history by using a new method for extracting precise
information from tiny "spherules" embedded in layers of rock.

The spherules were created when asteroids crashed into the Earth, vaporizing
rock that expanded into space as a giant vapor plume. Small droplets of
molten and vaporized rock in the plume condensed and solidified, falling
back to Earth as a thin layer. The round or oblong particles were preserved
in layers of rock, and now researchers have analyzed them to record precise
information about asteroids impacting Earth from 3.5 billion to 35 million
years ago.

"What we have done is provide the foundation for understanding how to
interpret the layers in terms of the size and velocity of the asteroid that
made them," said Jay Melosh, an expert in impact cratering and a
distinguished professor of earth and atmospheric sciences, physics and
aerospace engineering at Purdue University.

Findings, which support a theory that the Earth endured an especially heavy
period of asteroid bombardment early in its history, are detailed in a
research paper appearing online in the journal Nature on Wednesday (April
25). The paper was written by Purdue physics graduate student Brandon
Johnson and Melosh. The findings, based on geologic observations, support a
theoretical study in a companion paper in Nature by researchers at the
Southwest Research Institute in Boulder, Colo.

The period of heavy asteroid bombardment - from 4.2 to 3.5 billion years ago
- is thought to have been influenced by changes in the early solar system
that altered the trajectory of objects in an asteroid belt located between
Mars and Jupiter, sending them on a collision course with Earth.

"That's the postulate, and this is the first real solid evidence that it
actually happened," Melosh said.

"Some of the asteroids that we infer were about 40 kilometers in diameter,
much larger than the one that killed off the dinosaurs about 65 million
years ago that was about 12-15 kilometers. But when we looked at the number
of impactors as a function of size, we got a curve that showed a lot more
small objects than large ones, a pattern that matches exactly the
distribution of sizes in the asteroid belt. For the first time we have a
direct connection between the crater size distribution on the ancient Earth
and the sizes of asteroids out in space."

Because craters are difficult to study directly, impact history must be
inferred either by observations of asteroids that periodically pass near the
Earth or by studying craters on the moon. Now, the new technique using
spherules offers a far more accurate alternative to chronicle asteroid
impacts on Earth, Melosh said.

"We can look at these spherules, see how thick the layer is, how big the
spherules are, and we can infer the size and velocity of the asteroid,"
Melosh said. "We can go back to the earliest era in the history of the Earth
and infer the population of asteroids impacting the planet."

For asteroids larger than about 10 kilometers in diameter, the spherules are
deposited in a global layer.

"Some of these impacts were several times larger than the Chicxulub impact
that killed off the dinosaurs 65 million years ago," Johnson said. "The
impacts may have played a large role in the evolutional history of life. The
large number of impacts may have helped simple life by introducing organics
and other important materials at a time when life on Earth was just taking
hold."

A 40-kilometer asteroid would have wiped out everything on the Earth's
surface, whereas the one that struck 65 million years ago killed only land
animals weighing more than around 20 kilograms.

"Impact craters are the most obvious indication of asteroid impacts, but
craters on Earth are quickly obscured or destroyed by surface weathering and
tectonic processes," Johnson said. "However, the spherule layers, if
preserved in the geologic record, provide information about an impact even
when the source crater cannot be found."

The Purdue researchers studied the spherules using computer models that
harness mathematical equations developed originally to calculate the
condensation of vapor.

"There have been some new wrinkles in vapor condensation modeling that
motivated us to do this work, and we were the first to apply it to asteroid
impacts," Melosh said.

The spherules are about a millimeter in diameter.

The researchers also are studying a different type of artifact similar to
spherules but found only near the original impact site. Whereas the globally
distributed spherules come from the condensing vaporized rock, these "melt
droplets" are from rock that's been melted and not completely vaporized.

"Before this work, it was not possible to distinguish between these two
types of formations," Melosh said. "Nobody had established criteria for
discriminating between them, and we've done that now."

One of the authors of the Southwest Research Institute paper, David Minton,
is now an assistant professor of earth and atmospheric sciences at Purdue.

Findings from the research may enable Melosh's team to enhance an asteroid
impact effects calculator he developed to estimate what would happen if
asteroids of various sizes were to hit the Earth. The calculator, "Impact:
Earth!" allows anyone to calculate potential comet or asteroid damage based
on the object's mass.

Cheers,

Jeff Kuyken
Meteorites Australia
www.meteorites.com.au
Received on Sun 29 Apr 2012 12:21:55 AM PDT


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