[meteorite-list] 'Dino Killer' Asteroid Was Half the Size Predicted? CHICXULUB II

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Thu, 10 Apr 2008 18:47:46 -0500
Message-ID: <014601c89b65$478db2d0$8250e146_at_ATARIENGINE>

Hi,

    After having just posted (CHICXULUB I) that the most
common size estimate for the Chicxulub impactor was
10 km, here comes:

> The most recent computer models predicted a size of
> 9 to 12 miles (15 to 19 kilometers) across...

    Those whacky computer modellers are at it again!

    To get up to this size, you either have to get the encounter
velocity down to hardly moving with respect to the Earth
(12 km/sec) or pump up the energy of the impact from the
"old" estimate of 500 zettaJoules to 1500 to 2000 zettaJoules,
or a bit of both.

    Or you just order up a fresh long-period comet only 3200
meters across but moving at 72 km/second!


Sterling K. Webb
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----- Original Message -----
From: "Ron Baalke" <baalke at zagami.jpl.nasa.gov>
To: "Meteorite Mailing List" <meteorite-list at meteoritecentral.com>
Sent: Thursday, April 10, 2008 6:13 PM
Subject: [meteorite-list] 'Dino Killer' Asteroid Was Half the Size
Predicted?



http://news.nationalgeographic.com/news/2008/04/080410-sea-meteorites.html

"Dino Killer" Asteroid Was Half the Size Predicted?
Ker Than
for National Geographic News
April 10, 2008

The meteorite that wiped out the dinosaurs
might have been less than half the size of what previous models predicted.

That's the finding of a new technique being developed to estimate the
size of ancient impactors that left little or no remaining physical
evidence of themselves after they collided with Earth.

Scientists working on the technique used chemical signatures in seawater
and ocean sediments to study the dino-killing impact that occurred at
the end of the Cretaceous period about 65 million years ago.

They also looked at two impact events at the end of the Eocene epoch,
roughly 33.9 million years ago.

In what could be a major scientific puzzle, the team's new size estimate
for the dino-killing meteorite is a mere 2.5 to 3.7 miles (4 to 6
kilometers) across.

The most recent computer models predicted a size of 9 to 12 miles (15 to
19 kilometers) across.

The team notes that their findings could also mean that the makeup of
the impactor is different from what scientists commonly assume.

"We are hoping this will lead to further work," said study leader
Gregory Ravizza of the University of Hawaii in Honolulu.

Impact Fingerprints

The fiery passage of asteroids and comets through Earth's atmosphere
leaves chemical traces in the land, sea, and air.

The most common types of meteorites to hit Earth are chondrites, stony
objects that originate in the asteroid belt.

Chondrites contain two different versions, or isotopes, of the naturally
occurring element osmium: osmium 187 and osmium 188.

Seawater and sediments also contain the two osmium isotopes, but the
ratio of osmium 187 to osmium 188 is usually much larger in the ocean
than it is in chondrites.

When a small- to medium-size meteorite enters Earth's atmosphere, much
of the object is vaporized and the osmium ratio in seawater around the
world is temporarily decreased.

Over time, this osmium imprint is transferred to sediments at the ocean
bottom, creating a more enduring record of the impact.

The new technique therefore looks for osmium spikes in ocean sediments
and analyzes the isotope ratio. Scientists can then predict when an
impact event occurred and the size of the projectile.

The research is detailed in tomorrow's issue of the journal Science.

Dramatic Upheaval

In addition to the smaller Cretaceous impact, the team estimates that
two known meteorites from the late Eocene were smaller than previously
believed.

Boris Ivanov, an impact modeler at the Russian Academy of Sciences, said
that if the new size estimates prove correct, they would create a
"dramatic controversy" within the impact physics community.

"Most numerical modeling specialists believe the current modeling gives
us fidelity of a factor of a few times the mass of a projectile with
assumed average impact velocity," Ivanov said.

Study co-author Francois Paquay, also at the University of Hawaii, said
that more work needs to be done to confirm the latest estimates.

"We think the discrepancy is important and it will need to be addressed
in future [scientific] meetings," Paquay said.

Jay Melosh, a planetary scientist at Arizona State University who was
not involved in the study, called the new method a "potentially
powerful" technique for filling gaps in the geologic impact record.

"It's a very valuable contribution to the tool kit of ways we have of
estimating the presence of impacts in the geologic record," Melosh said.

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Received on Thu 10 Apr 2008 07:47:46 PM PDT