[meteorite-list] Double Whammy: Asteroids Delivered One-Two Punch

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Jul 27 16:27:31 2004
Message-ID: <200407272011.NAA10240_at_zagami.jpl.nasa.gov>

http://www.space.com/scienceastronomy/asteroid_shower_040726.html

Double Whammy: Asteroids Delivered One-Two Punch
By Robert Roy Britt
space.com
26 July 2004

A pair of 35-million-year-old craters on Earth thought to have been
carved by comets now appears to be the result of a broken asteroid that
generated a slowly delivered shower of debris over millions of years.

One crater is in Chesapeake Bay
off the Maryland coast. The other, called the Popigai crater, is in
north-central Siberia. Estimates of their age suggest they were created
a mere 10,000 years apart.

Scientists had thought a comet shower of some sort had left the two scars.

Asteroid go platinum

A new study of the Popigai crater finds an abundance of elements from
the platinum group, a signature common to asteroids and not thought to
suggest comets, which hold more water ice and lower concentrations of
metals.

The timing of the two impacts suggests the Chesapeake crater may have
been caused by the same parent asteroid, which in this scenario broke
apart in space and showered the Earth with many fragments large and
small, explained geology professor Phillipe Claeys of Vrije Universiteit
Brussel in Belgium.

"The chance to have over a short time scale the impact of an asteroid
and that of a comet occurring together is unlikely -- not impossible,
but very unlikely," Claeys told SPACE.com. He said objects up to 3 miles
wide (5 kilometers) -- big enough to form the craters in question -- hit
Earth once every 25 to 30 million years.

Other studies have
found that when asteroids collide and break apart, their fragments can
move from the asteroid belt between Mars and Jupiter to reach Earth's
vicinity over long time periods. Earth can then be hit by successive
pieces on its continual trip around the Sun.

Slow storm

Other evidence reveals extra space dust particles, perhaps the product
of the asteroid's breakup in space, falling on Earth for
2.5-million-years around the same time as the two large impacts, Claeys
said.

That suggests a slow rain of debris created increased rates of dramatic
fireballs in the sky and possibly other large impacts. The drama might
have spanned at least 10,000 years, he said, and other than the
Chesapeake and Popigai craters, there would be little evidence today.

The asteroid that broke up to produce the asteroid shower "was much
bigger" and "should have produced many more fragments than the two that
came on an Earth collision course," Claeys said. "The Earth is
two-thirds water, and we know little about craters in the deep ocean,
but perhaps there were also one or two other projectiles that fell in
the ocean during the period."

The uptick in activity would not have been noticeable over the course of
the typical human lifetime.

The study, detailed Friday in the journal Science, was led by Roald
Tagle of the Museum fuer Naturkunde in Berlin. The researchers plan to
look for similar hard evidence that an asteroid built Chesapeake Bay.
Received on Tue 27 Jul 2004 04:11:20 PM PDT