[meteorite-list] Multiple Impacts 65 Million Years Ago?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Oct 20 14:14:12 2004
Message-ID: <200410201813.LAA17430_at_zagami.jpl.nasa.gov>

http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=1253
 
Multiple Impacts?
Astrobiology Magazine
October 20, 2004

Summary: The Chicxulub meteorite impact is largely credited with
the extinction of 50 percent of the world's species, including the
dinosaurs. But could there have been more than one meteorite impact 65
million years ago?

Multiple Impacts?

by Leslie Mullen

Rather than a single meteorite impact 65 million years ago, could Earth
have been hit with a scattershot of several rocks from space?

It may have happened before. There is evidence that about 35 million
years ago, at least five comets or asteroids collided with Earth. If the
effects of a single large meteorite impact seem overwhelming, imagine
how life on Earth would reel from a barrage of rocks from space.

One way such impact clustering happens is to have a single bolide break
up as it approaches a planet. The comet Shoemaker-Levy 9 provides a
recent example. Before striking the planet Jupiter in 1994, the comet
was torn into 21 different pieces by Jupiter's immense gravity. These
fragments struck Jupiter over 5.6 days, some creating large fireballs as
they entered Jupiter's vast gaseous atmosphere.

Earth's gravity is no where near as powerful as Jupiter's, so the same
scenario would not happen to Earth. Yet many asteroids are thought to be
rubble piles, loosely bound by gravity, and such an asteroid could rip
apart as it approached our planet.

But if an asteroid "rubble pile" broke up before it entered Earth's
atmosphere, the pieces would only result in one crater, or at most two,
because most of the pieces would fall into the same hole.

"Once such a rubble pile enters Earth's gravity it's too late," says
Christian Koeberl, a geochemist at the University of Vienna in Austria.
"It would only get into the attraction field of Earth's gravity a few
hours before it hits at best, and this is not time enough to spread it
out appreciably."

However, having an asteroid break up as it approaches Earth is not the
only way to end up with multiple craters.

Within the asteroid belt that orbits the sun between Mars and Jupiter,
collisions between asteroids sometimes occur. The resulting fragments
can then rain down on Earth. Simon Kelley, a geologist at the Open
University in England, says that such a collision occurred 470 million
years ago, and many of those fragments traveled to Earth. In fact, some
of the fragments still are impacting Earth today.

A similar shower of fragments can come from a collision within the Oort
cloud, a comet-filled region in the outer-most portion of the solar
system. Kelley says such a cometary shower may be responsible for the
cluster of impact craters dated to be 35 million years old, including
two of the largest impact craters on Earth: the 100 kilometer Popigai
crater in Siberia, and the 90 kilometer Chesapeake Bay crater off the
shore of Maryland. This cometary shower is thought to have lasted for 2
to 3 million years.

Looking over the Planetary and Space Science Centre's Earth Impact
Database, several of the crater dates overlap. Much of this overlap
reflects the limitations of current dating techniques, where ages can't
be narrowed down further than hundreds of thousands of years. But it is
possible that some of the craters point to a multiple impact scenario.

The Boltysh crater in the Ukraine may be proof that multiple impacts
occurred during the Cretaceous-Tertiary (K-T) extinction. Kelley has
dated the crater to be about 65 million years old.

The Boltysh crater had previously been assigned ages ranging from 88
million to 105 million years old, a variation that arose due to the
different dating methods used. Kelley used Argon-Argon dating to
determine the correct age for the crater. The crater rocks melted in the
heat of impact, and when they cooled they trapped argon out of the air.
Different isotopes of argon decay at different rates, so by measuring
the ratio of argon isotopes, Kelley was able to estimate when the rocks
melted, within a margin of error of plus or minus 600,000 years.

The margin of error prevents scientists from saying Boltysh was
definitely part of the same meteorite strike as Chicxulub. And Kevin
Pope of Geo Eco Arc Research points out that while Chicxulub has been
linked directly to the K-T boundary and extinctions by the stratigraphy
of its ejecta, the same is not true for Boltysh.

"Craters the size of Boltysh form every few million years, so the fact
there is a crater this size close to the boundary is no surprise," says
Pope.

The Boltysh crater measures only 24 kilometers in diameter - compared to
Chicxulub's nearly 200-kilometer-wide monster - so even if the smaller
meteorite that made this crater hit around the same time as Chicxulub,
its effects wouldn't have been as catastrophic.

But if Boltysh did occur around the same time as Chicxulub, it could aid
in our understanding of the K-T extinction event. Craters can act as
time capsules, preserving information about the environment at the
moment of impact.

Like Chicxulub, the Boltysh crater is buried underground. But while
Chicxulub filled with seawater, the Boltysh crater became a fresh water
lake. Kelley is writing a paper with Dave Jolley at the University of
Sheffield on the microflora and fauna they have found in the Boltysh
crater fill. They hope to determine how rapidly life recovered in the
vicinity of the impact, and how that recolonization occurred.

As for other K-T impact craters, Kelley notes that the record is very
poorly dated. He plans to study other craters, currently dated to be
from the Devonian (around 380 million years ago) to the Eocene (34
million years ago), to see if their ages are accurate. By determining
the correct dates for impact craters, scientists will be able to better
understand how often multiple impact events have occurred in the past.

So do multiple impacts play a role in mass extinctions? Kelley says that
as far as we know, they don't. For instance, Kelley says there is no
evidence that the barrage of comets 35 million years ago led to a mass
extinction event.

"The effects would have been truly devastating locally, but they didn't
amount to global catastrophes," says Kelley. "You can argue that minor
extinctions are associated, but not a K-T-like event."

A single large meteorite impact like Chicxulub may be more harmful to
life than a cluster of several smaller meteorites or comets spread out
over a million years or less. Yet determining why certain species die
out can often be difficult. Extinctions are a natural part of the cycle
of life, and may occur due to a whole host of interrelated factors,
including competition for food, climate change, and even sea level
change. Perhaps tossing a few meteorites into the mix also can upset the
scales, tipping some species too far off balance to recover.
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Received on Wed 20 Oct 2004 02:13:53 PM PDT