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Impact
in Australia Nearly nearly wiped out life on Earth
14 May 2004
The crater was
probably caused by an asteroid about 1 km in diameter that crashed
into the earth about 142 million years ago. The isolated circular
feature within the crater consists of a central ring of hills
about 4.5 km in diameter. Image courtesy of NASA |
Evidence is mounting that 251 million years ago, long before the
dinosaurs dominated the Earth, a meteor the size of
Mount
Everest
smashed
into what is now northern Australia,
heaving rock halfway around the globe, triggering mass volcanic
eruptions, and wiping out all but about ten percent of the species on
the planet. The "Great Dying," as it's called, was by far the
most cataclysmic extinction event in Earth's history, yet scientists
have been unable to finger a culprit as they have with the dinosaur
extinction.
Researchers
at the
University
of
Rochester
and the
University
of
California
at Santa
Barbara (UCSB) points the finger for the Great Dying squarely at the
heavens."This is very likely the impact site we've been looking
for," says Robert Poreda, professor of earth and environmental
sciences at the
University
of
Rochester
.
"For years we've been observing evidence that a meteor or comet hit
the southern hemisphere 251 million years ago, and this structure
matches everything we've been expecting."
n
2001, Poreda and Luann Becker, research scientist in geological sciences
at UCSB, announced that they had detected in 251-million-year-old strata,
specific isotopes of helium and argon trapped inside buckyballs--a
cage-like formation of carbon atoms--that could only have come from space.
Since they were laid down in this same strata around much of the globe,
the implication was that a giant meteor had struck the Earth, vaporized,
and settled around the southern hemisphere. This past November, the same
three authors found actual pieces of the meteorite that struck the Earth
251 million years ago.
Extraterrestrial
buckyball - the carbon balls are thought to originate from open
space, and their presence on Earth, evidence for meteor impact. |
The
Permian Extinction
Just like with the Cretaceous-Tertiary extinction (K-T) that wiped out
the dinosaurs, many experts disbelieved the theory that a cosmic impact
could be responsible, but now it is largely accepted. The Permian
extinction is one of five major extinctions throughout Earth history
that have significantly affected the evolution of life, and 251 million
years ago the planet witnessed the biggest one to date.
Needle
in a haystack
The team knew that the chances of finding the crater, even one from an
impact large enough to nearly wipe out life on Earth, would be difficult
because the majority of the Earth is covered by ocean. Had the meteor
struck there, its telltale crater would have long ago disappeared. As
luck would have it, an oil-drilling exploration team in 1970 found a
"dome" in the area of Bedout, just off the northwestern coast
of
Australia
. Now
covered by 2 miles of sediment, this area was most likely dry land 251
million years ago. Frequently, such domes herald large oil deposits, but
in this case the drilling team found only volcanic rock. The core
samples were shelved and forgotten for 25 years, until in 1995 a report
in a journal aimed at the oil industry mentioned that the rock might
have been formed from a meteor impact.
Geophysical
analysis shows the rock strata underlying the dome at Bedout is fractured
exactly the way the team expected--showing rock strata older than 251
million years old broken apart, with younger rock above laid down without
the fractures. Simulations of a six-mile wide rock striking the area
suggest a crater rim should be visible about 60 miles from the central
dome, and despite the extreme age of the impact site and the rearrangement
of continental plates since then, there is evidence of a rim at that
distance. The team has plans to explore the geophysical outlay of the
region with more scrutiny.
Coincidentally,
the crater, at 120 miles across, is almost exactly the same size as the
Chicxulub crater in the
Caribbean
that has
been identified as the impact site of the meteorite that dealt the
dinosaurs their death blow. It's likely that the bodies that struck at
each site were of the same size and traveling at similar speeds.
"There
have been five mass extinctions throughout the Earth's history," says
Poreda. "Now we have very strong evidence that massive meteor impacts
happened precisely at two of those extinctions."
The
research was funded by NASA and the National Science Foundation.
Source: University
of Rochester
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