- In one of the most awesome sights of
prehistory, the earliest dinosaurs may have looked up from their evening
meals to witness a mountain hurtling through the sky. Blazing white-hot
and moving at 61,000 kilometers per hour, the giant comet or asteroid screamed
through Earth's atmosphere-possibly close enough to snap the tops off any
high peaks in its path. Then the object disappeared back into space, missing
the planet by the thinnest of margins.
-
- These late Triassic reptiles could not
have guessed that the sky streaker would return shortly-this time, in force.
The extraterrestrial menace split into a series of large chunks that slammed
sequentially into Earth, like bullets from a machine gun.
-
- Although this scenario sounds like a
product of the impact-obsessed movie industry, it is actually emerging
from scientific studies of five ancient craters in North America and Europe.
A team of geologists proposes that all five formed within a few hours of
each other 214 million years ago, with possibly planet-wrenching consequences.
-
- "I can't imagine that this event
would not have had a catastrophic effect on Earth," says John G. Spray,
whose work with two colleagues has connected the separate events. "When
you add up the three biggest craters, the energy released [in these impacts]
would have been comparable to, if not more than, that of the Chicxulub
impact, which wiped out the dinosaurs 65 million years ago," says
Spray, an impact geologist at the University of New Brunswick in Fredericton.
-
- Spray's road to discovery started in
France in 1994, when he visited a cryptic scar in Earth's crust. Called
the Rochechouart impact structure, the feature measures about 25 kilometers
in diameter and is so worn by erosion that it no longer looks like a crater.
-
- While at Rochechouart, Spray collected
samples of rock that had been melted by the impact and sent them to Simon
P. Kelley of the Open University in Milton Keynes, England. Using a technique
that relies on the slow radioactive decay of potassium to argon, Kelley
determined that the rock had melted between 222 million and 206 million
years ago, most likely at 214 million years ago.
-
- "That age struck me straight away
because it's the same age as the Manicouagan impact structure here in Canada,"
says Spray. "So then we got to thinking that if those two have very
similar ages, let's look at the list of 150 or so impact structures that
are documented and see if any of those have similar ages. And we found
three others, making five altogether."
-
- From the vantage of space, the Manicouagan
feature is easily visible, thanks to the Quebec authorities, who flooded
sections of the crater to make a reservoir. An almost perfectly circular
ring of water now surrounds the raised center of the crater, creating a
lake that looks like an eyeball on satellite images and even appears on
ordinary maps.
-
- The complete Manicouagan structure is
100 km in diameter, placing it within the top six largest craters known
on Earth. Geologists had previously dated it as 215 million to 213 million
years old.
-
- In Spray and Kelley's search for craters
of a similar age, they found a third large example called the Saint Martin
impact structure, located northwest of Winnipeg, Manitoba. Mostly hidden
beneath younger rocks, the crater measures about 40 km across. Researchers
have dated it to 219 million years ago, with a large uncertainty of 32
million years on either side.
-
- Their analysis also picked up a 15-km-wide
crater in Ukraine and a 9-km-wide one in North Dakota, both of which have
loosely determined ages that could overlap with the other three impacts.
-
- Armed with these five roughly coeval
craters, Spray sought to plot the impacts on a map of Earth. "But
the problem, of course, is that the present-day orientation of the [continents]
was not how they were 214 million years ago," he says.
-
- Earth's outer shell is broken up into
a dozen large plates that continuously migrate around its surface. To determine
the craters' ancient locations, Spray worked with David B. Rowley of the
University of Chicago. When Rowley located the craters on a map of the
world 214 million years ago, Manicouagan, Rochechouart, and Saint Martin
fell almost perfectly in line.
-
- Even more surprising, says Spray, all
three had the same paleolatitude of 22.8¡N, meaning that the line
through them parallels Earth's equator. Moreover, Saint Martin and Rochechouart
lay almost equidistant on either side of Manicouagan, forming a remarkably
regular string 4,462 km long.
-
- The two remaining craters do not lie
on this paleolatitude, but their positions are also noteworthy. When the
researchers drew lines connecting each small crater with the larger one
nearest it, the lines were parallel, running northeast to southwest, report
Spray, Kelley, and Rowley in the March 12 Nature.
-
-
-
-
- The new study has had far more impact
than previous claims regarding crater chains on Earth. In the past, most
researchers have discounted reports of craters that line up. Spray is now
trying to tighten his case by precisely dating the Saint Martin structure
to see if it formed at the same time as Manicouagan and Rochechouart. "It's
not absolutely proven that ours is a chain. If Saint Martin comes out within
the error of the other two, I'll be satisfied."
-
- Even that evidence might not convince
skeptics, who think the alignment of craters may be coincidental. "They
sort of have the same age, and they sort of line up," says Richard
Grieve of the Geological Survey of Canada in Ottawa. Although more dating
work can refine the ages of these features, he says, "I don't think
we're ever going to be able to say for sure based on isotopic ages, because
they can't get these down to a day or a week." At best, radiometric
techniques can indicate whether these impacts took place within a million
years of each other.
-
- Dennis V. Kent of Columbia University's
Lamont-Doherty Earth Observatory in Palisades, N.Y., has tried to check
the hypothesis by looking at magnetic studies of rocks from Manicouagan
and Rochechouart conducted in the 1960s and early 1970s. As rocks cool
from a molten state, they record a snapshot of Earth's magnetic field,
which occasionally turns over, switching north and south magnetic poles.
If the craters had the exact same age, reasoned Kent, magnetic particles
in their rocks should have the same orientation.
-
- According to the available evidence,
however, rocks from the two sites have opposite orientations, indicating
that the direction of Earth's magnetic field reversed in the interval between
impacts. Geomagnetic researchers think that it takes thousands of years
to reverse the field.
-
- Kent says his simple test argues against
the likelihood that these two craters formed hours apart, leaving unexplained
the straight line with the Saint Martin crater. "That alignment is
interesting, I confess," he says.
-
- Spray contends that the magnetic test
results may not be as clear-cut as they initially seem. Because the Manicouagan
crater is so much larger than Rochechouart, the molten rocks would have
cooled much more slowly at the Canadian site, taking thousands or hundreds
of thousands of years longer to lock in a record of Earth's magnetic field.
"There are a number of reasons why the differences in polarity of
the two [craters] can be explained. I don't think it necessarily means
that they did not in fact form at the same time," he says.
-
-
-
-
- If Spray and others can strengthen their
claim of an impact chain, it would raise some problems. Standard theories
say there is almost no chance of finding such a string of craters on Earth.
-
- "It's intriguing, and if it's true,
it causes great headaches for us theoreticians," comments H. Jay Melosh
of the University of Arizona in Tucson. Earth's gravity is not considered
strong enough to capture a comet except in very unlikely situations.
-
- In 1994, planetary scientists received
a vivid lesson on how crater chains can form on a larger planet. That July,
comet Shoemaker-Levy 9 plowed into Jupiter, raising a line of dark welts
(SN: 12/17/94, p. 412).
-
- The comet had started out as a single
body that strayed too close to Jupiter sometime this century and was trapped
in an orbit around the planet. On a close pass by Jupiter in 1992, the
planet's gravity tugged the weakly constructed comet apart, severing it
into at least 20 large pieces aligned like a string of pearls.
-
- On the comet's next pass, in 1994, the
fragments plunged into the atmosphere one by one over a period of 6 days.
Because Jupiter rotated several times during this span, the identically
aimed fragments struck different points, all falling on the same line of
latitude.
-
- These observations originally led Spray
to suppose that a similar process might explain the string of late Triassic
craters on Earth. Planetary scientists, however, regard this scenario as
highly unlikely because Earth's gravity is so much weaker than Jupiter's.
"The probability for this type of capture is 100,000 times less for
Earth than it is for Jupiter," says Melosh.
-
- A potential solution comes from William
F. Bottke of Cornell University. Last year, Bottke and his colleagues proposed
that a crater chain might develop when an object passes so close to Earth
it almost scrapes the surface. At that intimate distance, it would pass
through the atmosphere and fall prey to an array of destructive influences,
such as intense air friction, that could combine to shatter the object.
-
- Flying through the atmosphere, the asteroid
or comet would lose enough energy to enter an elongated orbit around the
planet. On the next pass, these now well-separated fragments would sequentially
strike Earth. The return could take several days or months, says Bottke.
-
- Melosh notes that this model has merit,
but he is unconvinced that it would actually work.
-
- Bottke admits his model cannot explain
every aspect of Spray's discovery-for instance, why the two smaller craters
lie off the main line of the other three-but says it requires more study.
When he first came up with the idea, there was no solid evidence that this
type of event had actually happened on Earth. "It seemed like a neat
scenario, but until we found an [example], it was kind of hard to get excited
about it. Now, we get to do all the fun stuff and rework it."
-
-
-
-
- It first glance, the multiple impact
scenario ends in true Hollywood fashion: with death on an epic scale and
the survival of a charismatic character.
-
- The close of the Triassic period is notorious
among paleontologists as a tumultuous time. One of the five biggest extinction
events in the fossil record, the late-Triassic crisis wiped out the dominant
reptiles of the time and helped spur the rise of a hitherto minor group
called the dinosaurs, which went on to dominate Earth for 150 million years.
-
- On closer inspection, however, the neat
story line dissolves. The biggest burst of extinctions took place at the
boundary between the Triassic and Jurassic periods, just about 202 million
years ago, says paleontologist Paul Olsen of Lamont-Doherty. Some 12 million
years separate the impacts from the most prominent Triassic die-offs.
-
- Spray and his colleagues suggest a tentative
link between the impacts and an earlier wave of extinctions, which occurred
at the boundary between the Carnian and Norian stages of the Triassic.
Geologists have not dated this time precisely, he says. "Although
it is generally held to be 220 million years ago, it could easily be close
to 214 million," contends Spray.
-
- Paleontologist Michael J. Benton of the
University of Bristol in England disputes that point. "Nobody has
suggested [the Carnian stage] goes to 214. There is no secondary evidence
that impacts had anything to do with the Carnian-Norian extinctions."
-
- Olsen echoes the skepticism. The age
of the Manicouagan crater, he says, "falls in the middle of the Norian,
but there is no evidence of anything going on in the middle Norian. There
are no extinctions."
-
- Geologists might find the lack of association
even more interesting than a link between the impacts and extinctions.
A string of five large body blows to Earth may not be enough to knock life
for a loop.
-
- The Manicouagan crater is a little over
half the size of the Chicxulub crater, but according to impact theories,
it is large enough to cause many of the same effects. With Manicouagan
and the other four craters, says Spray, the energy released in that
series of strikes should compare with the large Cretaceous collision.
-
- Many researchers, however, are starting
to think that size does not matter-above a certain point. Location may
be the more important factor in determining the killing potential of a
large impact.
-
- By this rationale, the Cretaceous crash
claimed so many species because the body slammed into a relatively rare
rock type, a thick carbonate platform loaded with sulfur-rich rock. The
crash filled the atmosphere with tiny, light-blocking sulfuric acid droplets,
which eventually dropped into the oceans and turned the surface waters
toxic. Carbon liberated during the crash enhanced Earth's greenhouse effect
and warmed the planet.
-
- "I'm beginning to think that Chicxulub
might be unique because of the target rocks and all the sulfur that went
into the atmosphere," says Grieve.
-
- If Earth suffered several simultaneous
hits 214 million years ago with few lasting biological effects, then scientists
may have overdramatized the threats of life-ending strikes from space.
That lesson, however, will not make a splash in Hollywood.
-
-
-
-
- The crater chain that wasn't
-
- A series of eight craterlike formations
runs through the U.S. heartland, forming a remarkably straight ine that
passes from Kansas through Missouri into Illinois. Two years ago, geologists
Michael R. Rampino and Tyler Volk of New York University argued that the
structures were all impact craters created at the same time by a string
of comets or asteroids.
-
- The midcontinent features would have
constituted the first known crater chain on Earth, but many geologists
dismissed the claim. In the April Geology, John Luczaj of Johns Hopkins
University in Baltimore presents what may be a death blow to the idea.
If multiple impacts formed the line, then they must have identical geologic
ages, he argues. The evidence, however, points to ages ranging from 500
million to 100 million years.
-
- "Clearly they are not the same age,
and since that is the fundamental test for an impact string, you can rule
out that hypothesis," says Luczaj.
-
- Although many geologists agree, Rampino
and Volk respond that the dates of the craters need to be better established.
-
- Crater specialists add that only two
of the structures in Missouri show convincing evidence of having formed
during impacts. The other examples don't have any obvious impact evidence
and may have a volcanic origin, says H. Jay Melosh of the University of
Arizona in Tucson. -R. Monastersky
-
- From Science News, Vol. 153, No. 20,
May 16, 1998, p. 312. Copyright Ó 1998 by Science Service.
-
-
-
- References:
-
- Luczaj, J. 1998. Argument supporting
explosive igneous activity for the origin of "cryptoexplosion"
structures in the midcontinent, United States. Geology 26(April):295.
-
- Rampino, M.R., and T. Volk. 1996. Multiple
impact event in the Paleozoic: Collision with a string of comets or asteroids?
Geophysical Research Letters 23(Jan. 1):49.
-
- Spray, J.G., S.P. Kelley, and D.B. Rowley.
1998. Evidence for a late Triassic multiple impact event on Earth. Nature
392(March 12):171.
-
- Further Readings:
-
- Cowen, R. 1994. After the crash. Science
News (Dec. 17):412.
-
- Monastersky, R. 1997. The call of catastrophes.
Science News (March 1):S20.
-
- Sources:
-
- William F. Bottke
- Center for Radiophysics & Space
Research
- Cornell University
- Department of Astronomy
- 306 Space Sciences
- Ithaca, NY 14853-6801
-
- Richard Grieve
- Geological Survey of Canada
- Department of Energy Mines Research
- 1 Observatory Crescent
- Ottawa, ON K1A 0YS
- Canada
-
- Dennis V. Kent
- Lamont-Doherty Earth Observatory
- Columbia University
- P.O. Box 1000
- 61 Route 9W
- Palisades, NY 10964-1000
-
- John Luczaj
- Johns Hopkins University
- Department of Earth and Planetary
Sciences
- Baltimore, MD 21218
-
- H. Jay Melosh
- University of Arizona
- Lunar and Planetary Laboratory
- Tucson, AZ 85721
-
- John G. Spray
- University of New Brunswick
- Department of Geology
- Fredericton, NB E3B 5A3
- Canada
|
Sightings HomePage