Posted on 01/24/2002 6:05:31 AM PST by Scully
Scientists basically agree that an asteroid struck the Earth some 65 million years ago and its impact created the Chicxulub crater in Yucatan, Mexico. More controversial is the link between this impact and a major mass extinction of species that happened at the geological (K-T) boundary marked by the impact.
But what mechanism did the impact trigger to cause mass extinction? The conventional theory is that impact dust obscured the sun, shutting down photosynthesis and snuffing out life. Kevin Pope from Geo Eco Arc Research shows in the February issue of GEOLOGY that the assumptions behind this theory are amiss, and therefore damage estimates from future asteroid impacts are also amiss.
This latter point became a recent issue when a large asteroid passed near the Earth on January 7 and news reports exaggerated its potential impact effects.
“Based on the old, inaccurate dust numbers, which erroneously suggested that a medium-sized asteroid (1-2 km in diameter) could cause global climate change and famine, scientists calculated that one's chance of getting killed by an asteroid impact were about the same as dying in a plane crash,” Pope said. “My new impact dust estimates indicate that death by an asteroid is far less likely and that such medium-sized asteroid impacts would not have catastrophic global effects. But of course the regional effects would still be devastating.”
To truly understand the influence of impact dust, scientists need to find a way to directly measure the amount of small dust particles in such places as the K-T boundary. In the meantime, Pope studied patterns of coarse dust particles to create a model that showed how the small dust particles were dispersed. Incorporating these geological observations with new theoretical work, Pope asserts that very few of the particles are of the size that it would take to shut down photosynthesis for any significant length of time and therefore the original K-T impact extinction hypothesis is not valid. He believes it may have been sulfate aerosols produced from impacted rocks and soot from global fires that could have shut down photosynthesis and caused global cooling.
“The original studies of the clay layer found at the K-T boundary assumed much or all of this layer was derived from fine impact dust,” he said. “More recent studies of this layer have shown this not to be the case. Furthermore, earlier estimates were based on extrapolations of data from surface atomic bomb blasts, which had about 100 million times less energy than the Chicxulub impact. Extrapolation over eight orders of magnitude is risky business.”
Pope was involved in the “discovery” of the Chicxulub crater in 1989-1990 when he worked at the NASA Ames Research Center. (Oil geologists had discovered the crater and reported the finding in 1981, but it was basically ignored.) He was using satellite images to map water resources in the Yucatan with Adriana Ocampo and Charles Duller when they found the semi-circular ring of sink holes. They thought the crater might be the K-T impact site and published their hypothesis in the May 1991 issue of NATURE.
I did a search, sorry of this is a repeat.
“The original studies of the clay layer found at the K-T boundary assumed much or all of this layer was derived from fine impact dust,” he said. “More recent studies of this layer have shown this not to be the case.
That was my question also. Is there a layer of iridium dust so thick in outer space that as the earth drifted through it placed a layer on the surface?
That would be an awfully large, awfully thick cloud of dust, no?
You would think if it was that big and thick that astronomers would be able to see it, wouldn't you?
Or was it the fallout from two alien races having a war on earth? Nuetral ground so they don't screw up their own planets.
We may never know.
A subtle difference.
This makes it sound as if there just simply wasn't enough impact dust to encircle the globe. I doubt that regional fires could have done it either, particularly in view of the clay layer being continuous (with some exceptions) worldwide. Again, I'm left with my original question: Where did the iridium come from?
If you read closely, this guy's conclusions are based on mathematical models, not measurements. It is probably about as sound as the global warming hypothesis which is also based on modelling. One GLARING supposition is the assumption that large dust particles can be used to model small ones--highly unlikely.
I've read that it likely came from the meteor.
First, here's the abstract(which is all I can read with out a subscription):
ABSTRACT Most of the 3-mm-thick globally distributed Chicxulub ejecta layer found at the Cretaceous-Tertiary (K-T) boundary was deposited as condensation droplets from the impact vapor plume. A small fraction of this layer (<1%) is clastic debris. Theoretical calculations, coupled with observations of the coarse dust fraction, indicate that very little (<1014 g) was submicrometer-size dust. The global mass and grain-size distribution of the clastic debris indicate that stratospheric winds spread the debris from North America, over the Pacific Ocean, to Europe, and little debris reached high southern latitudes. These findings indicate that the original K-T impact extinction hypothesis?the shutdown of photosynthesis by submicrometer-size dust?is not valid, because it requires more than two orders of magnitude more fine dust than is estimated here. Furthermore, estimates of future impact hazards, which rely upon inaccurate impact-dust loadings, are greatly overstated.
It's clear from the abstract that he's creating a distribution of grain sizes. The pure fact of physics is that different sized dust grains scatter and reflect certain wavelength light more effectively than others. In this case, he's finding more coarse grains than fine grains, and he's finding 100 times less of the fine grains necessary to reflect the visible light that plants use best to grow. Pretty straightforward stuff, and more complex then earlier models.
When talking about medium sized impacts, speed matters.
Yup, same except Cosmic Winter. See: Clube & Napier.
(It's hot and humid as the devil here today) Clouds are moving north like a bat outta hell.
by Nick Nuttall
Copyright 1990 Times Newspapers Limited The Times, May 24,
1990, Thursday
-------------------------------------------------------------------------------- Over the next few weeks the Taurid stream, a procession of vast cosmic rubble and dust that snakes around the Sun and out towards Jupiter, will swing through Earth's orbit for the first of its bi-annual crossings.
Within the stream are probably thousands of bodies including asteroids, mountain-and island-sized boulders, smaller meteoroids, Encke's Comet and assorted fragments of celestial refuse.
The exact number, size and location of objects, however, remains a mystery and according to Dr. Mark Bailey, research Fellow in astronomy at Manchester University, it is likely that for every object which is confirmed, there are nine others that have so far eluded detection.
All that is certain is that the rubble, believed by some astronomers to have been formed by a collision in the asteroid belt of a defunct comet which was captured by the solar system up to 30,000 thousand years ago, will bisect Earth's orbit in late June and again in November.
According to astronomers such as Dr. Victor Clube, of Oxford University's Department of Astrophysics, the coming and goings of the Taurid stream should be a source of concern to politicians, planners and anyone who cherishes life on Earth.
A ''catastrophist'', Dr. Clube is one of many astronomers who are convinced that within this celestial procession lie the seeds of mass destruction an Armageddon of biblical proportions. ''The matter requires urgent attention. It is crucial that everyone is woken up to the danger,'' Dr. Clube says.
The chilling scenario envisaged is of Earth and one of the 46,000mph objects in the Taurid stream colliding during one of the orbital crossings.
Dr. Clube says: ''It is analagous to a nuclear war with a megatonnage of the same order and all the effects of nuclear war with debris from the impact causing sunlight to be blocked causing a Dark Age or Ice Age.''
He has coined the phrase ''Multiple Tunguska Bombardment'' to describe the worst nightmare which, he believes, will eventually happen. Tunguska refers to a Siberian River near which, in June 1908, a 100 yard body from the Taurid stream ploughed into Earth, exploding and devastating an area 25 miles wide with the impact of a 20-megaton bomb.
Fortunately the encounter occured in an unpopulated part of the globe but if the impact had been on London it would have devastated the city, killing millions. The Tunguska event may have been only a chance occurence.
Yet, according to Dr. Clube and Dr. Bill Napier, of the Royal Observatory in Edinburgh, whose book Cosmic Winter is published next month, the history of Earth is littered with subtle evidence that cosmic debris have consistently intervened, often with catastrophic consequences.
One of the most popular theories to explain the sudden demise of the dinosaurs is that, 65 million years ago, a huge asteroid ploughed into the planet, triggering either a nuclear-style winter or huge fires.
This popular theory was given a boost only last week when scientists at the University of Arizona reported the discovery of an apparent 180-mile-wide crash site in the Caribbean of an asteroid six miles wide. They claim this could be linked with the great reptiles' extinction.
Dr. Clube ascribes other events including the Old Testament story of Noah and his Ark to a Dark Age linked with colliding heavenly bodies. He also believes that climatic changes, including fears of present global warming, may have a cosmic component.
There is sufficient evidence, he says, to indicate that collisions happen within centuries and millenniums rather than millions and billions of years, with multiple encounters more likely than sceptics claim.
Dr. Clube emphasizes that predicting when a bombardment may occur is impossible without more scientific evaluation of the Taurid stream.
''We are probably a little safer at the moment because the intersecting orbits are far away. But we are on the inward run and in 500 years we will start getting close again,'' he says.
According to Dr. Clube, the last time that the stream was closest within Earth's orbit was in the first millennium BC, from about 500BC up to 0AD, the time of Christ.
It is vital to overcome complacency about the threat from cosmic debris, he says. This complacency is relatively new, as pagan and ancient civilisations such as the Babylonians were firm believers in the threat of of cosmic destruction. Part of the blame for this complacency rests with the breakup, in 1845, of Comet Biela without any easily visible effect on Earth.
''This rather relaxed attiude to comets, which has persisted to the present day, helped turn 19th century opinion against a prevailing catastrophist view of evolution,'' Dr. Clube says.
''Indeed, the eventual disintegration of Comet Biela into dust made it no longer out of place for biologists and geologists to explain evolution in processes that were non-violent and slow-acting.''
''In short, it became fashionable to assume that the world is safe when in fact multiple Tunguska bombardments, releasing around five-thousand megatons, the equivalent of a full-scale nuclear war, may happen at intervals of about 1,500 years, producing a Dark Age,'' he says.''To suggest the planet is safe is absurd.''
The Oxford astrophysicist is not alone in his views. Similar concerns were echoed last week by the respected American Institute of Aeronautics and Astronautics (AIAA).
The institute is calling for studies aimed at defending the Earth from asteroid attack, including the possible redeployment of nuclear weapons to shatter incoming celestial bodies.
The call comes in the wake of thawing East-West relations and what is being claimed as a recent, potentially disastrous near-collision.
Last year, 1989 FC, a cosmic boulder bigger than an aircraft carrier, passed within 400,000 miles of Earth, a mere whisker in astronomical terms, before being noticed by astronomers.
''Such an object could cause a disaster of unprecedented proportions if it had struck. Although the probability is very small, its consequences in terms of the casuality rate could be enormous,'' the institute argues in a paper it released about the problem.
Apart from putting nuclear warheads on standby for intercepting and shattering asteroids, the institute is calling for studies into power units that could attach and divert the celestial boulders away from Earth.
''We have the technology needed to detect and track such an object and possibly to divert if from an impending impact. We would be derelict if we did nothing,'' the institute says.
Dr. Clube is hoping to get access to an infra-red telescope to study the Taurid stream during the November crossover.
In 1983, a satellite revealed what appeared to be dust following Comet Encke, but some scientists, including Dr. Clube, now believe that this contains the single large missing body, perhaps as large as 20 miles wide, shrouded in dust and boulders.
The best chance of detecting the defunct comet might come in 1994 when the American National Aeronautics and Space Administration (Nasa) is expected to launch the infra-red telescope, ISO.
Dr. Bailey says: ''We are learning more about these objects almost every week. We are realizing that there are quite a large number of fairly large objects, ranging in size from just a few hundred yards to six miles across, which are in Earth's collision orbit.''
Along with Dr. Bailey, Dr. Clube supports the institute's call for improved monitoring. But both British astronomers are concerned at suggestions of shattering incoming asteroids.
They believe that there is the danger that by solving one large threat, it may create scores of smaller ones. Cosmic Winter by Dr. Victor Clube and Dr. Bill Napier. Published in June by Basil Blackwell (Pounds 16.95). The Origin of Comets by Dr. Mark Bailey, Dr. Victor Clube and Dr. Bill Napier. Pergamon Press.
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