Skip to comments.More Theories on Tunguska
Posted on 10/28/2002 4:59:07 PM PST by aruanan
On 30 June 1908, in the remote Tunguska forest of Siberia, a vast explosion charred and flattened trees across an area nearly as large as Rhode Island. Scientists have long been mystified as to the cause, although prevailing wisdom has it that it was an extraterrestrial chunk of ice or rock (Science, 20 August 1999, p. 1205).
CREDIT: VITALII ROMEIKO
But two scientists last week rejected the "E.T. hypothesis" at a conference on environmental catastrophes in London. Andrei Ol'khovatov, formerly of the Soviet Radio Instrument Industry Research Institute, noted that no one has ever found definitive traces of extraterrestrial material. There's no impact crater, and some trees near the epicenter were left untouched.
Wolfgang Kundt, an astrophysicist at Bonn University, Germany, proposed an alternative scenario: a massive gas explosion. A large natural gas deposit lies below the site, a well-known fact unconnected to the event until now, he said. Kundt has modeled a Tunguska "outgassing" and says it would fit with eyewitness accounts.
"The geophysical hypothesis could be the answer," says Jesús Martínez-Frías of the Institute of Astrobiology in Madrid, Spain. But Ol'khovatov believes the explosion was caused by a "strong coupling between subterranean and meteorological phenomena" that science is not yet ready to understand.
Presumably there is a campfire or other ignition source in the immediate vicinity.
The odds of this being the culprit behind Tunguska are very remote.
I agree, dog.
If we were talking about methane collecting in a valley from a shallow gas bearing sand, then suddenly exploding due to static electricity, I could buy that.
But I can't buy gas collecting in an area the size of Rhode Island!
The second possibilty is equally remote:
A deep high pressure sand is somehow suddenly relieved of it's overburden pressure and blows out.
The blowing out of the amount of gas necessary for that kind of explosion in that short of a time period is an unrealistic scenario.
Keep that hat handy. Art's replacement is a lot like Art.
It augurs poorly that I forgot the difference between auger and augur.
Do you have anything on the recent meteorite impact?
Actually I gotta go for meteor of some type. I saw an interview with a couple of eyewitnesses who saw the big streak in the sky.
Well, if you decide to change your name, "KneelBeforeRod" is still available.
Not that you'd probably want it - the porn angle is pretty hard to avoid with a name like that... :^)
Scientists Studying Two Big Craters On Earth Find Two Causes
CHAMPAIGN, Ill. -- Two of the three largest impact craters on Earth have nearly the same size and structure, researchers say, but one was caused by a comet while the other was caused by an asteroid. These surprising results could have implications for where scientists might look for evidence of primitive life on Mars.
Susan Kieffer of the University of Illinois at Urbana-Champaign, Kevin Pope of Geo Eco Arc Research and Doreen Ames of Natural Resources Canada analyzed the structure and stratigraphy of the 65 million-year-old Chicxulub crater in Mexico and the 1.8 billion-year-old Sudbury crater in Canada.
Chicxulub is well preserved, but buried, and can be studied only by geophysical means, remote sensing and at a few distant sites on land where some ejecta is preserved. In contrast, Sudbury has experienced up to 4-6 kilometers of erosion, and is well exposed and highly studied by mining exploration companies because of its rich mineral resources.
By working back and forth with data from the two craters, the researchers were able to re-create the structures and then estimate the amount of melt in each structure. The amount of melt is critical for determining if long-lived hot-water circulation systems that might host life forms could have been formed after the impacts.
In their field studies, the researchers found that both craters were about 200 kilometers in diameter. In addition, they identified five ring-shaped structures with similar character and dimensions. A sixth ring -- the peak ring in the central basin -- was present at Chicxulub, but had been eroded away at Sudbury.
"While the size and structure of the two craters were similar, they differed greatly in the amount of impact melt that was produced," said Kieffer, who presented the team's findings at the annual meeting of the Geological Society of America, held Oct. 27-30 in Denver.
"Through field studies, we determined that Chicxulub has about 18,000 cubic kilometers of impact melt, approximately four times the volume of water in Lake Michigan," Pope said. Sudbury has about 31,000 cubic kilometers of impact melt, approximately six times the volume of lakes Huron and Ontario combined, and nearly 70 percent more than the melt at Chicxulub. These differences in volume have significant implications about the amount of heat available to drive hot-water circulation systems.
The researchers then used an analytical cratering model to examine possible causes for the huge difference in melt. According to the simulation results, the difference in melt volume could be readily explained if Chicxulub -- the impact crater that doomed the dinosaurs -- was formed by an asteroid and Sudbury was formed by a comet.
"Our calculation of 18,000 cubic kilometers of impact melt at Chicxulub agreed well with model estimates for an asteroid striking at a 45 degree angle," said Kieffer, the Walgreen Professor of Geology at Illinois. "None of the comet impact examples came close to agreeing."
"In contrast, the Sudbury impact melt volume of 31,000 cubic kilometers fell between model estimates for a comet striking at an angle of 30-45 degrees", Kieffer said. "Similarly, none of the asteroid impact examples came close to agreeing with the Sudbury melt volume."
Another clue to the cratersÕ origins lies in the impact melts themselves. The majority of the excess melt at Sudbury is in the form of a melt-rich breccia Ð called suevite Ð inside the crater. This material tends to form in impacts where the crustal target rock contains a lot of water. Sudbury has much more suevite in the preserved crater than Chicxulub.
"The mystery was that there probably wasn't a lot of water in the original rocks at Sudbury to account for the excess suevite," Kieffer said. "But in a comet impact of this size, somewhere around 1,400-2,000 cubic kilometers of water from the comet gets mixed into the impact melt, and that could play a major role in disrupting the melt and creating the excess suevite."
There is other independent evidence for an asteroid impact at Chicxulub, the team said, including the purported find of an asteroid fragment in an oceanic drill core, the amount of iridium spread around the world at the time of impact, and a telltale chromium 53 isotopic signature.
By studying the origin and structure of large impact craters on Earth, scientists might narrow the search for life on Mars. At Sudbury, for example, "there is evidence of a huge hydrothermal system that was driven by the heat of the impact melt," Ames said. "As a result, there was widespread hot spring activity on the crater floor possibly capable of supporting life."
The researchers are interested in "extrapolating these conclusions about comet and asteroid impacts to Martian conditions and asking where we might go to look for similar hydrothermal systems that could have hosted primitive life forms on Mars," Kieffer said. "Our next step is to model these hot-water circulation systems that were set up by the impact melts with fluid flow controlled by structures (fractures) inside the crater, and then extrapolate the results to Martian conditions."
The National Aeronautics and Space Administration and the Natural History Museum of Los Angeles County funded this work.
Note: this topic is from 10/28/2002. Thanks aruanan.