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Sunday, 1 August 2010

Asteroids, Comets & Meteor Strikes

Asteroids, Comets & Meteor Strikes

"Fresh" Crater Found in Egypt Changes Earth Impact Risk Estimate?


Geophysicists at work in the Kamil Crater
Geophysicists work in the newfound Kamil crater in an undated picture courtesy Museo Nazionale dell'Antartide Università di Siena

A small impact crater discovered in the Egyptian desert could change estimates for impact hazards to our planet, according to a new study.

One of the best preserved craters yet found on Earth, the Kamil crater was initially discovered in February 2010 during a survey of satellite images on Google Earth. Researchers think the crater formed within the past couple thousand years.
The Italian-Egyptian team that found the crater in pictures recently visited and studied the 147-foot-wide (45-meter-wide), 52-foot-deep (16-meter-deep) hole. The team also collected thousands of pieces of the space rock that littered the surrounding desert.

Based on their calculations, the team thinks that a 4.2-foot-wide (1.3-meter-wide) solid iron meteor weighing 11,023 to 22,046 pounds (5,000 to 10,000 kilograms) smashed into the desert—nearly intact—at speeds exceeding 2.1 miles (3.5 kilometers) a second.

There are no hard numbers for how many meteors this size might currently be on a collision course with Earth, but scientists think the potential threats could be in the tens of thousands.

Current impact models state that iron meteors around this size and mass should break into smaller chunks before impact. 

Instead, the existence of the newfound crater implies that up to 35 percent of these iron giants may actually survive whole—and thus have greater destructive power. 

Egypt Crater Still Shows Splatter

Estimating impact hazards to Earth isn't an exact science, since only 176 impact craters have been discovered so far, according to the Earth Impact Database, a resource maintained by the University of New Brunswick in Canada.
Most models are based on the number of impact craters on the moon, which has almost no atmosphere and so doesn't experience the same erosion processes as those on Earth.

"Current models predict that around a thousand to ten thousand such craters should have formed [on Earth] in one million years," said study co-author Luigi Folco, a scientist with the University of Siena in Italy.

"The reason why they are rare, however, is that, on Earth, weathering rates are high—small craters are usually easily eroded or buried."

Folco and colleagues were particularly surprised to find that the newfound, bowl-shaped crater has a prominent splatter pattern of bedrock shot up by the original impact blast.

Known as ejecta rays, these features are more often seen on other planets and moons with thin atmospheres.

The exact age of the Egyptian crater is still uncertain, the team reported this week in the online edition of the journal Science. Geologic evidence points to a relatively recent event, Folco said—although it's unlikely that any humans were around to witness the impact.

"During our field work we could see that some of the bedrock material ejected from the crater overlies prehistoric structures in the area," Folco said.
"We know from literature that the human occupation of this region ended about 5,000 years ago, with the onset of hyperarid conditions. Therefore we think that the impact occurred afterwards."

Meteor Threat Greater Than Realized

If future meteors like the Egyptian rock are more likely to remain intact, their energy on impact would be more focused, causing greater damage, said John Spray, a crater expert with the University of New Brunswick who isn't connected to the study.

Still, the probability of such a meteor hitting something critical for society, such as a major city, would be reduced, because the falling rocks would not be as spread out.
"Overall, the threat from impacts is probably greater than people realize, but historically there is very little information on this, and we just have not been collecting data for all that long," Spray said.

"Our knowledge is very limited, so events such as these are quite important for helping us understand the frequency and nature of impacts that affect our planet."
by Andrew Fazekas for National Geographic News
@ http://news.nationalgeographic.com/news/2010/07/100722-science-space-egypt-kamil-crater-meteor-meteorite-impact-hazard/

Comet "Shower" Killed Ice Age Mammals?

 Ancient extinction could be linked to annual Taurid meteors, study says.


A fragmented comet
A Hubble Space Telescope picture shows a comet breaking apart on April 18, 2006. Image courtesy NASA, ESA, H. Weaver (APL/JHU), M. Mutchler, and Z. Levay (STScI)

The comet that created the annual Taurid meteor shower was also responsible for snuffing out large mammals in North America 13,000 years ago, a controversial new study says.

The geologic record shows that global temperatures plummeted by as much as 14 degrees Fahrenheit (8 degrees Celsius) just as Earth was thawing out from the last ice age.

This cold snap probably led to the extinction in North America of large animals such as saber-toothed cats and wooly mammoths. But scientists have been unsure what triggered the abrupt change.

For more than 15 years, astrobiologists Bill Napier and Victor Clube have argued that the culprit was a 30- to 60-mile-wide (50- to 100-kilometer-wide) comet that entered the inner solar system—the region between the sun and the asteroid belt, just past Mars's orbit—20,000 to 30,000 years ago.

In the new study, Napier, of Cardiff University in the U.K., suggests that the huge comet settled into a new, faster orbit around the sun and began to break apart, creating fragments that pummeled Earth about 13,000 years ago.

His model suggests that those pieces might still be visible today as the Taurid complex, a debris cloud that sends tiny meteors streaming through Earth's atmosphere in late October and early November. (See pictures of the Perseid meteor shower.)

The study, according to Napier, further bolsters the comet-impact theory, which many astronomers continue to dismiss.

"We are looking at this as an actual, reasonable, astrophysical mechanism," said Napier, whose paper is now online at arXiv.org and will appear in an upcoming issue of the Monthly Notices of the Royal Astronomical Society.

Napier made the connection between the Taurids and the mammal extinctions after hearing about evidence for microscopic nanodiamonds at Ice Age sites across North America. Such tiny diamonds are thought to form only during extraterrestrial impacts.

"When we see the evidence of nanodiamonds on the ground, it clicks," Napier said.
Also, a barrage of fiery comet fragments would explain the evidence for ancient wildfires found at many North American sites from this time period.

But many astronomers say the idea is improbable, because space rocks big enough to cause such a catastrophe aren't known to orbit so close to the Sun.

"So this would be something pretty unusual," said Jay Melosh, an impact expert at Purdue University in Indiana, who remains unconvinced by the new study.

What's more, if an existing debris cloud did cause the ancient extinctions, there should still be threatening chunks in the inner solar system [like the dangerous Taurid Streams and others – and Venus? – Ed], argued Alan Harris, a senior researcher at the Space Science Institute in Colorado who studies impact hazards.
"An abrupt comet [or asteroid] shower is bit like, say, a stove fire that can fill the kitchen with smoke in a few seconds but takes many minutes to hours to clear out," Harris said in an email.

Surveys for such comet chunks show that "they simply aren't there in the numbers needed [to account for a giant, disintegrating comet]," Harris said.

"And what we do see are not in coherent or clustered orbits that could possibly have come from a single body" within the last 20,000 to 30,000 years.

According to Harris, the claim that the Taurid complex is linked to the North American extinctions "is just the most current window dressing for a broad hypothesis that has not yet been proven."

India Asteroid Killed Dinosaurs, Made Largest Crater?


The dinosaurs' demise may have been due to an asteroid double-whammy—two giant space rocks that struck near Mexico and India a few hundred thousand years apart, scientists say.
For decades one of the more popular theories for what killed the dinosaurs has focused on a single asteroid impact 65 million years ago.

A six-mile-wide (ten-kilometer-wide) asteroid is thought to have carved out the Chicxulub crater off Mexico's Yucat√°n Peninsula, triggering worldwide climate changes that led to the mass extinction. 

But the controversial new theory says the dinosaurs were actually finished off by another 25-mile-wide (40-kilometer-wide) asteroid. That space rock slammed into the planet off the western coast of India about 300,000 years after Chicxulub, experts say. 

"The dinosaurs were really unlucky," said study co-author Sankar Chatterjee, a paleontologist at Texas Tech University in Lubbock.
Chatterjee thinks this second asteroid impact created a 300-mile-wide (500-kilometer-wide) depression on the Indian Ocean seafloor, which his team began exploring in 1996. 

His team has dubbed this depression the Shiva crater, after the Hindu god of destruction and renewal. 

"If we are correct," Chatterjee said, "this is the largest crater known on Earth." 

Dinosaur-Killer Asteroid Boosted Volcanoes?

The Shiva asteroid impact was powerful enough to vaporize Earth's crust where it struck, allowing the much hotter mantle to well up and create the crater's tall, jagged rim, Chatterjee estimates. 

What's more, his team thinks the impact caused a piece of the Indian subcontinent to break off and drift toward Africa, creating what are now the Seychelles islands (see map). 

The Shiva impact may also have enhanced volcanic eruptions that were already occurring in what is now western India, Chatterjee added. 

Some scientists have speculated that the noxious gases released by the Indian volcanoes, called the Deccan Traps, were crucial factors in the dinosaurs' extinction. 

(Related: "'Dinosaur Killer' Asteroid Only One Part of New Quadruple-Whammy Theory."

"It's very tempting to think that the impact actually triggered the volcanism," Chatterjee said. 

"But that may not be true. It looks like the volcanism was already happening, and the [Shiva] impact just made it worse." 

by Ker Than for National Geographic News
@ http://news.nationalgeographic.com/news/2009/10/091016-asteroid-impact-india-dinosaurs.html

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