NEO News (01/02/09) More on Recent Comet Showers

From Dave Morrison's NEONews:

NEO News (01/02/09) More on Recent Comet Showers

A paper just published in Science and associated news reports have stimulated new interest in the idea that a cosmic impact caused the events of 12,900 years ago that included global cooling, the extinction of some North American megafauna such as the mammoths, and the end of the Clovis human culture. However, the new evidence of possible nanodiamond deposits still does not address the fundamental problems of the model when it was first proposed two years ago: that the hypothesized impact event is not consistent with what we know about cosmic impacts.

The authors of this work hypothesize that a shower of comets struck North America equivalent to thousands of Tunguska-size atmospheric or surface explosions. As described in the three news stories following, it is suggested "They would have seen a brilliant flash followed by others in quick succession. The sky would be a canopy of fire, and shock waves would flatten trees. Miniscule diamonds would drizzle over tens of thousands of kilometres, a third of the way around the planet."

The problem is that there are no craters or other evidence of large-scale impacts. The concept of a comet shower focused on North America makes no sense to me. A large object, whether comet or asteroid, would be expected to reach the ground, not explode in the atmosphere. Even if we imagine a loosely bound comet braking up in the atmosphere a few seconds before impact, the lateral dispersion of the fragments would be only of the order of the thickness of the atmosphere, or about 100 km. There is no known way to break up and disperse an incoming comet thousands of kilometers before it hits the atmosphere. If instead this were part of a general comet shower filling the inner solar system, then the evidence would not be limited to North America, and in addition we would still see the dynamical evidence of the remnants of this comet shower. There should also be a clear signature of this event in Greenland and Antarctic ice cores. So whatever happened 12,900 years ago, the hypothesis put forward by these authors does not seem viable. One could add that the size of this proposed impact would make it a one-in-several-million year event, and hence not something we would have expected within the past 13,000 years.

Three press reports follow, from the New York Times, the Washington Post, and the BBC.

David Morrison

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DIAMONDS LINKED TO QUICK COOLING EONS AGO
By Kenneth Chang, New York Times
Published: January 1, 2009

At least once in Earth's history, global warming ended quickly, and scientists have long wondered why. Now researchers are reporting that the abrupt cooling - which took place about 12,900 years ago, just as the planet was emerging from an ice age - may have been caused by one or more meteors that slammed into North America. That could explain the extinction of mammoths, saber-tooth tigers and maybe even the first human inhabitants of the Americas, the scientists report in Friday's issue of the journal Science.

The hypothesis has been regarded skeptically, but its advocates now report perhaps more convincing residue of impact: a thin layer of microscopic diamonds found in rocks across America and in Europe.

"We're up over 30 sites, as far west as offshore California, as far east as Germany," said Allen West, a retired geology consultant who is one of the scientists working on the research.

The meteors would have been smaller than the six-mile-wide meteor that struck the Yucatán peninsula 65 million years ago and led to the mass extinctions of the dinosaurs. The killing effects of the hypothesized bombardment 12,900 years ago would have been more subtle.

Climatologists believe that the direct cause of the 1,300-year cold spell, known as the Younger Dryas, was a sudden rush of fresh water from a giant lake in central Canada to the North Atlantic.

Usually a surface current of warm water flows northward in the Atlantic toward Greenland and Europe, then cools and sinks, returning south in the deep ocean. But the fresh water, which is less dense, blocked the sinking of the cold, salty water in the North Atlantic, disrupting the currents. That sudden change in plumbing has long been known, but what caused it has never been satisfactorily explained. The authors of the paper in Science say it was meteors.

At each site the scientists looked at, the diamond layer in the rocks correlates to the date of the hypothesized impact. Within the layer, the scientists report finding a multitude of diamond particles, all encased within carbon spheres. "We've yet to find a single diamond above it," Dr. West said. "We've yet to find a single diamond below it."

Perhaps more telling, the scientists reported last month at a meeting of the American Geophysical Union in San Francisco, the carbon atoms inside some of the diamonds are lined up in a hexagonal crystal pattern instead of the usual cubic structure. The hexagonal diamonds, formed by extraordinary heat and pressure, have been found only at impact craters and within meteorites and cannot be formed in forest fires or volcanic eruptions, Dr. West said. Last year the scientists presented other evidence of an impact, including elevated levels of the element iridium.

At least some skeptics are not convinced. "The whole thing still does not make sense, and there are lots of contradictions," said Christian Koeberl, a professor of geological sciences at the University of Vienna in Austria. His chief reservation is that there is no crater. "A body of this size does not just blow up without a trace in the atmosphere," Dr. Koeberl said. "Physics won't have it."

Proponents have suggested that the meteor hit an ice sheet a couple of miles thick or that there was a series of smaller objects that exploded in the air. But Dr. Koeberl said something hitting an ice sheet would still generate a hole in the ground underneath, and he questioned whether smaller impacts or air explosions would produce the shock waves needed to make diamonds. An impact should also have left remnants of melted rocks and shocked minerals, Dr. Koeberl said.

But if true, the hypothesis could explain the disappearance of ice age mammals like mammoths and argue against the alternative idea that the animals were hunted to extinction by humans. It might also help explain the disappearance of the Clovis people, a culture named after a distinctive arrow point discovered in a mammoth skeleton in Clovis, N.M., who are believed to have arrived in the Americas more than 13,000 years ago.

Douglas J. Kennett, a University of Oregon archaeologist who is the lead author of the Science paper, said no Clovis points or bones of the extinct animals had been found above the diamond layer. "It seems those two things synchronously end," he said. Dr. Kennett said there also appeared to be a gap of several centuries between the disappearance of the Clovis and the resettlement by other people.

Gary Huss, a scientist at the University of Hawaii, Manoa, who was one of the early reviewers of the paper in Science, said though the scientists had not proved their case, they had offered enough evidence that the idea warranted a closer look by others. "They have a hypothesis that explains several things that hard to explain any other way," Dr. Huss said. "Diamonds are less convincing by themselves, but they strengthen their case considerably."

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GEMS POINT TO COMET AS ANSWER TO ANCIENT RIDDLE
By Joel Achenbach, Washington Post
Friday, January 2, 2009

Nanodiamonds, such as these in the black layer of sediment at the Murray Springs archaeological site in Arizona, may explain the extinction of large animals, the disappearance of the Clovis culture and the climate change of an epoch known as the Younger Dryas.

Something dramatic happened about 12,900 years ago, and the continent of North America was never the same. A thriving culture of Paleo-Americans, known as the Clovis people, vanished seemingly overnight. Gone, too, were most of the largest animals: horses, camels, lions, mammoths, mastodons, saber-toothed cats, ground sloths and giant armadillos.

Scientists have long blamed climate change for the extinctions, for it was 12,900 years ago that the planet's emergence from the Ice Age came to a halt, reverting to glacial conditions for 1,500 years, an epoch known as the Younger Dryas.

In just the last few years, there has arisen a controversial scientific hypothesis to explain this chain of events, and it involves an extraterrestrial calamity: a comet, broken into fragments, turning the sky ablaze, sending a shock wave across the landscape and scorching forests, creatures, people and anything exposed to the heavenly fire.

Now the proponents of this apocalyptic scenario say they have found a new line of evidence: nanodiamonds. They say they have found these tiny structures across North America in sediments from 12,900 years ago, and they argue that the diamonds had to have been formed by a high-temperature, high-pressure event, such as a cometary impact. "This is a big idea," said Douglas J. Kennett, an archaeologist at the University of Oregon and the lead author of a paper on the Younger Dryas impact hypothesis published today in the journal Science.

The hypothesis has been hotly contested, as would be expected for a catastrophic tale that, so far, lacks anything as compelling as a crater. Nor are there signs of deformation in rock debris that is a signature of the massive impact that, 65 million years ago, apparently wiped out the dinosaurs.

But Kennett and his colleagues say that they have found these diamonds at the layer of sediment that marks the start of the Younger Dryas. They are not found above or below that layer. These diamonds are measured in nanometers -- mere billionths of meters -- and one of them would not suffice for an engagement ring unless the recipient had an extremely small finger. Indeed, these diamonds are visible only with the aid of the most advanced microscopes.

The wide distribution of the nanodiamonds could be a sign that the comet broke into pieces in space and that the fragments burned up explosively over a broad area of North America. The heat and pressure from the event transformed carbon on the planet's surface into the tiny diamonds, the scientists said.

"Imagine these fireballs exploding in the air. A Clovis hunter standing and looking at these things would have seen a canopy of fire as these things came in and exploded," said Allen West, a geophysicist and one of the paper's co-authors. "There would have been no sound. There would have been massive explosions. Brilliant light, brighter than the sun. There would have been radiant heat -- it would have been capable, at the very least, of giving him serious burns and, at the maximum, of incinerating him."

The hypothesis of a catastrophic impact at the start of the Younger Dryas has incited abundant skepticism in the scientific community. NASA space scientist David Morrison, an expert on impacts, said he doubts that a comet could have broken up in the manner proposed by the Kennett group.

"They talk rather blithely about a comet disintegrating in the atmosphere," Morrison said. Referring to the nanodiamonds, he said: "They may have discovered something absolutely marvelous and unexplained. But the impact hypothesis just doesn't make sense."

Morrison posed several questions: "What size impact does it take to produce diamonds? What size crater would that be? Where is it? If it hit in the ocean, would it have had the same effect? These are all questions one can ask."

Kennett's father and co-author, James Kennett, an earth scientist at the University of California at Santa Barbara who has devoted much of his career to studying the Younger Dryas, said: "I think it's totally reasonable that there should be skeptics. What we're arguing is that this impact hypothesis explains three major things that have been enigmatic and not particularly resolvable."

Those three things are the extinction of the megafauna, the disappearance of the Clovis culture and the climate change of the Younger Dryas. The general thought has been that climate change played a key role in wiping out the large animals and perhaps undermining the Clovis people, though some scientists have argued that the animals were hunted to extinction (the Pleistocene overkill hypothesis). But the fossil record has been puzzling, for many species of megafauna had survived multiple ice ages until the cool spell of the Younger Dryas.

For decades, scientists have believed that meltwater at the end of the ice ages formed a huge lake in central North America, known to scientists as Lake Agassiz. At some point, the water from that lake may have surged into the North Atlantic and shut down the dominant ocean current that brought warmer water toward higher latitudes. That, in turn, could have created a long-term climate change.

The impact scenario incorporates the meltwater scenario. The scientists say that the impact could have destabilized and melted the edges of the ice sheet resting on the northern tier of the continent. An impact would also have created a short-term environmental disaster. Dust from the impact and soot from continent-spanning wildfires could have risen into the atmosphere, blocked sunlight and dramatically hampered plant growth. With vast portions of the landscape burned, large animals requiring a great deal of food may have died off, even if they had survived the initial catastrophe.

The younger Kennett acknowledged that work must be done to firm up the claim: "It's a hypothesis. . . . Basically, there's a suite of data that suggest that something like this occurred, but it still needs to be tested."

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DIAMOND CLUES TO BEASTS' DEMISE
By Molly Bentley, BBC Science reporter
January ,, 2009

The controversial idea that space impacts may have wiped out woolly mammoths and early human settlers in North America has received new impetus. Nano-diamonds and other exotic impact materials have been unearthed in thin sediments, Science magazine reports. The age of these materials coincides with the start of a millennium-long climate cooling event known as the Younger Dryas - some 13,000 years ago.

Many large animals vanish from the archaeological record at this time. It is also the period in Earth history that sees the demise of Clovis culture - the prehistoric civilisation that many regard as the first human occupation of North America. Taken together, it all makes for a compelling story, claims the team behind the latest research.

The group used transmission electron microscopy (TEM) to identify tiny impact diamonds found at a range of sites - four of them Clovis archaeological digs - across North America. Diamonds form through intense pressure and heat.

"We've discovered nano-diamonds that are not normally produced through average processes on the surface of the Earth," said James Kennett, a geologist at the University of California, Santa Barbara, and author on the Science paper. "They indicate there was an extra-terrestrial event on Earth 12,900 years ago," he told BBC News.

Scientists last year reported the discovery of five types of nano-diamonds along with impact material such as iridium and magnetic microspherules in the Younger Dryas impact layer, a thin blanket of sediment 12,900 years old.

The new analysis with TEM, they said, confirmed an abundance of diamonds in carbon spherules - melt material that forms in a fraction of a second - and the identification of lonsdalite, or hexagonal diamonds, associated with meteorite explosions.

The sheer number of diamonds - up to a million times that found in neighbouring sediment - and their presence inside spherules, refutes the speculation that the material is the normal rain of meteorite debris, says Allen West, a retired geophysicist in Arizona and a co-author. "There is no other way that hexagonal diamonds could have ended up in a carbon spherule in this number," said Dr West.

The absence of some traditional impact material and visible craters in North America led researchers to speculate that a meteoroid or comet disintegrated before exploding in a cluster of airbursts.

Researchers argue that the airbursts could have triggered a series of dramatic climate shifts - including colder temperatures and an abrupt change in vegetation - that would have made survival difficult for large mammals and Clovis hunters.

Sceptics of the impact theory are not won over by the latest data. While scientists agree that something dramatic occurred on Earth 12,900 years ago, the theory that it was an exploding space rock has been cast by some as long on dramatic flair, short on compelling evidence.

Nicholas Pinter, a geologist at Southern Illinois University, said he had yet to see classic evidence of an asteroid impact. The so-called discrete layers of material were not of a uniform age, he said. Microspherules, for example, rain down all the time and are present throughout the geological record. "My graduate student found some on his mailbox," said Dr Pinter. "The Younger Dryas impact looks like an increasingly desperate fishing expedition for supporting evidence".

While Dr Kennett proposed that ordinary carbon was forged into diamonds in the intense pressure of an airburst, Dr Pinter said nano-diamonds are now being identified at other locations and times without credible evidence of any impact.

The suggestion that they could have been produced by an airburst event is "untested and highly implausible," he argued. "Time will tell, but so far the Younger Dryas impact looks like an increasingly desperate fishing expedition for supporting evidence," said Dr Pinter.

Impact theorists maintain that the diamonds peak in abundance in the impact stratum. The thumb width layer appears in a number of sites across North America including Murray Springs in Arizona and the Channel Islands off the coast of Southern California. It lies beneath a black mat of biomass formed during the Younger Dryas.

The bottom-most film contains charcoal and soot, thought to be associated with impact fires, said University of Oregon geo-archaeologist Doug Kennett, son of James Kennett and another author on the Science paper, who has studied sedimentary vegetation and charcoal records.

No mega-fauna skeleton or Clovis artefact has been found above the impact layer or the black mat, he said. "The black mat covers them like a blanket," said Dr Kennett.

Before they disappeared, woolly mammoths and other massive beasts such as sabre-toothed cats, giant sloth, camels, and teratorns (predatory birds with a nearly four-metre wingspan) roamed North America.

Doug Kennett doubts the theories of over-hunting, climate change and disease used to account for their extinction. There are not enough Clovis kill sites to suggest that the animals were over-hunted, for example, he said. The animals' disappearance coincides with that of Clovis artefacts in the archaeological record 12,900 years ago. Prehistoric Clovis Indians lived broadly across North America for a few hundred years.

They were big game hunters, who introduced a sophisticated new Stone Age technology - the fluted spear point, known today as the Clovis Point. The Paleo-Indians vanish at the onset of the post-Ice Age Younger Dryas, or Big Freeze, that snapped Earth back to near glacial conditions, where it lingered for about 1,200 years.

The causes of the woolly mammoth extinction, the collapse of Clovis culture and the onset of the cold snap have long been debated. But only the impact theory accounts for the simultaneous occurrence of all three, said Doug Kennett.

Others are wary of the link. Jeff Severinghaus, a geochemist who studies ice cores at Scripps Institution of Oceanography, is sceptical that an impact could have led to the temperature plunge. He said records from Greenland suggested the cooling began earlier than 12,900 years ago. However, he is keeping an open mind. "I'm still in a wait-and-see mode," added Dr Severinghaus.

Cosmic impacts are known to have profound climate consequences. In this case, scientists propose that flash heat and pressure from an explosion destabilised the edges of the Laurentide ice sheet that covered North America, adding fresh water to the North Atlantic, and slowing the conveyor of warm water that heats Western Europe. Impact debris kicked into the atmosphere would have cooled the Earth and led to a number of ecological disruptions, including abrupt shifts in vegetation.

Critics say that with an impact comes a crater, such as at Chicxulub in the Yucatan, Mexico, which supports the theory that dinosaurs were wiped out by an asteroid impact 65 million years ago. But an airburst in which an impactor explodes in the Earth's atmosphere - such as that over Tunguska, Siberia, in 1908 - might not produce a crater, said Dr James Kennett. The energy released in the Tunguska blast was at least five megatons, said Dr West. The Younger Dryas impact would have been much larger. "Imagine 1,000 to 10,000 atomic bombs detonating within a few minutes over two continents," he said.

Had the Clovis people witnessed the event, said Dr West, they would have seen a brilliant flash followed by others in quick succession. The sky would be a canopy of fire, and shock waves would flatten trees. Miniscule diamonds would drizzle over tens of thousands of kilometres, a third of the way around the planet.

Nasa (Ames) space scientist David Morrison says the abundance of nano-diamonds is an "interesting mystery", but he does not think they were produced by cosmic airbursts. A comet or asteroid that fragmented in Earth's atmosphere might have time to disperse over a few hundred kilometres, but certainly not thousands of kilometres across a continent, he said. "I know of no mechanism that would break up a comet and distribute it over North America in the way they suggest," Dr Morrison told BBC News. "It violates what we understand about cosmic impacts," he said.

Scepticism is necessary when building a new scientific theory. But, Dr West said, there was particular resistance to that of a Younger Dryas impact because the event occurred in modern human history and was so abrupt. "People still like to think of geological processes happening slowly over time," he explained. "It's unsettling that something happening in a few minutes could flip our climate and cause widespread extinctions."

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NEO News (now in its fifteenth year of distribution) is an informal compilation of news and opinion dealing with Near Earth Objects (NEOs) and their impacts. These opinions are the responsibility of the individual authors and do not represent the positions of NASA, Ames Research Center, the International Astronomical Union, or any other organization. To subscribe (or unsubscribe) contact dmorrison@arc.nasa.gov. For additional information, please see the website http://impact.arc.nasa.gov. If anyone wishes to copy or redistribute original material from these notes, fully or in part, please include this disclaimer.