The airburst

Many indigenous tribes of eastern and central North America have legends about a catastrophic explosion in mid-air in the distant past.

"The Miami tell of a horned serpent that flew across the sky and dropped rocks onto the land before plummeting into the river," said University of Cincinnati anthropologist Kenneth Tankersley, himself a member of the Piqua Tribe of Alabama.  "The Shawnee refer to a 'sky panther' that had the power to tear down a forest.  The Ottawa talk of a day when the Sun fell from the sky."

Tankersley led a team of researchers who believe they know why these legends exist.  Some time between 252 and 383 C.E., they say, a comet hit the upper atmosphere over eastern North America and exploded.  The impact rained micrometeorites over most of the continent -- and, they say, would have scorched an area the size of New Jersey.  The results would have looked like the aftermath of the Tunguska Event of June 1908, in which a similar airburst flattened an estimated eighty million trees in an area of over two thousand square kilometers.

The tipoff was similar to what the father-and-son team of Luis and Walter Alvarez found in the clay layer marking the Cretaceous-Tertiary boundary, and which caused them to suspect an asteroid collision as the cause of the Cretaceous Extinction: a layer of material that is high in two rare heavy metals, platinum and iridium.  Iridium, especially, is far more common in asteroids and comets than it is on Earth, and its presence in a dust layer is suspicious, to say the least.

The impact, Tankersley et al. theorize, is part of what led to the decline and eventual collapse of the Hopewell Culture, an interconnected network of tribes that extended from what are now Manitoba and Ontario all the way down to northern Florida.  They were characterized by a particular style of pottery, jewelry, and arrowheads, but even more by the distinctive mound-building that we still see traces of in Ohio, Indiana, and elsewhere.

An 1848 map of the Chillicothe Earthwork in Ohio [Image is in the Public Domain courtesy of the Smithsonian Institution]

Something, however, triggered this confederation of cultures to go into rapid decline.  By 500 C.E. mound-building had ceased completely, and there's evidence that the trade routes that linked the different member groups were no longer being used.  The constituent tribes themselves appeared to decrease dramatically, and it took centuries for the population to rebound.

The cause, say Tankersley et al., is an airburst -- which would have destroyed completely any communities within a hundred or so miles, but would have caused extensive damage much farther away, likely triggering crop failures.  Given the fact that in most pre-technological cultures, one bad harvest was all that it took to trigger famine and starvation, something like this would have been catastrophic.

It's fascinating that like the Cascadia rupture and subsequent tsunami that made its way into the legends of the indigenous people of the Pacific Northwest, an entirely different cataclysm seems to have been recorded in the oral histories of the people of central and eastern North America.  The day the sky exploded -- with the result that an entire civilization collapsed.

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The imaginary fireball

The subject of today's post isn't anything new; it was just new to me, and, I suspect, will be to a good many of my readers, as well.  I found out about it from a long-time loyal reader of Skeptophilia, who sent me a link about it with a note saying, "Okay, this is interesting. What think you?"

The link was to a 2008 article that appeared in Phys.org entitled, "Cuneiform Clay Tablet Translated for the First Time."   The tablet in question is called the "Sumerian planisphere," and was discovered in the ruins of Nineveh by a British archaeologist named Henry Layard in the middle of the nineteenth century.  From where it was found, it was dated to around 700 B.C.E., and although it was recognized that part of what it contained was maps of constellations, no one was quite sure what it was about.

The Sumerian planisphere [Image is in the Public Domain]

The researchers were puzzled by the fact that the arrangements of the stars in the constellations were close to, but not exactly the same as, the configurations they would have had at the time it was made, but then they concluded that those would have been their positions 2,400 years earlier -- and they claimed the text and maps didn't just show the stars on any old night, but on a sequence of nights chronicling the approach of a comet or asteroid.

Which, ultimately, hit the Earth.

They claim the collision site was near Köfels, Austria, and triggered a five-kilometer-wide fireball.  Why no huge crater, then?  The answer, they say, is that the steep side of the mountain gave way because of the impact, and a landslide ensued.  Organic matter trapped in the debris flow gave an approximate date, but once deciphered, the Sumerian planisphere's detailed sky maps (including the position of the Sun, the timing of sunrise, and so on) supposedly pinpointed the exact day of the impact: the 29th of June, 3123 B.C.E.

Between the planisphere and the geometry of the collision site, the researchers claimed that the comet came in at a very shallow angle -- their estimate is about six degrees -- clipped the nearby peak of Gamskogel, and exploded, creating a five-kilometer-wide moving fireball that finally slammed into Kófels head-on.

You may be wondering why Sumerian astronomers had any particular interest about an impact that occurred almost four thousand kilometers away.  They have an answer for that, too; the shallow impact angle created a sheet of superheated debris that arced away from the impact site, and right toward what is now the Middle East.  A 2014 paper by Joachim Seifert and Frank Lemke concluded that the greatest amount of damage didn't occur right at the collision site, but where all that flaming debris eventually landed -- in Mesopotamia.

"The back plume from the explosion (the mushroom cloud) would be bent over the Mediterranean Sea re-entering the atmosphere over the Levant, Sinai, and Northern Egypt," said Mark Hempsell of the University of Bristol, who is the chief proponent of the Köfels collision hypothesis.  "The ground heating though very short would be enough to ignite any flammable material - including human hair and clothes.  It is probable more people died under the plume than in the Alps due to the impact blast."

The dust and ash from the event caused a hundred-year-long "impact winter" that triggered droughts, leading to a several-centuries-long famine that ultimately caused the collapse of the Akkadian Empire.

Okay, so that's the claim.  There are, unfortunately, a host of problems with it, beginning with those pointed out by the scathing rebuttal by Jeff Medkeff in Blue Collar Scientist.  The first issue is that there is "impact glass" -- vitrified shards of debris partially melted by a collision -- in central Europe, but it dates to much longer ago (certainly more than eight thousand years ago).  There is no impact debris to be found between central Europe and the Middle East anywhere near 3,100 B.C.E., no scorched pottery shards or charred bones that would be indicative of a rain of fire.  An asteroid or comet "clipping" a mountain -- and then generating a plume of debris that was still superheated four thousand kilometers downstream -- would have sheared off the entire mountain top, and there'd be clear evidence of it today.  Last -- and most damning -- the Köfels formation has been studied by geologists and found to be not a single event, but a series of landslides, none of which show convincing evidence of having been triggered by an impact.

The scientists involved don't even seem sure of their own chronology; the Phys.org article says 3123 B.C.E. (the 29th of June, to be exact), while the Seifert and Lemke paper says the impact occurred almost a thousand years later (in 2193 B.C.E.).  The latter date at least is closer to the claimed civilization-destroying effects; the Akkadian Empire fell in around 2154.  It seems likely, though, that the collapse of the Akkadians (and various others, including the Indus Valley Civilization, the Egyptian Old Kingdom, and the Chinese Liangzhu Culture) was due to a drought called the "4.2 Kiloyear Event."  The cause of that is uncertain, but probably wasn't an impact (again, because of the lack of clear stratigraphic evidence).  The most likely culprit was a shift in cold-water currents in the North Atlantic changing patterns of rainfall, but even that is speculative.

As far as Hempsell's even more outlandish claim -- that the Köfels impact generated the story of the destruction of Sodom and Gomorrah -- I won't even go into details except to say that there is evidence of a much smaller airburst explosion where the cities were allegedly located, but once again, it's from a different date (around 1650 B.C.E.).  As for any other evidence of the biblical "Cities on the Plain," it's slim to nonexistent.  Archaeologist Israel Finkelstein, of Tel Aviv University, called the tale of the destruction of Sodom and Gomorrah "an etiological story, that is, a legend that developed in order to explain a landmark.  In other words, people who lived in the later phase of the Iron Age, the later days of the kingdom of Judah, were familiar with the huge ruins of the Early Bronze cities and told a story of how such important places could be destroyed."

So given the (1) lack of any reasonably reliable evidence, (2) a chronology that even the researchers don't seem to be able to keep straight, and (3) plausible alternative explanations for the supposed societal aftereffects, I'm afraid I'm gonna be in the "don't think so" column on this one.  As dramatic as it would be if the astronomers of Sumer documented the approach and ultimate collision of a comet or asteroid, a collision that ultimately showered flaming debris over the entire Middle East, I think we have to set aside the drama of an imaginary fireball for the cold light of reason.

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The rain of glass

A couple of weeks ago I looked at the rather unsettling fact that the seeming benevolence of our home planet is something of an illusion.  As I write this, I'm sitting in a warm house with the calm, clear sunshine sparkling on frost-covered grass, hardly a cloud in the sky, and it's difficult to imagine it ever being any different.  While I don't believe a thoroughly pessimistic outlook helps anything or anyone, it does bear keeping in mind how fragile it all is -- if for no other reason, so that we value what we have.

I started thinking about how quickly and unpredictably a place can go from tranquility to devastation when I ran across a paper that appeared in the journal Geology two weeks ago.  In it, I learned about something I'd never heard about -- a seventy-five-kilometer-wide patch of the Atacama Desert in northern Chile that is covered with shards of black and green glass.

The Atacama Desert is a strange place in and of itself.  Other than the dry valleys of Antarctica, it is far and away the most arid place on Earth; the average rainfall is around fifteen millimeters per year, and there are parts of it that are down in the nearly-unmeasurable range of one to three millimeters.  The few plants and animals that live there have dry-climate adaptations that beggar belief; they get most of the water they need using condensation from fog.  The reason for the peculiar climate is a combination of a more-or-less permanent temperature inversion produced by the South Pacific Anticyclone and the cold, northward-flowing Humboldt Current, combined with a two-sided rain shadow caused by the parallel Andes Mountains and Chilean Coast Range.  It's so dry and barren that it was used by NASA as one of the places to test the Mars Lander's ability to detect the presence of microscopic life.

The aridity is what allowed for the discovery that was the subject of the November 2 paper.  Geologists Peter Schultz (Brown University), R. Scott Harris (Fernbank Science Center), Sebastián Perroud (Universidad Santo Tomás), and Nicolas Blanco and Andrew Tomlinson (Servicio Nacional de Geología y Minería de Chile) analyzed the peculiar shards that cover the patch on the northern end of the desert, and found out that they were all formed in one event -- the mid-air explosion of a comet about twelve thousand years ago.

The authors write:

Twisted and folded silicate glasses (up to 50 cm across) concentrated in certain areas across the Atacama Desert near Pica (northern Chile) indicate nearly simultaneous (seconds to minutes) intense airbursts close to Earth’s surface near the end of the Pleistocene.  The evidence includes mineral decompositions that require ultrahigh temperatures, dynamic modes of emplacement for the glasses, and entrained meteoritic dust.  Thousands of identical meteoritic grains trapped in these glasses show compositions and assemblages that resemble those found exclusively in comets and CI group primitive chondrites.  Combined with the broad distribution of the glasses, the Pica glasses provide the first clear evidence for a cometary body (or bodies) exploding at a low altitude.  This occurred soon after the arrival of proto-Archaic hunter-gatherers and around the time of rapid climate change in the Southern Hemisphere.

The dry climate is why we even know about this event.  Cometary collisions almost never leave a crater; given that comets are mostly made of various kinds of ice, the heat of friction from the atmosphere causes them to evaporate and finally explode, creating an airburst but no solid-object impact.  The airburst can be devastating enough, of course.  The 1908 Tunguska Event, the largest such occurrence in recorded history, flattened eighty thousand trees in over two thousand square kilometers of Siberian forest, and registered on seismographs all the way around the world in Washington, D.C.  If Tunguska had happened over a major city, there wouldn't have been a person left alive or a building left standing in the blast zone.

Like Tunguska, at the time and place of the Atacama airburst, there weren't many people in the danger zone.  There was, however, a lot of sand, and the heat from the collision melted it into glass -- indicating temperatures in excess of 1,700 C.  In a climate with ordinary amounts of rainfall, the glass would have been degraded and eroded, but here, it rained out of the sky and then has just kind of sat there for the intervening twelve thousand years.

"It was clear the glass had been thrown around and rolled," study lead author Peter Schultz said, in an interview with Science News.  "It was basically kneaded like bread dough."

The glass shards (the dark bits) in the northern Atacama Desert [photograph by Peter Schultz]

It would have been quite a spectacular thing to witness (from a safe distance), and you have to wonder how the survivors explained it.  "It would have seemed like the entire horizon was on fire," Schultz said. "If you weren’t religious before, you would be after."

So that's our disquieting scientific research for the day.  The reassuring news is that we've gotten pretty skilled at mapping the asteroids, meteors, and comets out there in the Solar System, and none of them seem to be headed our way, at least not for a good long while.  Which is a bit of a relief.  As often as I complain about how dull it is to live in a part of the world where the biggest excitement of the day is when the farmer across the road lets his cows out into the field, this isn't the kind of change of pace I'm really looking for.

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If Monday's post, about the apparent unpredictability of the eruption of the Earth's volcanoes, freaked you out, you should read Robin George Andrews's wonderful new book Super Volcanoes: What They Reveal About the Earth and the Worlds Beyond.

Andrews, a science journalist and trained volcanologist, went all over the world interviewing researchers on the cutting edge of the science of volcanoes -- including those that occur not only here on Earth, but on the Moon, Mars, Venus, and elsewhere.  The book is fascinating enough just from the human aspect of the personalities involved in doing primary research, but looks at a topic it's hard to imagine anyone not being curious about; the restless nature of geology that has generated such catastrophic events as the Yellowstone Supereruptions.

Andrews does a great job not only demystifying what's going on inside volcanoes and faults, but informing us how little we know (especially in the sections on the Moon and Mars, which have extinct volcanoes scientists have yet to completely explain).  Along the way we get the message, "Will all you people just calm down a little?", particularly aimed at the purveyors of hype who have for years made wild claims about the likelihood of an eruption at Yellowstone occurring soon (turns out it's very low) and the chances of a supereruption somewhere causing massive climate change and wiping out humanity (not coincidentally, also very low).

Volcanoes, Andrews says, are awesome, powerful, and fascinating, but if you have a modicum of good sense, nothing to fret about.  And his book is a brilliant look at the natural process that created a great deal of the geology of the Earth and our neighbor planets -- plate tectonics.  If you are interested in geology or just like a wonderful and engrossing book, you should put Super Volcanoes on your to-read list.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]

Comet tales

So most of you have probably been following the amazing landing of Philae, from the Rosetta comet study mission, on the comet 67-P Churyumov-Gerasimenko.  It was a triumph of technology -- hitting a four-kilometer-wide target traveling at over 100,000 km/hr from 500,000,000 kilometers away.  Unfortunately, the landing site proved problematic; Philae is apparently in shadow, and its solar cells have been unable to charge the batteries, resulting in a loss of transmission.  There's still hope that as the comet approaches its perihelion, the lander may be exposed to enough light to start functioning again; but at the moment, it's out of commission.

All of which has the conspiracy theorists in a lather.

I should have been expecting this.  They've always had a bee in their bonnet about NASA, whom they suspect of being in collusion with aliens and the Illuminati and heaven knows who (or what) else.  Never mind that Rosetta was a mission from the European Space Agency, not NASA; facts have never mattered much to these folks.

But now we have allegations that the ESA (working with evil, evil NASA) is hiding the mission's true purpose.  In an email, allegedly from an ESA whistleblower, we read:

Do not think for one moment that a space agency would suddenly decide to spend billions of dollars to build and send a spacecraft on a 12-year journey to simply take some close-up images of a randomly picked out comet floating in space.  Comet 67P is not a comet…  Some 20 years ago NASA began detecting radio bursts from an unknown origin out in space…  It would later be known that these had likely come from the direction of the now named comet 67P.

Mmm-hmm.  I've looked at the photographs that came in before the lander died, and they look pretty much like random dirty ice to me.  Which, coincidentally enough, is what comets are made of.

[image courtesy of the European Space Agency]

The whole thing reached another level of silliness when it was announced that the comet was "singing."  The scientists, who would probably be happier not to have their research characterized this way in the media, found that the comet's magnetic field was oscillating at about 40 millihertz, and  after speeding it up by a factor of 10,000, it can be turned into a sound audible to human ears.  The oscillation is still unexplained, but is thought to be an effect caused by ionized particles interacting with the solar wind.

Hoo boy.  An unexplained "song," plus a mission to a comet, plus a good imagination, and you have the makings of a great conspiracy theory.  Scott Waring, who has made Skeptophilia before for his claims that there are alien bases on the Moon and that a digital photographic glitch from a NASA photograph of the Sun proved that a huge cubical alien spaceship was harvesting the Sun's energy, has weighed in thusly:

In my opinion, this is not a code. It is how a species of aliens communicate to one another without speaking — [something like a] form of telepathy put into primitive radio signals … It's the only way this species can communicate to us.  This is their thoughts [because] they don’t talk.  Is it a message of greetings, or is it a warning of what’s to come?  We, the people of the world, need to find out.

Well, if it's a message, the aliens need to work on their language skills.  You can listen to the "song" here.  Remember: this is sped up by a factor of 10,000, so whatever this bit of the "alien communication" is, it would take ten thousand times longer to listen to if you played it at its actual speed.

And, of course, the failure of the lander's batteries has added a whole extra layer of suspicion.  No way would scientists design a multi-million-dollar probe that could so easily lose contact.  It's still transmitting, say the conspiracy theorists; but what it's sending back is so shocking that the scientists don't want us to know about it.  You know, aliens and spaceships and whatnot.

The usual stuff.

What's funny about all of this is that if there really is this great big conspiracy, covering up First Contact with an alien race, the head honchos at NASA and the ESA are being pretty sloppy about it.  First they make the mission public, with thousands of press releases and so on; they post photographs of the comet all over the place.  Then they make the unfortunate announcement of the lander's radio silence.

Why go through all of these gyrations, when they could just have launched the thing in secret in the first place, and not told us anything about it?  It's not like some amateur astronomer is going to look through his backyard telescope and see Philae sitting on the surface of the comet, or anything.

So if these people are in a conspiracy, they should resign and let someone take over who actually knows how to run one.  Because they're kind of an embarrassment.  Will Smith and Tommy Lee Jones did a much better job in Men in Black, with their little memory-wipe devices.  Also, those were some cool alien languages.  I'd learn to speak those, if I could make noises like that.