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Saturday, April 11, 2015

Geological Cataclysm - The Chixulub meteor impact

When I was growing up, everyone knew about dinosaurs and their sudden extinction.  No archaeologist or geologist could come up with a valid explanation of what caused that extinction.  All anyone knew was that the Mesozoic Era abruptly ended and the Cenozoic Era ("The Age of Mammals") began.  Dinosaurs were the dominant terrestrial vertebrate for 135 million years - and then one day, pffft... they were gone.  But why???

It took an experimental physicist to answer the question that had baffled mankind for over a century. That physicist was Luis Alvarez.  Luis Alvarez, along with his son Walter (a geologist), came up with a whacky theory:  A large meteor had hit the earth, and caused immediate massive climate change, exterminating many of the species on the planet.

The scientific community disliked the idea of course - probably because it smacked of Deus ex Machina, that is, the intervention of an outside force.  At the time, scientists didn't seem to view the earth as part of a larger system that encompassed huge earth-crossing asteroids.  This is odd, because:
  •  People find small meteors all the time
  • There is a damn big meteor crater near Winslow, Arizona, so clearly we are not immune to large impacts. 
  • Then there is this little gem that happened about a decade before the Alvarez team announced their theory...  

The notion that a massive meteor might have smacked the earth in prehistoric times and killed damn near everything should not have been a big leap of faith, and yet it was.

Luis Alvarez pointed out that meteorites, being extra-terrestrial, have a slightly different composition of than normal earth dirt.  If, as he theorized, a large portion of the meteor had vaporized upon impact, there should be a fine layer of foreign material at the depth where archaeologists no longer found dinosaur bones.

The theory went like this:  At some point long in the past, the entire earth was a fiery ball of molten rock and metal.  The heavier metals of course migrated to the center (Iron, nickel, etc), while the lighter slag on top solidified (silica, carbon, metal oxides, calcium) to form the earth's crust.
If there had been a meteor impact, he suggested, we might find a fine layer of heavier metal at the layer of dirt marking the end of the Mesozoic Era.

Specifically we were looking for Iridium, which is rare in earth's crust (having sunk to the core long ago while the planet was still molten), but which is more abundant in meteors, because meteors never experienced segregation of Iridium.

Sure enough, all around the globe, there is a fine layer of Iridium right where the boundary exists.

This layer is found  Everywhere.  In.  The.  World.  That's a global cataclysm.

The Iridium is embedded in a layer of clay, which is believed to be ejecta caused by the impact.  The asteroid itself was believed to be 6.2 miles in diameter, about the size of Manhattan.  The energy released by the impact was equivalent to about 100 Trillion tons of TNT.  The largest man-made detonation ever was the Tsar Bomba, a 50 Million ton device.  The Chixulub explosion was 2,000,000 times bigger than that...

The explosion is believed to have lofted enough particulate into the upper atmosphere to block out quite a bit of the sunlight reaching the earth.  This would cause plants and plankton to die, which in turn would cause a die off in land-going herbivores and plankton-feeding fish.  This in turn would lead to the loss of carnivorous animals and fish.  A planetary die-off.

When the asteroid theory was first proposed, there was no known crater that fit the criteria for this event.  This is not too surprising, because unlike the moon, we have weather that is constantly eroding the surface of the earth, and erasing features like that.  However... 

Petroleum companies in 1978 had been gravity-mapping the earth in search of oil deposits, and inadvertently gathered the data that geologists needed to find a crater.  Here is the gravity anomaly data that shows where the crater hit the earth:

Here's the location on a larger map:

Below is an artist's rendering of what this might have looked like from orbit.  Impressive... and deadly.

How about that?  Might just have been nasty enough to kill off the dinosaurs.

Now you know why we are scanning the heavens for earth-crossing asteroids.  New ones are being found all the time.  A link to the discovery of brand new metor hazards is here:

A more user-friendly version is here.  Scroll down to the table of Near Earth Asteroids.  It might give you a sense of how often we have close calls that don't make the news...

In the years since Luis Alvarez first proposed the meteor-impact theory we've been treated to some interesting impacts that don't make it seem like such a far-out idea:

The Chelyabinsk meteor in 2013:

And comet Shoemaker-Levy 9 breaking into pieces and then walloping the planet Jupiter in 1994 in a series of impacts and leaving massive marks that could be seen even in small telescopes.

I think we can say this mystery has been solved!

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