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Prehistoric meteor shower?
nprev
post Dec 18 2007, 03:23 PM
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Problem is that the metallic residue does match meteoritic compositions (including Ir enrichment), from what I read. Plus, it seems as if the event may have affected a fairly large geographical area, the possibility of animals being not too injured afterward (also mentioned in the article) to still wander far & wide notwithstanding.

Hmm. Maybe the shrapnel particles don't have to be traveling too fast...maybe they just need to be very, very hot to penetrate the skin & bones (yuck, I know--wouldn't feel good at all for the poor critters). Whatever would blow a metallic meteorite into smithereens like this, you can bet that the smithereens are gonna be pretty warm for awhile. I'm betting on a large iron/nickel body with an unusually high impact velocity, say several tens of km/sec, that maybe took a shallow chord through the atmosphere that allowed it to heat up enough to blow up...


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djellison
post Dec 18 2007, 03:30 PM
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But surely a meteorite fragment that small would cool in the 'flight' phase?

Doug
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tasp
post Dec 18 2007, 03:33 PM
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OK, read some more on this, and will gladly back off the lightning idea.

However, do these 'tiny bits' have sufficient range in the earth's atmosphere to pepper mammoths, but whatever accelerated them is insufficiently energetic to kill the mammoth??

I am having trouble seeing the mammoth in the 'sweet spot' of the calamity for it to wind up this way. Terminal velocity of these tiny bits in air isn't all that high (watch Mythbusters) and whatever is accelerating them is 'worse' than they are, isn't it ?

{everyone loves a mystery, don't they!}



Just out of curiosity, have any soft minerals on Mars examined by the microscopic imager have any features that might be interpreted as produced in a similar event ?? For instance, if a sulfate rock (if that would be comparable in hardness to tusk) has little pits on the upper surface in the size range noted in the article, whatever is doing this in Alaska might be generally operative on Mars??
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nprev
post Dec 18 2007, 03:40 PM
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QUOTE (djellison @ Dec 18 2007, 07:30 AM) *
But surely a meteorite fragment that small would cool in the 'flight' phase?

Doug


Not sure; lots of variables. Metal doesn't cool very rapidly in air anyhow, and if the frags aren't really traveling that fast then they'll cool even less. Grenade fragments are hot as hell for several minutes after detonation, for example...

Tasp, it also appears that these things (whatever they are) didn't necessarily kill all the animals that they hit. Some of the bones showed signs of long-term healing.


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helvick
post Dec 18 2007, 09:07 PM
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QUOTE (nprev @ Dec 18 2007, 03:40 PM) *
... Metal doesn't cool very rapidly in air anyhow, and if the frags aren't really traveling that fast then they'll cool even less. Grenade fragments are hot as hell for several minutes after detonation, for example...

Yes but grenade fragments travel a few tens of meters at most - if they were to travel through hundreds or thousands of meters of air they would cool rapidly.

I'd like to see someone work out a viable model that creates thousands\millions of small high velocity (or high temperature) fragments at ground level from a meteor entering the atmosphere _without_ noticable blast effects. I'm going to see what I can get from just running velocity\drag\density and range numbers.
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nprev
post Dec 19 2007, 12:42 AM
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Yeah, I was thinking about what sort of model would work as well. The "shrapnel" hypothesis is thin as paper; if it can work at all, looks like it needs all sorts of very special circumstances. Still, can't think of anything better.


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lyford
post Dec 19 2007, 12:58 AM
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QUOTE (nprev @ Dec 18 2007, 04:42 PM) *
Still, can't think of anything better.

Cro Magnon Blunderbuss? unsure.gif


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nprev
post Dec 19 2007, 02:05 AM
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At this point, I'd almost rate that just as likely as magic exploding metal meteoroids... tongue.gif ...this is a weird one for sure.


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AndyG
post Dec 19 2007, 11:25 AM
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QUOTE (lyford @ Dec 19 2007, 12:58 AM) *
Cro Magnon Blunderbuss? unsure.gif


A flintlock, for certain.

Andy
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ngunn
post Dec 19 2007, 11:59 AM
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Can't help noticing the resemblance between this shower of missiles and Don Burt's impact-generated condensation spherules. Any comment on this story dburt?
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algorimancer
post Dec 19 2007, 02:45 PM
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I wonder whether this may have more to do with the mammoth using their tusks to dig in the ground (as I seem to recall that modern elephants do) and in the process embedding the metal fragments from a meteorite which just happened to be buried in the soil. In this case the "burn marks" may simply be oxidation rings and/or due to inflammation around the embedded fragments. This may also correlate with the dorsal distribution of the fragments - I would envision the mammoth pushing the tusks into the ground, then lifting. The same explanation works for the bison horns. I'm having a really tough time accepting them as due to the impact event, as any event which yields small particles moving fast enough to do this would likely also be throwing out a lot of big rocks and heat, and the bigger rocks would travel further than the small particles.
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ngunn
post Dec 19 2007, 03:34 PM
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QUOTE (algorimancer @ Dec 19 2007, 02:45 PM) *
the bigger rocks would travel further than the small particles.


Maybe not if they are fallout from a mushroom cloud.
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algorimancer
post Dec 19 2007, 04:17 PM
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QUOTE (ngunn @ Dec 19 2007, 09:34 AM) *
Maybe not if they are fallout from a mushroom cloud.

And my assumption was that all the particle sizes would have begun with the same velocity, and thus the smaller particles would be decelerated by the air faster than the larger ones. The opposite is also true, that the smaller particles would be accelerated more than the larger particles by an expanding volume of gas.

I could believe the impact hypothesis more easily in the context of your notion of fallout from a mushroom cloud (or secondary impact) if I could be persuaded that the terminal velocity of particles of this size would be sufficient to embed them within the tusks. This falls under the old "if you drop a penny from the top of the Empire State Building" question, or alternatively the "if you shoot a bullet straight up into the air and it falls back" question. Seems like Mythbusters addressed this and found that falling bullets or pennies would hurt (like paintballs) but not cause serious injury.
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hendric
post Dec 19 2007, 04:19 PM
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QUOTE (helvick @ Dec 18 2007, 03:07 PM) *
Yes but grenade fragments travel a few tens of meters at most - if they were to travel through hundreds or thousands of meters of air they would cool rapidly.


Doesn't that depend on their speed though? Above a certain speed, they are going to get hotter, and below that speed, they are going to cool off. I could easily see that switchover at 1-2km/sec. Granted, it might be quickly decelerating, but a metal fragment is pretty darn dense. Bullets can travel for miles, so I can see a small, mostly metallic fragment, starting off hot at 3-4 km/sec, travelling for quite some distance, potentially tens of miles, and still be hot enough to burn bone upon impact.

Dr. Burt, where are you on this? These fragments are proof, so to speak, of an impact surge! How does their size compare to the blueberries and other impact surge fragments?

With that said, it's interesting there are actually different sizes of fragments in the same tusk. I'm sure a reasonable model could be made of how far away the impact would have to be to allow fragments of the minimum size to penetrate the tusks. At the least, the model should give a minimum distance.


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helvick
post Dec 19 2007, 04:44 PM
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QUOTE (ngunn @ Dec 19 2007, 03:34 PM) *
Maybe not if they are fallout from a mushroom cloud.

Small particles falling out of a debris\mushroom cloud will never exceed the terminal velocity for such a particle which will be too slow to penetrate flesh and bone. Some examples:
Nickel Iron fragment - Density 8000kg/m^3, 1cm diameter, mass 4g, volume 0.5cc, Drag Coeff 0.7, Terminal Velocity 35m/sec (80mph)
Drop that to a 1mm diameter and the terminal velocity drops to 11m/sec (25mph)
Drop that to 0.1mm diameter and the terminal velocity drops to 3.5m/sec (8mph)

I need a bit more time to figure out range vs initial velocity but rest assured that it happens very fast.

If you take the 1mm grain and assume an initial velocity of 5km/sec the initial deceleration due to drag is just below 2000 km/sec^2. That's not going very far - frankly I'd be surprised if it went more than 50m before hitting it's terminal velocity.

There is also the fact that the loss in kinetic energy needs to be bled off as heat and at higher velocities the particle will just atomize.
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