[meteorite-list] Mammoth Stew - first you cut up the Mammoth

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Thu, 20 Dec 2007 16:39:56 -0600
Message-ID: <005b01c84359$3f5032f0$b64fe146_at_ATARIENGINE>

Hi, Jason, and the List,

    Don't want to do a full-court press here or have
a knock down drag out. The point of "atmosphere
not a factor in that size range" is merely that an
impactor that makes a one kilometer crater is not
going to be stopped from reaching the surface by
the Earth's atmosphere; that's all.

    As for the angle of an impact, that is a datum
obscured by the event. There is no way to derive
the angle of impact from the resulting crater in any
except the smallest and weakest events. Any angle
of incidence over 15 degrees produces a round,
level crater. At 15 degrees or less, elliptical craters
are produced.

    Meteor Crater (aka Barringer) is low angle:
http://adsabs.harvard.edu/abs/1995Metic..30Q.567R
Gene Shoemaker's classic 1960 analysis that
established it was an impact crater, still a hot
dispute "in the day." A history of the dispute,
and in particular the finding that angle of incidence
has no effect on crater formation can be found in:
http://www.enotes.com/earth-science/barringer-meteor-crater
    "Barringer and his 12-year-old son set out to
experiment with the formation of such craters by
firing bullets into clumps of rock and mud. Regardless
of the firing angle, the Barringers demonstrated
(and published their results in both popular and
scientific magazines) that the resulting craters
were substantially round. More definitive proof
was subsequently provided in 1924 by calculations
of astronomers who determined that forces of
impact at astronomical speeds likely resulted in
the explosive destruction of the impacting body.
Importantly, regardless of the angle of impact,
the result of such explosions would leave rounded
craters."

    Such experimental crater-forming data show
that impacts above a certain energy level (a rather
low level, too) preserve only the fact of the total
energy required to produce them, as any really big
explosion does. Such factors as angle of incidence,
direction of travel, velocity of impactor, and/or type
of impactor may inferred from circumstantial traces
in a recent, well-preserved crater, if such traces are
present. Usually this only happens in "small" craters,
like the Arizona one. The bigger the crater, the less
the chance of such indicators surviving.

Jason wrote:

> the likelihood of an impact occurring at an angle
> of ~30 degrees or less is simply unlikely. Possible,
> but unlikely - as in odds are that it would not occur
> this way. I'm not an idiot.

    Impactors large enough to make a respectable crater
do not have an entry affected by aerodynamic forces
nor altered by gravity after they encounter atmosphere
(and very little before). They do not deviate from their
cosmic vector in any substantial way. ANY angle of
incidence with the surface of the Earth is totally random
and ANY angle of incidence is as equally likely as ANY
other. Basic knowledge. Look it up. The "slow" 16 km/s
500-meter rock heads for the future 3.98 mile diameter
crater site as straight as a bullet from a gun, pretty much.

    If you average all the directions together, you get 45
degrees, just because you have to pick some direction!
That's what everybody does. I just got tired of typing in
"45," so I typed in "30" -- it makes no difference. 30,
60, all the same. This is an event with only ONE parameter:
Kinetic Energy.

    You've obviously read too much about the little rocks
that leave littler rocks behind for us to collect, with their
long curving paths and their aerodynamic braking, their
"falls" and all that meteorite frew-fah. Crater-makers are
not like that at all. For all practical purposes, think "straight
line" segment of a very large radius curve.

Jason wrote:

> An airburst of a ridiculously large (lets say a 1 km
> object, for the hell of it) size would devastate, what,
> maybe a thousand square miles? Few thousand?
> Not enough to do any real damage...

    Tunguska was an object of perhaps 100 meters; its
zone of greatest destruction, the tree-flattening-and-
charring zone, was 2200 square kilometers, or 850
square miles. The lethal-to-humans zone (LD 50) was
about ten times bigger than that. The flash was as bright
as the Sun 80 miles away. It was observed 600 miles
away. The barometric shock wave traveled around the
planet 2.5 times before dying out. (Stop beating the
horse...)

    And you think that an object one thousand times
bigger (10^3) would be "not enough to do any real
damage"? My only suggestion: think again.

    As for the general cause of all this argument, some
possible impact events in "recent" times proposed
by Firestone and an increasing number of collaborators,
that can only be determined by decades of field work
yet to be done and, hey! we can't do it. All we are doing
is sitting around like a bunch of ancient Greeks arguing
from "basic principles." This inductive approach (the
Greeks' greatest logical flaw) is essentially of very limited
utility.

    What we can do is follow the hunt and wait for the
results, science spectators that we are, and enjoy the
arguments in the meantime. Then, even if the evidence
favors impact, we'll have unending decades of argument
about whether it could have "exterminated" anything. I
expect the present Dino and Permian disputes to extend
well beyond all our lifetimes.

    What I do know is that, twenty miles north of my house,
there was once a wall of ice that towered into the sky above
low clouds and bird flight, and it ain't there any more. And
I haven't seen a mammoth in ages, even tho Thomas Jefferson
asked Lewis and Clark to keep an eye out for'em.

Jason wrote:

> Floods would leave geologic evidence, no?

    I live on the 200-foot-high edge of a canyon 20 to 30 miles
wide that was carved out in a geological wink of the eye (few
hundred years) by the melting of a mile-thick glacier, and I
think, as I watch an ice cube drip, that was awfully quick.
The mechanism proposed is a long slow melt as conditions
warmed, the formation of vast glacial lakes, the rupture
of ice dams and gravel-and-rubble dams, followed by very
and rapid drainage. The evidence is the Mississippi River
Canyon with its wave-formed canyon walls that undulate in
500-foot-long periods. Hey, there's still a little water left in
the bottom of the Canyon -- the Mississippi River. It's just
a trickle now...

    The Mississippi River is a spanking brand-new river;
there was never any river here before the end of the last
glaciation, nor any of the rivers between the Rockies and
the Appalachians; North America always drained sluggishly
north to Hudson Bay before the glaciations. As for impacts,
even huge ones, melting the glaciers? Forget it. You know
how much energy it takes to melt a cubic mile of ice? The
equivalent of 330 MegaTons of TNT, which is the thermal
portion of a GigaTon impact, to remove one cubic mile
from the thousands and thousands of cubic miles of ice
that made up the ice cap.

    What does a GigaTon impact take? A 300 meter rock,
more or less, leaving a 3-mile crater on the ice cap, less
than a quarter-mile deep. A pinprick to a glacier, a scratch,
a mosquito bite, a jumbled melt zone that quickly re-freezes
and doesn't leave a trace. But it might have considerable
atmospheric effect and deposit traces for thousands of
miles in every direction.

    No meaningful damage to the physical planet is not
the same as no meaningful damage to the creatures that
live on it. Your hypothetical one kilometer impactor is
a 50 GigaTon impact with an 8 to 10 mile crater about
a half-mile deep; it leaves a big lake for a while, but its
atmospheric effects spread around the entire globe and
might have serious effects on climate. But neither of these
will leave any trace "on the ground" other than the distant
deposits of the type talked about. A physically trivial
event but perhaps not biologically trivial.


Sterling K. Webb
-----------------------------------------------------------------------
----- Original Message -----
From: "Jason Utas" <meteoritekid at gmail.com>
To: "Meteorite-list" <meteorite-list at meteoritecentral.com>
Sent: Thursday, December 20, 2007 3:11 AM
Subject: Re: [meteorite-list] Mammoth Stew - first you cut up the Mammoth


Hola Sterling, E.P., All,

> Concerning recent impacts (<12,000 years old), what
> I've noticed over the years is that some people go
> into denial, and those denial mechanisms are sometimes
> really pretty bizarre. It's tough to accept on a gut
> level that as things now sit you, your family, your
> friends, everyone you ever knew or loved can be blown
> off the face of the Earth in an instant without a
> minutes warning.

I think you missed my point, because I accept this fully.

> But that's exactly how it is,

Yeah...

> and
> that's exactly what happened to some of our fellow
> human beings in the recent past.

Well, probably, though we have no real proof of their having been
blasted to death *anywhere.*

> So, Jason, you wrote:
>
> >Right, but seeing as the effects from the event of
> >which we speak differ greatly from those of your
> >comparison, it seems an unworthy one to make. Yes,
> an >unknown phenomena might create such a set of
> effects
> >as are geologically evident, but just saying "it's
> >possible" is something that I acknowledge as well; we
> >all know that Tunguska events occur and that,
> >evidently, astronomical events that create the
> >geological evidence that we've found occur. But that
> >still in no way ties the two together.
>
> Fact is, Jason, the Tunsguska impactor was viewed
> coming in, and spherules from it have been recovered.
> Sorry to disappoint you, but it wasn't a flying
> saucer.

No shit. It was most likely a piece of a burnt-out comet. My point
was that saying that there was some sort of cosmic event that left no
geologic trace of an impact and saying that Tunguska left no trace
(whatsoever) proves nothing, given that 1) the geologic evidence for
each event is completely different, and that 2) we have no proof that
what occurred was due in any way to an event similar to that which
occurred in Tunguska.

> Sterling, you wrote:
>
> >Atmosphere not a factor in that size range.
>
> Yes it is. Another factor in lunar crater distribution
> is later coverage by dust and removal by later
> impacts.

I'm not sure what you're trying to say here...that craters on Earth
are hidden by ejecta blankets from other craters that have hidden
them, or that ejecta doesn't move as easily (or moves more easily?)
through an atmosphere...what?

> Jason, you wrote:
>
> >I don't know where you draw the 1km crater line, as,
> >in my opinion, such a body might well break up if it
> >entered the atmosphere at a shallow angle, but who's
> >to say....
>
> Well, airbursts can be more devastating then ground
> hits, in terms of overall effect. We know compression
> propagation in impactors, and 1 kilometer crater seems
> to be a good guess as to airburst versus ground
> impact.

No.
Sorry, but that's only true to a limit. A 10km impactor isn't going
to explode in the atmosphere for any reason that I could conceivably
think of, and it's going to make one hell of a lot of destruction upon
impact...
A 1km object would most likely break up (as opposed to explode in a
violent airburst) if it managed to enter the atmosphere at a slim
angle without glancing off, as the energy required to vaporize such a
large amount of matter is simply too large as to be reasonable. I
don't think there's a great chance of it breaking up in general, but
if we're talking about cometary material, I do hear that it's
friable...

> Jason, you wrote:
>
> > A thirty-degree impact is highly unlikely,
>
> unsupported and most likely wrong.

It's less likely than an impact occurring at a greater degree measure,
unless I'm mistaken.
My point is that looking at all possible impacts, the likelihood of an
impact occurring at an angle of ~30 degrees or less is simply
unlikely. Possible, but unlikely - as in odds are that it would not
occur this way. I'm not an idiot.

> > and I'm thinking that an iron impactor would do a
> bit > more damage than a comet.
>
> wrong. See airburst versus ground impact, above.

Wrong, because such a comet would (in all likelihood) not airburst. See
above.
Also, since we're talking about airbursts, why not look into them vs
craters.

An airburst of a ridiculously large (lets say a 1km object, for the
hell of it) size would devastate, what, maybe a thousand square miles?
 Few thousand? Not enough to do any real damage on the scale that you
mention (death of 90% of all humans on the continent, etc). Climate
change due to dust released by such an airburst might cause trouble I
suppose, but it was an ice-age anyways; they knew how to deal with
cold.

An impact, on the other hand, should it hit either water or land,
would be infinitely more destructive. Instead of 1km^3 of material in
the atmosphere (since we're talking about a comet anyways, much of
that would be water, not particulates), you would get that 1km^3 as
well as whatever ejecta that was thrown out or vaporized (causing
global wildfires, etc - the sort of fires that would occur *only
locally* should we be talking about an airburst, because the fires
would only occur in the area underneath the airburst). The climate
change potential is exponentially higher, as is the amount of general
damage produced, not to mention how widespread it winds up being.

> >Do you, by any chance, know what the composition of
> >the dust layer (if it would suggest such a thing)
> >points towards the composition of the body having
> >been?
>
> You're confusing two different impacts here, the iron
> one at 31,000 BCE and the cometary one at 10,900 BCE.
> Why?

I've already taken care of the iron thing; you failed to address any
of my thoughts regarding the physical impossibilit*ies* of these
traces of iron embedded in the tusks having come from an airburst of
any sizable nature, and, since your only hope for a cometary event at
10,900 BC revolves around some sort of super-airburst that seems
(according to the laws of physics), impossible, unless, of course,
we're talking about an ice-impact (which I view as physically
possible, though unlikely, given that I would believe that other
geologic evidence would result from such a necessarily large impact
(though I suppose the Great Lakes might have been formed by the
subsequent flood following the impact), etc, etc.
You'd get a big volume of water from such an impact, and I would guess
that surrounding glaciers would be fractured by the impact for
hundreds of miles.
Floods would leave geologic evidence, no?

You have a dust layer, I'll grant you that, but drawing conclusions
from this without any other evidence seems like nothing more than
folly to me; find a piece of evidence and draw off of it, but simply
saying "this happened" because there's a layer of cosmic dust (the
likes of which we have never really seen before and thus cannot justly
identify) is pointless. By all means, explore, excavate, dig, do
whatever you'd like.
But try working out the physics of it. A cometary airburst of a body,
say, 1km in diameter, simply doesn't make any sense. Physically
speaking (I'm currently taking college-level physics), it just doesn't
make sense. Maybe you know of some laws regarding atmospheric
resistance that I don't, but unless some such laws exist, I'm
disinclined to believe just about everything you say.

> >I just don't see much metallic residue coming from a
> >comet, though I suppose there would be some.
>
> It seems to me that the cores of the cometissimals in
> a comet have a nice metal content. That's where the
> iridium is, after all.

No.
We don't know much about cometary composition, but there's no reason
(at all) to suspect that they formed around iron cores, as they are
undifferentiated bodies. Even chondrites, which formed much closer in
to the newly born sun (and thus have higher metal contents overall),
such as the H or L chondrite parent bodies, didn't have concentrated
iron cores (they'd be achondrites if this were the case). If they had
such cores, comets would look like Eucrites, and they wouldn't contain
any water. In general, it takes a body ~300km or more in diameter to
form an iron core.
Comets don't have metal cores, as far as we know. They're probably
CI1 material, through and through, though maybe with a bit more ice
thrown in.

> Sterling, you wrote:
>
> > 5. You say, "most of the craters were formed before
> > the [recent?] timeframe." Well, that's exactly what
> > the argument's about, isn't it? This is the
> > comfortable, "that's all in the past" argument.
>
> You've got it: denial. Did I ever tell you the one
> about Homo wushan?

They were mostly in the past. Impact rates have been declining. I'm
not saying that there's no chance that we could be wiped out tomorrow.
 What I'm saying is that the odds are better for us living out the
next day than they were, say, two billion years ago.
That's a fact.

> Jason, you wrote:
>
> >It's not all in the past, nor have I ever said such a
> >thing. That said, there were more impacts two
> billion > years ago than there are today, and you know
> that as > well as I do.
>
> Jason, if you're not in denial, then why try to make
> statements about the recent impacts, and then divert
> from the two impacts under discussion to the long term
> impact rate?

Because you're misunderstanding just about everything I say?

> Sterling, you wrote, most excellently:
>
> >Let's review the cratering history of the solar
> >system. After initial accretion, a tapering off.
> >Then, at 3.8 to 3.9 billion years ago, an intense
> >episode, the "Late Bombardment," followed by an
> >exponential decline for more than 3 billion years.
> >Then, at 0.6 billion years ago, cratering rates begin
> >to rise dramatically, until 0.4 billion years ago,
> >when they have increased fourfold in 0.2 billion
> >years. They again decline. until 125-100 million
> years >ago, when they increase, roughly doubling.
>
> A great summary, which leaves us with significant
> questions concerning meteorites and their parent
> bodies - and answers which are only now gradually
> being accepted.

So you say that impact rates are less today than they have been
(generally) in the past.
Hmmmm....sounds like......
Denial!

..Right.

> > The role of comets, stellar encounters, Oort Cloud
> > shenanigans, asteroidal family dustups is all
> unclear > and yet to be pinned down. Good old
> ignorance.
>
> Yes indeed. Lack of science budget helps in this too,
> as well as the behaviour of certain individuals
> (ahem!) who control those budgets.

Well, that and the fact that we simply haven't been observing for long
enough.
We have lots of information, but no Kepler to put it all
together...much of the research we're doing today is being wasted on
trying to find planets like Earth that exist unattainable hundreds or
thousands of light years away, instead of trying to figure out to save
Earth itself; we seem to be sidetracked...there's enough money to
manage it, but we really don't want to because, well, it's boring.

> Jason, you wrote:
>
> >What I'm trying to say is that given that there are x
> >number of craters on the moon, we would need to know
> >the age of each to determine a good number for the
> >rate of impacts over the past, say, 50,000 years,
> >which would be relevant to this discussion.
>
> Undoubtedly this information will be provided by the
> next series of manned landings on the Moon.

Well, hopefully, but the point is that I find it odd to believe impact
estimates in general, when I have little understanding of how
(predictably, or in general) they have been conceived.

> Jason, you wrote:
>
> >Well, then climate change could well have been the
> >culprit - but what cause the climate change is, I
> >think, still open to discussion.
>
> Actually, no it is not. That discussion has finished.
> When you have a layer of extra-terrestrial material
> and impactites such as has been demonstrated to exist,
> fact replaces speculation.

Well, assuming that the age of the layer and the climate change
correspond, and from my understanding, with regards to the supposed
10,900BC event, they do not.

> >And that those iron bits, if they are from a
> >meteorite, didn't come from the culprit that caused
> >the climate change,
>
> No one said they did, except you.

You strongly implied a connection between the two, above in this same
message, not to mention previous emails.

> Two different impacts, two different impactors. One
> that killed some mammoth, and another one that
> extincted them. Two impacts.

"killed some mammoth"
You're looking at an event which was, at best, something like
Sikhote-Alin, and might have sniped a herd or two of mammoth.
Anything bigger and you would get a crater as well as significant
heat damage to the bones. Physics. The fact that the metal was hot
when it hit the bones, and that it occurs in such tiny fragments - on
the top of the bones, implies that you're looking at a very low
airburst of a very friable body at a very high speed.
And yet it can't have been too high of an impact because the particles
are quite spread out, considering that they were moving still at a
great speed and would thus have deviated little from their original
path due to gravity. Oh - and it was a rather homogeneous, friable,
iron body, as there have been no large masses found embedded in
mammoth corpses, etc...seeing as we're talking about cosmic velocities
here, it seems logical to conclude that size distribution would matter
little in such an airbust that 1) must have been very low and 2) must
have been, not an airburst, but rather a breakup event such as
Sikhote-Alin, because none of the tusks show signs of being burned.

Regards,
Jason

> good hunting all,
> E.P. Grondine
> Man and Impact in the Americas
>
>
>
>
>
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Received on Thu 20 Dec 2007 05:39:56 PM PST