[meteorite-list] ARCTIC IRON, THEORIES, FACTS, AND MATHEMATICAL MODELS

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Thu, 27 Dec 2007 13:52:17 -0600
Message-ID: <09a101c848c1$fccf4830$b64fe146_at_ATARIENGINE>

Hi, Larry, List,

    That is exactly what the online impact calculator shows!
The impactor "arrives in a broken condition." But the fragments
do not spread enough to achieve a footprint bigger than the
crater that results from the impact.

    There is a subroutine to calculate the dispersal of fragments
impacting the ground, but it does not "kick in" in the examples
I ended up with, although I saw it in "unsuccessful" attempts
to copy-cat Canyon Diablo (the beginning of one trail of
parameters that lead to Campo del Cielo).

    If there are a few milliseconds where a number of cratering
events begin, I suspect that shortly they coalesce into a single
event. Although now that question has been raised, I wonder
if the not-quite circular shape of the crater should be attributed
to a "distributed" energetic event rather than one having a single
focus.

    The usual explanation -- at least the one I've heard -- is that
the orientation of the strata of the country rock "forced" this
non-circularity on the explosion. But even though the impact
of slightly separated portions of the impactor would produce
a single crater, it might betray traces of multiple "foci," like
a slightly squared off crater.


Sterling K. Webb
-----------------------------------------------------------------------------
----- Original Message -----
From: <lebofsky at lpl.arizona.edu>
To: "Sterling K. Webb" <sterling_k_webb at sbcglobal.net>
Cc: <meteorite-list at meteoritecentral.com>; "E.P. Grondine"
<epgrondine at yahoo.com>; "Michael Farmer" <meteoriteguy at yahoo.com>
Sent: Thursday, December 27, 2007 5:09 AM
Subject: Re: [meteorite-list] ARCTIC IRON, THEORIES, FACTS, AND MATHEMATICAL
MODELS


Hello Sterling:

We have a "regular" Asteroid Lunch here at U Arizona where we sometimes
even talk about asteroids and meteorites! Last week, I happened to ask Jay
Melosh (who I think originally wrote the LPL program) about the Canyon
Diablo impactor (I am reviewing a book for Meteorite magazine). The CD
impactor is thought to have broken up prior to impact, it was not one
solid chunk.

I do not know the details of the impact parameters, but this might affect
your results, but probably not all that much (energy is energy).

On Wed, December 26, 2007 8:01 pm, Sterling K. Webb wrote:
> Hi,
>
>
> With all this talk about Canyon Diablo, I thought
> it would be fun to try to model it with LPL Impact Calculator, While I
> thought it would be fun, it was actually exasperating. I started off with
> all the usual things about the impactor that one has read in the
> literature
> for decades. First, everybody says a 100 meter or bigger impactor. Well,
> the Calculator always returns a crater way too big, no matter what the
> angle or speed, for an impactor that big.
>
> So, you find that you need a much smaller impactor
> than that 100 meter job they're always talking about. Another truism is
> that it was a low-angle, low velocity object. Angle doesn't change things
> much, but speeds make a big difference. For all the impactors under 25
> km/s, the Calculator says "No Fireball, No Vaporization." And we know that
> there was a lot of vaporization... The Famous Spherules. So I tried a
> 75-meter impactor at
> 27 km/s and got roughtly the right size of crater. Smaller
> ones wouldn't vaporize and larger ones made bigger craters than the real
> crater.
>
> I was calculating for a distance of 50 km (thinking
> of EP's and Mike's 30 miles away), and I got the following at 50 km: A
> Richter force 5.8. The fireball is
> about 1000 meters in diameter, 5 times brighter than the Sun, puts out 2.2
> times as much heat as the Sun and lasts for 16 seconds.
>
> The blast overpressure at 50 km is 0.85 psi, the wind
> of the blast is 30 mph, and the sound as loud as heavy traffic. There is a
> light dusting of powdered ejecta less than a millimeter deep, but there
> will also be a buncha chunks with an average size of 3 inches. Ouch! It's
> about a 5 Megaton impact.
>
> I moved in to 20 km from the impact, and I was in big
> trouble! The fireball appeared 14 times bigger than the Sun, and the heat
> flux was 16.4 times that of the Sun -- first degree burns over most of the
> body. The blast overpressure is 3.5 psi; the wind is 120 mph; the ejecta
> blanket is under a half-inch thick, but the average tossed "chunk" is
> about a yard across! Still 5 Megaton...
>
> Out at 33 km (20 miles) is where things got survivable.
> No burns, only a 50 mph wind, no burst eardrums, and all you
> have to do is dodge 9 inch chunks of hot iron. Still 5 Megaton...
>
> I tried smaller but faster impactors (but with the same
> kinetic energy) and with this one small change, suddenly, there was no
> crater: the iron impactor airburst! Frankly,
> I always figured irons were more resistent. It fragmented
> at a height 94,000 feet. There's a small crater field of numerous small
> pits, but no crater at all. No fireball. No vaporization, 2 psi
> overpressure, a 75 mph gust of wind, windows shatter, and no one gets
> hurt. Still a 5 Megaton blast, but 20 miles up as well as 20 miles away.
>
> If Canyon Diablo had been this kind of an airburst,
> there would have been some big house-sized chucks sitting on the desert
> floor here and there, a dozen or two shallow pits, and millions of tons of
> small Diablo-ites scattered for many miles -- they'd sell for a penny a
> piece. It would be... Campo del Cielo! (which must have formed just that
> way).
>
> Fiddle as I might, I could never get a vaporization event
> out of an airburst; it takes a ground impact, and the greater the
> vaporization, the bigger the crater. From this I surmise that an
> iron-vapor event would leave a heck of a mark.
>
> Unless... it's on the surface of a 3000 meter ice cap, of
> course.
>
>
> Sterling K. Webb
> ------------------------------------------------------------------------
> ----- Original Message -----
> From: "E.P. Grondine" <epgrondine at yahoo.com>
> To: <meteorite-list at meteoritecentral.com>
> Sent: Wednesday, December 26, 2007 6:22 PM
> Subject: Re: [meteorite-list] ARCTIC IRON
>
>
>
> Hi Mike -
>
>
> One problem here is that we don't know where in Alaska
> or Siberia these fossils come from. (Firestone's team or the Calgary shop
> owners might know where the Alaskan tusks were found.) The straight
> between Alaska and Siberia is not that wide. I'm thinking that this thing
> has to range about maybe 300-500 miles, but who knows...
>
> I don't know what the spread pattern was for shperules
> from the Barringer imapact as Nininger measured it was, and I don't know
> if
> Nininger managed to find the
> whole field. Ballistic re-entry of irons over a fairly large range is
> still
> a possibility, if not a likelyhood.
>
> We had freezing sleet blowing horizontal here the
> other day in Illinois, and my guess is its going to be several months
> before Alaska becomes accessible...
>
> good hunting, E.P. Grondine
> Man and Impact in the Americas
>
>
> PS - I don't remember exactly the 1.8 psi range of
> death by blast from Barringer, but for sure the rain of molten iron and
> succeeding fires would have been fatal out to a considerable range. I
> suppose I should remember all of this dead on, but since my stroke that
> has
> not been possible.
>
>> Why would a crater the size of Canyon Diablo pepper
>> Mammoths in both Siberia and Alaska?
>> Meteor crater is big, but my god, you would not have
>> been killed if you were 30 or 40 miles away, and you think it would
>> shower iron with enough force to
> damage
>> things thousands of miles away? I am confused here. Michael Farmer
>>
>
>
>
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Received on Thu 27 Dec 2007 02:52:17 PM PST