[meteorite-list] Entry Dynamics in Peru

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Thu, 4 Oct 2007 23:43:53 -0500
Message-ID: <062701c8070a$55f58400$b92ee146_at_ATARIENGINE>

Hola, Doug,

    I think the variety of reports I've already posted
and then referred to several times, are all that I can
say to your dislike for my reconstruction of the
event. I confess to being somewhat mystified by
your comments.

> The size of the crater, which is rare or even unique...

    Quite to the contrary, it is a textbook normal conical
simple crater with a width/depth ratio of 3:1 (13.4 meters
wide and 4+ meters deep), just like "ideal" theoretical
crater.

> A much better comparison, btw, is Jilin.

    The Carancas crater bears no resemblance to Jilin,
none whatsoever. Jilin is not a crater. Jilin is not even
an impact pit. Jilin is a hole 6 meters deep and less than
2 meters wide. Jilin is a good example of your previous
metaphor of a marble dropped in a snowbank. It was
so slow-moving that it just poked a hole in the dirt.

> what model you have accounts for potato sized
> meteorites (and powder) scattered in and around
> meters from the impact

    The "incredible amount of meteorite powder" Mike
mentioned is not a derivation from a model; it's a witness
statement by someone who was there, an expert witness
at that.

    The mechanism is back-spalling. The shock wave of
impact, originating at the point of impact, extends both
forward into the target material and backward through
the impactor. If the speed of impact exceeds the speed of
sound in the meteoritic material, the expanding shock wave
shreds the meteorite and pushes the distrupting material
back, away from the impact.

    [I insert here the fact that the few tests that have been
performed on meteorites show that the speed of sound is
less in meteorites than in comparable terrestrial rocks. The
more porous the meteorite, the slower the speed of sound
in it. Carancas was a dead duck, I'm afraid.]

    In a truly violent impact, only the central rear portion of
the impactor survives as fragments. In less violent impacts,
the rear quarter, third or more of the impactor is fragmented
and ejected backwards (along with the powdered material
closer to the point of impact). It is found radially distributed
around the crater (or asymetrically if an oblique impact).

    I mentioned Canyon Diablo because Nininger first
elucidated the mechanism, I believe, although I cannot cite
chapter and verse. Googling, I discover that Jay Melosh
claims to have discovered it. Shame, shame. How quickly
they pick, not your bones, but your ideas... once you're dead.
http://www.gsajournals.org/perlserv/?request=get-document&doi=10.1130%2F1052-5173(2002)012%3C0029%3AGKGA%3E2.0.CO%3B2&ct=1

    Melosh's "Impact Cratering: A Geological Process" is
the standard work on impact mechanics. Amazon Canada has
used copy for only $665.77. I guess it's priceless knowledge.
Well, no; it has a price. And not in crummy US dollars either,
but those rare and valuable Canadian dollars!

> the ablative path for most meteorites stops much, much
> higher than 3800 meters!

    I cited the witness evidence that indicates the ablative path
continued to, or very near to, the crater, so this is another ditto.
And if it was ablating to the ground, it clearly wasn't in free
fall. I quote Jose Machero of INGEMMET (which I've done
before):

        "There was a strong explosion that was felt up to
Desaguadero city 20 km from the impact site. Some window
glasses of the Local Health Center (at 1 km from the site)
were broken."

    An impact that was felt 20 kilometers away does not sound
like "free fall" to me.

    I really like the graph.

    May a Lunar fall gently in your back garden.


Sterling K. Webb
--------------------------------------------------------------------------
----- Original Message -----
From: "mexicodoug" <mexicodoug at aol.com>
To: "Sterling K. Webb" <sterling_k_webb at sbcglobal.net>; "Meteorite List"
<meteorite-list at meteoritecentral.com>
Cc: <mqfowler at mac.com>
Sent: Thursday, October 04, 2007 9:39 PM
Subject: Re: [meteorite-list] Entry Dynamics in Peru


Not so fast Sterling :-) The size of the crater, which is rare or even
unique... doesn't make mucked-up analyses a requirement!

Short and simple as I just read your reply to me in which you somehow missed
the central point I asked about when you insisted that the crater contains
nothing but powder...let's take a little more of a scientific approach.

My prior post began, "Sterling what model you have accounts for potato sized
meteorites (and powder) scattered in and around meters from the impact, yet
strictly powder inside, especially for a meteorite that sheds like this one
particularly along its natural 'fault' lines."

Please answer that question clearly for my benefit rather than skipping and
speaking of Canyon Diablo and Barringer. A much better comparison, btw, is
Jilin.

As to the ancillary stuff...
Congratulations on ace Mountaineer Mike Fowler who mentioned that 50% of the
atmosphere is under 3.5 miles elevation - it jives within 100 meters to the
calculation I worked on and gives me the confidence I need for checking this
calculation. When you state that "only" 58% of the atmosphere's mass was in
the path of the Peruvian meteorite, just to keep a sensible argument going,
I would suggest you don't introduce bias via adjectives like "only" into the
interpretation. There is an incorrect implication that in this last 2 miles
of atmosphere, cosmic velocity is typically damped. ---not true.

According to my numbers, your 58% estimate was ok for the back of an
envelope, though a little exaggerated. I calculated it to be 62.1% using a
more accurate model (which agrees to M. Fowler's 3.5 mile figure within 100
m) for the atmosphere than your barometric formula. Rather than dump a
bunch of numbers on the list, let me just share this graph, which I just
generated that is useful from sea level to 25 kilometers altitude, so you
can graphically see how much atmospheric mass is traversed for any bolide
around at the Peruvian crater's around October. Don't forget that the
ablative path for most meteorites stops much, much higher than 3800 meters!

www.diogenite.com/Huanocollo.gif

This graphically gives a great idea of how much % of the atmosphere any
meteorite anywhere on Earth passes through to get to any altitude above sea
level, and if you look at it you can see how much of a fraction of the
atmosphere mass is traversed in any segment of the travel from 25Km on down.
Just compare the blue area to the white and you get the idea of of the
FRACTION of the atmosphere traversed. No arithmetic needed - the ratio of
blue to blue+white is the % of the atmosphere for any geographical elevation
and includes luminous paths too..

Sorry, but I can't accept your dismissing unscientifically the arrival of
any meteoritical material generally to the ground as difficult to on one
hand and then on the other calculate all these asides to things even you
don't want to know to such precision! 62% is 62%, not "only" anything. 62%
of the atmosphere is only where it starts in this case in Peru, but this is
another subject. I.e., if it comes in at around a 45 degree angle instead
of vertical, it passes through the full 100% since it doesn't take the
straight path, and you are back to square one. These meteorite was observed
to enter at an angle. Yes, I understand that "on average" meteorites
reaching sea level will go through more atmosphere, but this is a non-issue
when they are conveniently sized and in free fall for that 3800 meters. The
one effect I will agree that will cause a higher velocity, which has nothing
to do with retaining cosmic velocity, is that FREE FALL VELOCITY is greater
in thinner air. There is plenty to be said about that as you would imagine
such as a potential doubling of the energy of impact making a bigger crater
for something the size of Jilin. I don't think it is likely a huge ball is
at the bottom of the crater. Just that there are plenty of kilos that
weren't pulverized in the mucky crater.

Best health,
Doug
The numbers behind the graph, I could post if you want, along with the
modeled temperature in F and C of the atmosphere over its lat/lon. I used
the trapazoidal rule to estimate the percent of the atmospheric mass with
the midpoints of intervals of 200 meters altitude for 0 - 25 kim above sea
level.), and considered that the atmosphere ended at 100Km above sea level.
Received on Fri 05 Oct 2007 12:43:53 AM PDT