[meteorite-list] Norway Meteorite Impact Site Believed to be Found

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Tue Jun 13 12:24:31 2006
Message-ID: <006201c68ec3$d7f93c00$e242e146_at_ATARIENGINE>

Hi, Chris, List

    I don't want it to seem like I'm jumping on anyone
in particular, and I'm NOT jumping on any one in particular,
but folks are just not paying attention here.

    You don't get a seismic event from an airburst unless it
really big or very close to the ground (and to get close to
the ground, you have to be big, so it's the same thing).

    Even assuming a very hard dense stone (achondrite)
50 meters in diameter that produces a 1,000,000 tons
of TNT airburst, you get ZERO seismic signal. This,
when even a lousy 1 meter iron will reach the ground
and achieve a Richter 0.2 reading. Duh. Just not paying
attention, you guys.

    I've fiddled with sizes, materials, entry speeds, impact
angles, for hours. You can't get a small seismic signal
from a stone. You get the threshhold of seismiticity when
the stone gets big enough -- 70 meters (varies with speed
and angle of entry, naturally) -- to punch down below
the stratosphere and produce a 30 megaTon airburst,
not before. The transition across that threshhold is very
steep, from the first tremble to broken windows. The
airburst too small to produce a seismic signal WILL
smash forests and snap off small trees, which did not
happen in Norway. Such an airburst would leave a
huge strewn field with thousands of fragments. No
indication of that in Norway. It would also produce
a much louder sound signal than indicted by the reports.

    The ONLY way you get a smallish seismic signal is
with a small object that actually reaches the ground,
and only a small iron will do that -- NO small stone can.
Yes, an airburst will produce a LOCAL seismic signal,
but a seismometer 50 miles away will record nothing.
Remember the case of the airburst in England that
triggered a seismic signal when seismometers in London
only thirty miles away registered nothing? Can't remember
the date, but I'll search for it.) The seismometers that
measured this signal were hundreds of miles away.

    Yes, I'm using a mathematical model, but Melosh has
the rep as a top man, if not the top man. Do you suppose
anyone went and checked its assumptions and methods?
http://www.lpl.arizona.edu/~marcus/CollinsEtAl2005.pdf
Did you? You work this all out numerically with real-world
parameters, using his or your own model, and come up
those precise figures? "High" altitude, and "a lot" of energy?
How many joules is "a lot"? How high is "high" altitude?
Am I unreasonable in prefering his well-thought-out model
to an off hand opinion?

    I'm sure all the people who just blow this off are fine and
wonderful people, the Swede who doesn't like the Norwegian
and says so (nothing surprising there), our model of temperamce,
Mike Farmer, and you, and Marco, all fine people, I'm sure, but
does it sound like anyone is actually bothering to work this out,
or are you just assuming your instantaneous impression is sufficient
and superior to actual investigation, however crudely done using an
on-line modeling program? Disagree? Do a better job of
quantifying it.

    I mean no one has to work this out to see if an event was
actually possible. We all have better things to do. Me too.
I agree the "crater" is not definitive of impact, could be a
rock fall; newspapers and webnews is often wrong and
careless. It appears to be a poor crater candidate. Not
the point. There was an impact (not an airburst)
somewhere in that area.

    You and Marco are meteor/fireball experts, I know, but
the event that brings an object to earth is different, no matter
how similar in appearance. No recovered object has ever
been from a meteor stream. The very few fireballs that bring
a substantial object hide among countless fireballs that don't,
and they ALL LOOK ALIKE. Power Law: for every 1 meter
lump, 30 10 cm lumps, 900 1 cm lumps, 2700 1 mm lumps,
all bright and flashy. For every 1 meter iron (0.7% of falls),
140 1 meter stones that won't reach ground, 4200 10 meter
stones that won't reach ground... and 120,000 bright 1 cm
body fireballs for every 1 meter iron. THEY ALL LOOK
THE SAME, until the last minute of the outcome... So you're
arguing the odds from what you know best, but...

1. A seismic signal means an impact or an airburst..
2. There was no airburst large enough to produce this seismic signal.
Hence...

     Excuse me for being more candid than usual, and I have no
intention of provoking bad feelings, but you know that people
are not working this through, but responding in an off-hand way,
thoughtlessly. No one has to agree with my approximations (or
my wild guesses), but I don't see anybody presenting a calculation
that shows a differing outcome, that explains the few facts in a
quantitative way Show me numeric values for size, density, speed,
entry angle that produce the object you believe it was and a seismic
signal such as was reported. (That's a generic "you," referring
to anybody, like all the "you's" in this post.) Use the Melosh model.
Use your own model. Work it out as an individual case. Whatever.
Demonstrate a little rigor. Shove a digit or two around. Make it
a reasoned argument, people, even in this forum.


Sterling K. Webb
------------------------------------------------------------------
----- Original Message -----
From: "Chris Peterson" <clp_at_alumni.caltech.edu>
To: "Meteorite Mailing List" <meteorite-list_at_meteoritecentral.com>
Sent: Monday, June 12, 2006 5:39 PM
Subject: Re: [meteorite-list] Norway Meteorite Impact Site Believed to be
Found


It doesn't look anything like a crater. I really doubt this event was
anything other than a typical largish fireball. It exploded at high altitude
and released a lot of energy, and maybe some meteorites fell. The image
appears to show a small rockslide, and there is no reason to connect it with
the fireball at all.

Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com


----- Original Message -----
From: "Sterling K. Webb" <sterling_k_webb_at_sbcglobal.net>
To: "Meteorite Mailing List" <meteorite-list_at_meteoritecentral.com>
Cc: "Knut J?rgen R?ed ?degaard" <knutjo_at_astro.uio.no>; "Ron Baalke"
<baalke_at_zagami.jpl.nasa.gov>
Sent: Monday, June 12, 2006 2:44 PM
Subject: Re: [meteorite-list] Norway Meteorite Impact Site Believed to be
Found


Hi, All

http://www.aftenposten.no/english/local/article1348689.ece

    Great photo of the impact site. Very scenic. And absolutely
NOTHING to provide scale. Is that bleme in the rock 10 feet
across or 100 feet across? Well, there are some stunted arctic
bushes and part of a tree in the photo. Eyeballing it, I would say
it's about 10-20 meters across. Note that the "crater" is not just
that more regular circular feature, but is actually an elongated
ellipse stretching downslope.

http://www.lpl.arizona.edu/impacteffects/

    Race off to Melosh's on-line impact calculator, shamelessly
invent details: an iron (since it's small and a small rock would
be less likely to survive the atmosphere), a 25-degree angle of
incidence (it hit a semi-vertical surface; more likely at low angles),
hard crystalline rock as the target, of great depth (no-brainer),
an average-ish 25 km/sec entry velocity... Let's assume we're in
the Reisa Dale about 20 km away... skiing.

    Here's the results:

Distance from Impact: 20.00 km = 12.42 miles
Projectile Diameter: 1.00 m = 3.28 ft = 0.00 miles
Projectile Density: 8000 kg/m3
Impact Velocity: 25.00 km/s = 15.53 miles/s
Impact Angle: 25 degrees
Target Density: 2750 kg/m3
Target Type: Crystalline Rock
Energy before atmospheric entry: 1.31 x 1012 Joules = 0.31 x 10-3 MegaTons
TNT
The average interval between impacts of this size somewhere on Earth is 0.2
years
The projectile lands intact, with a velocity 0.719 km/s = 0.446 miles/s.
The energy lost in the atmosphere is 1.31 x 1012 Joules = 0.31 x 10-3
MegaTons.
Transient Crater Diameter: 13.6 m = 44.6 ft
Transient Crater Depth: 4.81 m = 15.8 ft
Final Crater Diameter: 17 m = 55.8 ft
Final Crater Depth: 3.63 m = 11.9 ft
The crater formed is a simple crater
The floor of the crater is underlain by a lens of broken rock debris
(breccia) with a maximum thickness of 1.68 m = 5.51 ft.
At this impact velocity ( < 12 km/s), little shock melting of the target
occurs.
Richter Scale Magnitude: 0.2
Mercalli Scale Intensity at a distance of 20 km:
Nothing would be felt. However, seismic equipment may still detect the
shaking
The air blast will arrive at approximately 60.6 seconds.
Peak Overpressure: 17.3 Pa = 0.000173 bars = 0.00246 psi
Max wind velocity: 0.0408 m/s = 0.0913 mph
Sound Intensity: 25 dB (Easily Heard) and barely audible up to 100
kilometers.

    Trying to check the consistency of this with the "story" we've gotten,
if we lower the size of the iron to 0.7 meters, we get only a 15-foot bleme,
and there is NO seismic trace, so it has to be bigger... Given the slope of
the mountain, the impact angle on the slope would be about 80 or 90
degrees, maximum for chunking out this shallow hole. If we reduce the
size of the iron to 0.95 meters, we get no seismic trace. At 1.05 meters,
Richter 0.3; at 1.0 meters, Richter 0.4; and so forth, so the seismic
numbers would really pin it down... Anybody know?

    The mass of a 1-meter iron sphere is 33,500 kilogrammes, or
34 English tonnes (or 34 American tons). That's about half-a-Hoba,
or about the mass of Cape York (Ahnighito). Dr. Hansen may think
?degaard was exaggerating, but if he thinks "it was a stone weighing
around 12 kilos (about 26 pounds)," that produced this impact
feature, he is much further off-the-mark than ?degaard. If this
reconstruction is vaguely correct, it lost much of its energy
passing through the atmosphere. The crater could be produced
by a kiloTon. I wonder if the two seismic traces:
http://www.astro.uio.no/ita/nyheter/ildkule06/ildkule06.html
have been calibrated on the Richter Scale?

   For comparison, a 1-meter hard stone fragments in a 15 kiloTon
airburst, and makes no crater. In fact, when you keep increasing the
size of the stone, you get up to a 100-meter stone with a 100 MegaTon
airburst with vast destruction and STILL no crater... Suggests it had to
be an iron to punch through to the ground if the input parameters are
even roughly correct (and the impact calculator model).
http://www.lpl.arizona.edu/~marcus/CollinsEtAl2005.pdf

    I opined that this was a terrible place to search for meteorites, but
IF it was an iron (only going about 1500 mph when it hit), I'd bet that
mountain slope down from that bleme is littered with -- guess what?
I could be dead wrong (as is most often the case). Right place for a
metal detector? CC: Mike Farmer. Wait a minute... if it was a 34
ton chunk of iron and it hit at "only" 1500-1700 mph, some of
those "fragments" might be really big. Look for 20 ton black rock
too, while you're at it... (Truth is, meteoritic iron, tough as it is, is
often quite brittle. Remember, inside it's only 50-100 deg K when
it hits, so maybe no big chunks.) Even if it was "only" a ton or two,
it would be worth looking for, obviously.

    Hey! It's summer in the Arctic, with sunshine 20-22 hours a day.
It's in the 50's F. (Take mosquito netting.)


Sterling K. Webb

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Received on Tue 13 Jun 2006 04:32:01 AM PDT