[meteorite-list] Entry Dynamics in Peru

From: mexicodoug <mexicodoug_at_meteoritecentral.com>
Date: Fri, 5 Oct 2007 02:13:50 -0500
Message-ID: <00f501c8071f$485e9d80$4001a8c0_at_MICASA>

Hi Sterling, my brief comments are inserted in your text in all lower case,

> 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.

i am not focused on your reconstruction of the event. just your comment
that everything in the crater is pulverized due to the incredible energy
released in impact. while this is a possible scenario, i hope more intact
material can be found. my objection really is the tendency for people to
jump to conclusions ruling out other posibilities based on heresay. i've
read the same reports you have over the same time period and cannot see any
clear cut evidence as you do. that's what i enjoy about the list, the
ability to debate all sides of a problem openly and come away with a new
angle in the process. plus the bickereing was getting to me so this was a
nice opportunity to contribute.

>
>> 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.

sterling, on earth, this crater has few if any known peers. I said size. i
did not mentioned anything about proportions.

>> 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.

jilin, nevertheless, is the largest individual stony meteorite to ever land
and be recovered, and we suspect it to be more comparable in size and
composition. trust me - that ton of rock just didn't land like a feather on
a waterbed, either.

>
>> 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.

ok. so i now understand your opinion hidden behind the tireless search
engine and fine postings - which is just that - opinion. because there is a
lot of powder around you think everything in the hole must be powder because
we get this graceful, textbook idealized impact that fits the barringer
crater. mike also was an expert witness to all the powder when a small
fragment of moss landed on the concrete in an industrial complex. still he
believed a meteorite is in the crater...so your logic is back to mike saw a
lot of powder and barringer crater had no big meteorite buried, so this is a
barringer situation.
>
> 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.

ok. something to think about at last. mike mentions many other theings -
the meteorite may have punched a hole into the water table below, too. as i
mentioned yesterday, the speed of sound controlling this idealized process
is significantly compromised when you don't get a dead stop. that was the
purpose of my comments yesterday you followed-up. please keep an open mind
when you mention the hole is strictly filled with powder and contains no
fragments.. your reconstruction isn't a reconstruction at all in the
scientific sense, as we lack too much at this stage to model it seriously
and that is why i am bothering to respond to this interesting discussion
which would determine whether there are more sustantial meteorites in the
hole - a very interesting question worthy of debate.

>
> [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.]

these negligable differences between meteorite and terrestrial are unlikely
to make the difference and i don't think you mean that either.

>
> 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).

maybe. these are all based on models and you are assuming an idealized
'truly violent impact' which is not an easy order. but we have perhaps 6
meters of penetration that still isn't "violent" in the terms of a Barringer
crater type scenario. A rim will get created whenever you excavate ... it
is partially a reflection of a gaussian distribution of displacement of
strewn material. i wouldn't read any more into this, though if i did it
would make for a good Science fantasy plot.

>
> 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!

yup, it would be nice to have an endowment for a better library. and i
appreciate your pet subject of jay melosh's impact modeling. does he have an
impact calculator somewhere on the internet you can provide a link for, that
includes the assumptions of his high energy violent models?


>
>> 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.

we also have witness evidence of boiling cauldren pits. in the excitement
is is easy to get carried away. dust sloughing off the rock in free fall
after its violtent entry commonly causes dust trails and is a likely
explanation, as well. if it truly were ablating down to the ground, i think
we would have some nice impact glasses or spheroids!!! now if mike saw
those, i'd be in your camp.


> 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.

ok, so there were some strong sonic booms. we don't know if they are from
the impact alone or flight alone or what. let's not get too carried away by
reports we need to be a bit skeptical about if we ever expect to get this
straight. the sonic booms for such a big meteorite can be big. an maybe
they forgot to caulk the windows. i know what that is like. every time a
car passes my place the windows begin to whine like the door is falling
down. (past tense, i siliconed them)
>
> I really like the graph.

thank you. perhaps it shouldn't be buried in these messages as it goes a
long way to answer some questions that sometimes pop up on the list.

>
> May a Lunar fall gently in your back garden.

thanks, again, and to you may a well oriented Venisian grace the mantel
above your hearth.

further comment. truly there is no reliable basis to discount the
probability of bona fide fragments in the crater. i have nothing against
your scenario as long as the mind remains open to the simplier though
debatably less dramatic explanations. in any case a couple of tons falling,
like in the case of Jilin, will certainly come in with a bang. i do believe
that this space rock impacted at about double the energy it would have at
free fall at sea level, due to the faster free fall velocity it had in the
air which is 38% thinner at 3800 meters. at this stage, a neat, supersonic
impact throughout the collision seems highly unlikely to me, even if the
free fall velocity was near the speed of sound initially. we can check free
fall velocity for a one meter sphere -that's two tons, btw. i think i put
the formula on the list in 2003 if you don't want to look for or derive it.
as i mentioned in my only post yesterday, if you double the time of impact
you half the rate of transfer of energy. the time of impact was probably
significantly increased due to the characteristics of the soil.

the great thing is, if the folks responsible for the crater now will do
their homework, we will get a definitive answer one way or another and can
revisit this issue with a refreshing investigative basis rather than the
usual unreliable accounts during the excitement..
>
>
> Sterling K. Webb

best health,
doug
Received on Fri 05 Oct 2007 03:13:50 AM PDT