Hi, Jason, EP, List,
Jason wrote:
> unless we're talking about a 1km body vaporizing
> like Tunguska, because, in terms of physics, I
> simply don't see why such a body would vaporize
> before striking the ground...
Relying on intuition rather than numbers is always
risky. It helps to model the event in question. You
have to interpret the results sometimes, but it's a big
help to understanding an event.
There's a really good tool available for "getting the
feel" of the impact parameters that might produce a certain
kind of result. For example, any 1000-meter impactor that
"airbursts" has to be very weak and fragile for that to
happen. So, you go to:
The Online Impact Calculator:
http://lpl.arizona.edu/impacteffects/
[The full discussion of the parameters of impact
used in the LPL model are in this document:
http://www.lpl.arizona.edu/~marcus/CollinsEtAl2005.pdf ]
You start with an iceball 1000 meters in diameter
with a density of 1.0. It multiply fragments 55 miles up,
without the fragments losing much speed at all. When
you look at the documentation, you will note that the
online calculator does not follow the fate of fragments,
so the model says nothing about what happens to them.
Obviously, a fragment in the same place at the same
speed as the original body will suffer same fate: it will
fragment, and its fragments will fragment... That's an
airburst, or at least a :rolling" series of them. What you do
get is a crater, though not big and not deep, from the
high energies of the event in the atmosphere.
Well, cometary bodies, particularly pristine ones that
formed as very small bodies, are not fully dense. They are
poorly consolidated; the technical term is "fluffy"! :-)
Their density is less than 1.0 (also true of some small
asteroidal bodies; made of rock, yet with densities less than
1.0). In a porous icy body, the density could be much lower.
Some individual cometary dust particles have densities of
0.01, a kind of "whispy" dust.
If we lower and lower the density we use in the model,
the energy falls off with the mass, the altitude of the breakup
goes up and up, to 80 miles or more, the craters vanish
despite energies that are still a Gigaton of TNT. We are
left with an airburst that would have tremendous atmospheric
effects. It would deposit a few 100 billions of tons of fine
dust at the top of the atmosphere (60 to 80 miles up) to
persist for years with climatically destructive results. (You
like sunshine?)
The interesting thing is that the investigation of such
events, looking for them in the historical record, and
attempting to find evidence for them, has been more
active in Europe than in the US. There is apparently a
cultural style in preferred impacts to worry about. We
Americans seem to prefer to worry about the big hammer,
the hard rock pounding the planet, and dismiss any event
that doesn't resemble the movie Armageddon so much.
We don't want to mess around with any subtle scenarios;
we want Apophis!
This is not to say that such non-cratering catastrophes
are "popular" in Europe; they're often pooh-poohed even
when there's substantial evidence:
http://en.wikipedia.org/wiki/Climate_changes_of_535%E2%80%93536
Before you know it, the US attitude leads to the "If it didn't
leave a crater, who cares?" The answer to that is: everybody
who eats food, when all food production ceases for two
years, for example. That sunshine is a handy thing to have,
if you plan to eat. (Well, at least it would cure global warming.)
I don't think it takes a great deal of head-scratching and
sophisticated agricultural economics to calculate what percent
of a population dies if 90% of your food crop fails (and 90%
of your livestock isn't fed and so forth). After you use up all
your canned goods, after the environmentalists have hunted
to extinction every edible species, threatened or not, what do
you do? (Please, bring your movie scripts around to the back
door...)
Jason, you made a lot of fun of EP for his reference
to a 90% death rate. It's true that EP paints sketchily and
with a broad brush. I'll grant you that. I dug up his original
reference at the end of an email. It goes like this: "...comet
impact which killed about 90% of the people living in North
America at the time. Most died due to hunger. But then,
there's not likely to be any strewn field from that..." He just
bounced through (in three sentences) the events I have spent
(way too) many paragraphs connecting.
Yes, it's an hypothesis, not a fact. In this area where solar
system mechanics and human history intersects, EVERYTHING
is an hypothesis. Some we like better than others, though; we're
not really dispassionate about them, and rarely treat them equally.
There's even a piece of evidence for his hypothesis which he
hasn't mentioned (yet). At some point in the distant past, ten
to twenty thousand years ago, there was a drastic reduction in
the population ancestral to native Americans.
It shows up in studies of mDNA and y-chromosome studies.
It is "interpreted" as the result of a single small-population
settlement migration into North America rather than multiple
waves of settlement by larger groups.* But obviously, it could
just as easily be the result of a drastic die-off that reduced the
population to a low number (with less genetic diversity) before
rebounding. When such a thing occurs in "animals" instead of
"humans," that is always the interpretation it is given. It's called
a "genetic bottleneck." Natives of North America are one of the
few human populations to show it in that time range:
http://en.wikipedia.org/wiki/Population_bottleneck
A much older population bottleneck has been associated
with a major volcanic event, a really big one, 70,000 years ago.
http://en.wikipedia.org/wiki/Toba_catastrophe_theory
There's something about spending part of Christmas Day
talking about 90% of everybody dying from hunger and doing
the typing of it between bites of fresh-baked apple pie. I feel a
little odd doing that. But I know what will fix that feeling: yes,
another piece of pie!
Happy Holidays!
Sterling K. Webb
----------------------------------------------------------------------------
* Here's the genetic study cited:
http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.genom.5.061903.175920?journalCode=genom
------------------------------------------------------------------------------------
----- Original Message -----
From: "Jason Utas" <[EMAIL PROTECTED]>
To: "Meteorite-list" <meteorite-list@meteoritecentral.com>
Sent: Tuesday, December 25, 2007 7:28 AM
Subject: Re: [meteorite-list] Stuff
Hola All,
1) Thanks Darren, and others who messaged me in private.
2) Pete,
By making such a message public, you most certainly meant it to be
offensive; otherwise, you would have addressed it privately. You
meant for everyone to see it, whether or not you gave the post
adequate thought before posting it...
That said, what I've been saying all along is that one simply can't
draw solid conclusions from a *lack of evidence.*
I haven't had any premeditated ideas about what caused these dust
layers/extinctions. I openly admitted several times that I have no
bloody idea what did it. I do, however, know some statistics, as well
as physics, both of which point towards certain scenarios being either
highly unlikely or, physically speaking, impossible.
I'm not saying that certain events didn't necessarily happen. In most
cases, it's easy enough to say that the observed effects do not match
a given possibility for what did, according to E.P., occur.
My issue isn't with such events occurring (unless we're talking about
a 1km body vaporizing like Tunguska, because, in terms of physics, I
simply don't see why such a body would vaporize before striking the
ground), but rather with the fact that he isn't looking at evidence
objectively, and also isn't being open-minded when it comes to all of
the possible scenarios that could create the observed geologic
results.
Because of a lack of solid evidence for any conclusion at this point
in time, simply put, no firm conclusions can be drawn.
And you call me biased....maybe to the 'theory' that KE = 1/2mv^2, I
suppose.
I've only been arguing based on impact dynamics and known facts.
E.P., in case you didn't notice, was the one pulling airbursts and
craters out from 'twixt his cheeks.
Admittedly, some of what I said could probably have been
better-explained, and I did mix-up 30 degrees from horizontal vs
vertical, but even Sterling made some mistakes in the discussion; it's
complex stuff that not many of us deal with on a day-to-day basis, and
as such, I'm of the opinion that ideas should be thrown around - with
the general acceptance that such ideas are nothing more than theory
and are in no way to be called 'facts.' Because saying that would be
*wrong.*
I love how you refer to me in the third-person as well...I find
it...interesting.....
Merry Christmas,
Jason
On Dec 24, 2007 7:35 PM, Peter A Shugar <[EMAIL PROTECTED]> wrote:
> It was not meant to offensive, but more of a comment on his stubborn
> refusal
> to look at tons of evidence that is so contrary to only his view. Or put
> this way,
> Jason against the world. If you can't convince the world to adopt your way
> of
> seeing things, then maybe it's time for you to change your views. This
> "the whole
> world but me is wrong" is a refusal to adapt and mayhap even learn
> something.
> Just watching and reading the MANY comments has given me a education I
> never
> could have afforded to go to college for.
> I do not want to sound as if I am a know-it-all because I am not. I want
> to learn, so
> when I bring my meteorite collection to school, I can make a presentation
> that will both
> instruct and inspire the kids to want to learn more. In order to do this I
> need to be
> willing
> to CHANGE MY MIND when the facts don't fit my way of thinking. This means
> to change
> the way I view the theory instead of trying to make the theory fit my
> point of view.
> If I can not adapt to this new theory, then I will admit to being two
> tacos and a
> burrito
> short of a combination plate myself. In addition, I will admit to being a
> stubborn old
> Coot.
> .I will apologize to the extent that I didn't mean to hurt him, but maybe
> to jar him
> into listening
> and becoming more rational in his reasoning.
> Pete
>
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