[meteorite-list] Re: Tektites

From: Kelly Webb <kelly_at_meteoritecentral.com>
Date: Thu Apr 22 09:43:31 2004
Message-ID: <3B4E9EFF.68633A40_at_bhil.com>

Hi,

    Glass's preliminary guess in 1968 that the Tunguska object had a
"composition similar to that of tektites" has been long superseded by
more recent and ongoing studies of the Tunguska microspherules, none of
which bear this notion out. They are largely metallic, not glassy. The
microspherules themselves lend no evidence either way on whether the
impactor was meteoritic or a comet chunk. Actually more meteoriticists
think the Tunguska Object was a stony meteorite, while more astronomers
think it was a comet; it seems to correspond to whichever choice is most
familiar to the theorist.

    The crucial issue in the possible airburst of an impactor is the
point at which the dynamic pressure of the atmospheric resistance [p =
0.6 x gas density x (velocity)^2] equals the crushing strength of the
impactor material. The Tunguska object, whatever it was, exploded at a
dynamic pressure of about 200 bars. Whatever it was, it WASN'T weak and
fluffy. That doesn't necessarily mean it wasn't a cometary fragment. We
don't know enough about comets to be sure.

    The weakest fireball objects burst at 0.1 bar, so obviously there is
weak and fluffy material out there, corresponding to interplanetary dust
with densities of 0.01 to 0.10 gm/cm^3. However, your assertion that a
10 kilometer weak'n'fluff would never reach the surface of the Earth is,
well, ridiculous. The factor in determining whether an object will
suffer ANY decceleration in the atmosphere depends on the total mass per
unit area of frontal surface of the object.

    Once an object is big enough to have more than 1.057 kilgrams of
mass for every cm^2 of frontal area, it's gonna reach the surface of the
Earth with its cosmic velocity relatively undiminished. The dynamics of
cometary impact are as well known as that of any other kind of impact.
That 10 kilometer fluffball of yours is just as deadly a hazard as any
other object with its mass and velocity, whether stone, iron or pure
neutronium [kinetic energy = mass x (velocity)^2]. A cometary fluffball
with the density of interplanet dust particles, if there were such a
thing, would reach enough mass to punch through essentially unimpeded at
the 500 meter size.

    Ignoring the numerical coefficients, we have a factor here composed
of density x diameter^3 / diameter^2. This reduces to density x
diameter. Even a little simple arithmetic would have revealed that your
fluffball would have a volume of 5 x 10^29 cm^3 and a frontal area of 8
x 10^11 cm^2 and hence would have to have a density of less than 10^-13
gm/cm^3 to be unable to penetrate the atmosphere, or in other words, a
density essentially similar to space itself or a very good laboratory
vacuum. So, in a way, you're right: balls of vacuum do not penetrate the
atmosphere!

    But, hey, we knew that already. That's what the backs of envelopes
are for. When we get these great notions, like, I'll bet there are big
comets so fluffy they won't penetrate the atmosphere, we do the
arithmetic first to find out whether it's in the ballpark (or even if
there is a ballpark). This notion is a complete non-starter.

    As for Michael Pain's article answering my questions, it was Michael
Pain's article that was summarized in the CCNet post which raised the
questions for me. I repeat, the notion of an impact of this magnitude
--- 10,000,000,000,000 tons of TNT equivalent --- having occurred so
recently and without leaving unequivocal and substantial evidence,
obvious traces, is ludicrous.

    It reveals the increasing intellectual poverty of impact dogma to
require an impact of this magnitude to create the Australasian field and
be unable to find any trace of that fresh hole --- 70 miles across and
12,000 to 20,000 feet deep --- in Indochina or anywhere else. You can't
bury all traces of a crater 1/3 the size of Chicxulub in less than a
million years. (Back to arithmetic: it would take depositation of 1-2
meters of sediment per century to fill the damn thing in and bury it in
so short a time.)

    And, tektites do not have the "composition" of terrestial rocks, any
terrestial rock. There is no match for tektites anywhere on Earth. Even
if by "composition," you mean "bulk composition," there is no match.
What you are referring to is a set of plausible hypotheses that certain
mixtures of terrestial materials subjected to certain very extreme
conditions might produce something like a tektite. It's quite possible
that they're produced that way, but it's hypothesis, not proof. Go grab
some rocks and a big electric vacuum furnace and cook me up a tektite.
That I'll believe. That would be proof. It's been tried, by the way,
many times, and nobody's ever succeeded in making a tektite. Unique and
perverse little things, that's why they're so interesting.

    The isotopic compositions do indicate that tektites are derived from
a differentiated body with a secondary crust. But the Earth is not the
only such body, only the one we're most familiar with.

Sterling K. Webb
---------------------------------------------------

Steve Schoner wrote:

Actually, virtually every question that you raise can be addressed in a
very interesting article found in METEORITE, Feb. 2001 Volume 7, No. 1.
"Source of the Australasian Tektites" by Michael Pain, p. 34-37, with
extensive references that if followed pretty much shatter the Lunar, or
the supposed "silicate meteorite" theory.

Then there is that abstract by Billy Glass, {Microtektites and the
Origin of the Australasian Tektite Strewn Field; Publications of the
Center for Meteorite Studies, ASU, May 1968, p. 19) where he writes
after doing a microprobe of Tunguska glass spherules sent to him by Dr.
Krinov "Thus it seems probable after this preliminary investigation that
some of the Tunguska material may have a composition similar to that of
tektites."

As Tunguska clearly demonstrated comet impact DO NOT have to leave a
crater. And being that they are composed mostly of ices (gas and water)
they will not leave much solids either. (the lack of a crater might be
like the old story-- the perfect murder weapon-- a blade of ice).

The dynamics of cometary impact is not known, and there are factors that
are not understood. First off we are not certain how solid a comet is,
and what range of structure they might have. These icy dust balls might
be so fragile that even a 2 or 10 km one could not reach the surface of
the earth intact, instead exploding in the air in a Tunguska type event.

This would certainly generate huge amounts of energy-- enough to blow
away the atmosphere to the ground, dissociate the hydrogen-oxygen bond,
and melt, if not completely vaporize source rocks.

Comets are the most likely source for the events that created tektites,
and we won't have the evidence for it until it happens again, or when a
sample return mission brings back a piece of a comet and the trace
elements match with those found in tektites.

But more importantly, sans water, tektites in and of themselves reveal
their terrestrial source as they have compositions that are identical
with terrestrial source rocks. This is especially true of the
microtektites.

The evidence for Lunar origin has evaporated, and the the evidence for a
tektite meteoroid parent body is non-existent.

Terrestrial, impact origin is being ever the more supported by the
evidence.

So-- who is it that is relying on "faith rather than reason" ????

Steve Schoner,
Received on Fri 13 Jul 2001 03:10:57 AM PDT