[meteorite-list] Carancas: Impact crater vs. impact hole

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Sat, 20 Oct 2007 16:19:51 -0500
Message-ID: <0e9801c8135e$f47019a0$b92ee146_at_ATARIENGINE>

Hi, Bernd, List,

    An excellent attempt at defining the difference,
but as Bernd has pointed out there are characteristics
of both in Carancas. But, Buchward is seeing things
from an "iron" perspective.

    An iron will make a much larger "impact pit"
than any stone ever could, as it takes much more
energy to vaporize iron than it does to vaporize rock.
(The boiling point of iron is 3134 degrees Kelvin,
almost double that of even the toughest rocks.)

    In practical terms, an exploding (vaporizing)
iron impactor would probably have to hit at 8 to
10 km/s to create a vaporizing explosion. Buchwald
mentions a velocity of 5 km/s as an upper limit to
an impact pit. This is because if all the impact energy
were converted to heat with full 100% efficiency, it
would take a 4.2 km/s impact to vaporize rock. In a
real impact, it would take 5 km/s or more to do it.

    In practice, if we had a vast range of craters to examine,
we would find "true" craters made by stones that were
smaller than any "true" crater made by irons, and iron
impact pits larger than the smallest "true" craters made
by stones.

    There's an interesting complication not often thought
about: an iron impact with insufficient energy to vaporize
its own iron can be "hot" enough to vaporize the terrestrial
rocks it impacts! So, it's possible that an iron impactor
could produce a vaporizing cratering explosion that leaves
the impactor (partly) intact! Perhaps this type of crater
would occupy the intermediate range between stone and
iron craters in the case of iron impactors.

    Carancas had "vaporizing" traits: the reported "bright
flash," the mushroom cloud, and the mysterious vapors
all point to a thermal event, but other signs of the heat of
a rock-vapor explosion event are missing. I believe that
what happened is that only the 5% (to maybe) 10%
troilite component of the impactor vaporized (an idea
first posted on the List by Piper R. W. Hollier -- going
to be academic here and credit my sources).

    Troilite vaporizes at a much lower temperature than
rock -- only 427 degrees C. -- but it would produce a
very satisfying explosive shock, excavating the crater,
powdering the impactor, releasing hot sulfur into the
air and the "wet" crater. Troilite is almost unknown in
the terrestrial environment because it breaks down
rapidly at Earth-normal temperatures; vaporized troilite
would chemically combine almost instantly. Even the
"bubbling" in the crater, which everybody immediately
dismissed, can be explained by the short-term reaction
of the troilite-generated (dilute) sulfuric acid in the crater
with the native carbonates and the production of
hydrogen sulfide.

    As for what might be found in the crater itself, I
suggest that nothing but the free iron component will
have remained there, probably having penetrated the
crater "bottom" as far as the native rock allows. Early
descriptions of big pieces picked by local institutions
show a 15% free iron content, much of it (by weight)
in large irregular concentrations (like Portales Valley).
That portion is likely in (or under) the crater still. IF
it were a ten-ton impactor, there could (might) be a
ton of iron down there. (Notice I used the big "if.")

    This type of "semi-vaporizing" explosion has never
been proposed before as far as I know, but the fit of this
"theoretical" model to Carancas is extremely convincing.
(Well, I'm convinced anyway.) An investigation of this
impact could actually contribute something to our scant
knowledge of impact mechanics in the real world, but
instead (if the MSNBC article is to be believed) we get
"ho-hum, another boring H4/5." My, are we spoiled,
or what?


Sterling K. Webb
--------------------------------------------------------------------
----- Original Message -----
From: <bernd.pauli at paulinet.de>
To: <meteorite-list at meteoritecentral.com>
Sent: Saturday, October 20, 2007 11:40 AM
Subject: [meteorite-list] Carancas: Impact crater vs. impact hole


Hello All!

BUCHWALD V.F. (1975) Handbook of
Iron Meteorites, Volume 1, pp. 33-34:

For the sake of clarity it should be noted here that
giant meteorites can form two types of craters.
The smaller crater is more properly called a large
impact hole and is geerated by relatively small
meteorites (< 50 tons) with relatively low velocities
not exceeding 5 km/s. Such meteorites cause
mechanical destruction of the ground and are themselves
usually broken into a number of fragments
upon impact. The major part of the meteortic fragments
will remain in the impact hole with
shattered rock and soil. Typical examples are the
100-1,700 kg iron meteorites of the Sikhote-Alin
shower that produced impact holes 6-27 meters in
diameter and buried themselves to depths of 2-8 meters.
The genuine craters discussed here are more than
100 m in diameter and were formed as the result
of an explosion at the moment of impact. The
projectile itself vaporized almost entirely, and
tremendous shock waves raced outward from
the focus.

********************************************************************************

As if to find a compromise between list members
who differ on what to call Carancas, the
associated "hole" in the ground there obviously
unites characteristics that place it inbetween what
should be called an impact hole and an impact crater.

What a pleasant coincidence! ;-)

Bernd

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Received on Sat 20 Oct 2007 05:19:51 PM PDT