[meteorite-list] MPOD Feb. 20th 2014

From: Rob Lenssen <rlenssen_at_meteoritecentral.com>
Date: Sat, 22 Feb 2014 13:03:56 +0100
Message-ID: <000701cf2fc6$29cffb80$7d6ff280$_at_planet.nl>

Thanks for sharing your thoughts Elton. Especially about the thermal
expansion of some constituent(s).

    " All these working theories would be one heck of a multi-diciplinary:
chemistry, aerodynamic, thermodynamic, calculus, etc, problem beyond most
doctoral thesis work."
Yes, it would be. And that is what - for me - it makes it fun to discuss :-)

    " If you remember there was a NWA(?) sometime back discussed on the list
that had lots of meteorite "pebbles" largely lacking fusion crust which were
welded to the back side of some of the larger stones. They were cemented by
the molten silicate glass."
I guess you mean Tamdakht (
http://www.lpi.usra.edu/meteor/get_original_photo.php?recno=5650643 )
 
    " Aerodynamic theory suggest however that the smaller object, having
less drag would catch up to the larger mass which may have been the case
above with the cemented NWA example"
I don't see a smaller object catching up with a larger one due to
differences in drag forces though. Although the smaller one will have less
drag in an absolute sense, it's drag will be much larger relative to its
mass. Therefor the smaller object will be slowed down much faster than the
larger one. Drag force is a function of the cross-sectional area "facing the
flow", while mass is a function of an objects volume. So approximately
squared versus cubed with respect to an objects size.

Have a nice weekend,
Rob Lenssen


-----Oorspronkelijk bericht-----
Van: MEM [mailto:mstreman53 at yahoo.com]
Verzonden: zaterdag 22 februari 2014 1:29
Aan: Meteorite-list at meteoritecentral.com; rlenssen at planet.nl
Onderwerp: Re: [meteorite-list] MPOD Feb. 20th 2014


Rob, Gregor, List. I have two or three? "shake and bake" working theories
I've been pondering . The first is based on the evidence of the porous
texture itself which usually represents out gassing of entrapped gases.
There were many claims of slickensides in Chelyabinsk however owing to
several factors they are more likely shock cracks lined with melt material.?
This "possible" coating/melt material would have been injected/emplaced in
the explosion phase of an earlier cosmic collision prior to its arrival on
Earth. I surmise that it contains some volatiles which are ordinary solids
in the coldness of space.? I have been in touch with one of the Chelyabinsk
researchers who is looking into the dark material lining these cracks. Some
of the bubbles could be related to gas being released from the surface as
the specimen underwent thermal and or mechanical stress.

?Even a small mass of gas could expand to make micro bubbles visible and
could also act to chill the bubbles preserving them.? For this to work isn't
exactly simple as normal ablation would tend to erase the bubbled area
faster then it could form--unless it were shielded within the wake of a
larger object.? Bubble expansion might also occur if the condensing
silicates carried atmospheric gas which either underwent extra thermal
expansion, else several micro-pockets coalesced into tiny visible ones and
or--also represent an oxidation byproduct( e.g gas) from the meteoroid (e.g.
metallic iron oxidizing with the atmosphere). I haven't worked through the
chemistry other than knowing that there is some oxidation in the heated
trail such as that producing fine magnetite crystals retained in the fusion
crust.


There is also a probable infrared radiation "roasting" of the specimen if it
were not exposed to an ablation cone head on but were in a "relatively: slow
or almost stationary" position out of the slipstream. It would be bathed on
all sides from the thermal radiation of the plasma trail behind a larger
object.? Much depends on the relative location of this specimen with gas
jets, plasma trail diameter, etc and with other larger specimens AND where
in the retardation timeline it was exposed.

There may be yet another working theory: If this specimen were trailing in
the wake of a much larger specimen which shielded it from ablation, it could
still be exposed to the fog of melted metal/silicate.? As I understand it,
the smaller stone would still be cosmically chilled at this point and in
such a case would have the micro silicate "blobs" condense on contact. As
the stream of metal, phosphate and silicate etc particles are irregular the
coating could be irregular.?


If you remember there was a NWA(?) sometime back discussed on the list that
had lots of meteorite "pebbles" largely lacking fusion crust which were
welded to the back side of some of the larger stones.? They were cemented by
the molten silicate glass.? Aerodynamic theory suggest however that the
smaller object, having less drag would catch up to the larger mass which may
have been the case above with the cemented NWA example. If the however
trailing stone were crossing the ionized /silicate fog zone at even a slight
angle, then it would pass through without colliding with the forward stone.
Depending on the relative velocity at the point this bubbly specimen found
itself, the bubbles were preserved as ablation had ceased.?

All these working theories would be one heck of a multi-diciplinary:
chemistry, aerodynamic, thermodynamic, calculus, etc, problem beyond most
doctoral thesis work.? There are a few other factors I can think of and many
I haven't thought of but perhaps this will inspire some additional brain
storming.
Regards,
Elton

PS And yes a vacuum would have been at play but I think it took thermal
expansion or some sort to produce the gas which formed the foam.






On Friday, February 21, 2014 5:00 PM, Rob Lenssen <rlenssen at planet.nl>
wrote:

Hi List,

Thank you for your on and off List replies. Main focus in the answers was
the extreme violence of the Chelyabinsk explosion.
Yes, this bolide, and the amount of energy released, was much larger than
average. I'm still wondering what mechanism could produce such a crust
though.

This kind of extreme (all-around) foamy crust is uncommon in typical
chondrite falls, but I have seen quite some examples in the case of
Chelyabinsk already. It might have something to do with the size of this
bolide. But again, what was the mechanism?

Like Gregor states in his MPOD contribution (here's the link again for your
convenience: http://www.tucsonmeteorites.com/mpodmain.asp?DD=2/20/2014&WYD=
), one would expect the stone to have been in a low pressure environment
when the crust solidified. Foaming typically requires an under-pressure,
like in the case of the back side of an oriented specimen.
This specimen however has foamy crust (almost) all around. Could it have
anything to do with the constituents of the fragment? Or did the sheer
magnitude of the Chelyabinsk "explosion" produce a vacuum uncommon to
smaller Falls? Could it like Gregor proposed have been flying in the low
pressure zone behind a larger piece?
Still wondering...

Best regards,
Rob Lenssen
Received on Sat 22 Feb 2014 07:03:56 AM PST