Vs: [meteorite-list] Re: Researcher Says... Tektite Events

From: Jarmo Moilanen <jarmom_at_meteoritecentral.com>
Date: Thu Apr 22 09:44:43 2004
Message-ID: <006301c0b622$a1da3770$9f9bedc3_at_impact>

Hi Kelly and List,

> Hi, Steve, Darryl, and List,

> Jay Melosh, the chief theorist of impact events in general, says =
it is impossible to produce tektites by jetting. He proposes that they =
form from deep rock below the crater on rebound decompression. But the =
associated craters show no evidence of any deeper excavation than =
non-tektite craters, and deep rock sources are compositionally unlikely. =
And, again, why doesn't this happen with every crater?

Melosh doesn't mention anything about deeper excavation. They are =
similar impacts than the others. He also doesn't mention rebound =
decompression but rarefaction waves (=3D release waves) which has a =
major role in tektite forming in his theory. Rebound decompression and =
rarefaction waves are, as far as I know, totally different things during =
cratering process.

Melosh theory is one of the best theory for tektite origin which I have =
seen so far. Of cource, I can't know if it is the right one. However, it =
does explain e.g. lack of siderophile elements, deplation of water and =
predominance for reduced Fe. He also mentioned that the melt droplets =
which will become tektites are rapidly accelerated with accelerations in =
the vicinity of 50 g. That should be enougt to throw tektites far =
enought to explain known tektite strew fields.

I think that the mechanism Melosh proposed should work with lunar =
impacts as well. Althought those "lunar impact tektites" should lack =
aerodynamic shape. So, it is possible that true "glass meteorites" may =
exist!=20

Everybody who is interested about Melosh theory about tektites should =
read (this is an abstract): H.J.Melosh, "Impact physics constraints on =
the origin of tektites", Meteoritics & Planetary Science Vol. 33, No 4 =
(Supplement), 1998...

As Kelly point out, it is hard to explain why some impacts produce =
tektites and some don't. However, in Melosh paper is mentioned that =
pressure needed for tektite production is 100 GPa. This kind of =
pressures are not present in all impacts but it doesn't fully explain =
this. Cometary impators has usually higher velocities (=3D> stronger =
shock and rarefaction waves) than asteroids, but could it explain this, =
I don't know since as Kelly says, there is not enough evidences to prove =
that. But we also must remember that many proved impact craters are =
lacking impact melts and melt matrix breccias too.

Rayleigh Taylor instability could explain why all tektites are =
relatively small. Ejected silicate fluid will fall into droples because =
that instability. If Rayleight Taylor waves increases in intensity in a =
violently exposive way, one could make a suggestion that bigger impact =
will produce smaller tektites... But how that instability will work with =
fluid?

> Sterling K. Webb
>=20
> > Though the idea of tectites being of lunar origin was held by =
Nininger, and others, the notion has with recent evidence fallen into =
disfavor. Dr. John Wasson has done, and is as I understand it, doing =
work on tectites. His research, as he explained it to me, indicated a =
terrestrial impact origin for these objects. A "atmospheric cratering =
event" such as what occured at Tunguska, but of a much greater =
magnitude, would generate enough heat and the conditions to create them. =
 Such an explosion would splash the atmosphere back so that the vacuum =
of space would reach the ground (even though no land crater was =
created). The enourmous heat pulse would have been such that the sands =
and rocks on the ground would be vaporized and then recondensed in that =
vacuum. All water found in earth material thus vaporized would be =
released, and not become part of the recondensing melt. The layered =
tectites are those that are closest to ground zero, and they most likely =
would not have attained ejection velocites sufficent
> > enough to throw them up into space. Those towards and closer to the =
edges would be expelled over and above the onrushing air before it came =
back into the void created by the initial explosion. Such events, if =
this case scenario is true, will not produce any large and visible =
crater on the earth's surface. I am no expert on tectites, but this is =
one of the best theories to explain their origin that I have heard thus =
far.

I have heard similar airblast theory fro tektites somewhere else and I =
can't buy it. How tektites are ejected in that kind of event or hw you =
can throw vaporized material? How vaporized material will recondence in =
the vacuum to form cm-sized tektites?.

Jarmo Moilanen
Received on Mon 26 Mar 2001 01:28:21 PM PST