[meteorite-list] Impact melt formation by low-altitude airburst processes, evidence from small terrestrial craters and numerical modeling, H E Newsom & MBE Boslough 2008 Mar 2p abstract: Rich Murray 2010.11.17

From: Rich Murray <rmforall_at_meteoritecentral.com>
Date: Wed, 17 Nov 2010 00:06:20 -0700
Message-ID: <AEB2CB6C05894786BF5CD58B62BFDAA2_at_ownerPC>

Impact melt formation by low-altitude airburst processes, evidence from
small terrestrial craters and numerical modeling, H E Newsom & MBE Boslough
2008 Mar 2p abstract: Rich Murray 2010.11.17
http://rmforall.blogspot.com/2010_11_01_archive.htm
Wednesday, November 17, 2010
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http://dl.dropbox.com/u/2268163/IMPACT%20MELT%20FORMATION%20BY%20LOW-ALTITUDE%20AIRBURST%20PROCESSES,.pdf

http://www.lpi.usra.edu/meetings/lpsc2008/pdf/1460.pdf free full-text

Title:
Impact Melt Formation by Low-Altitude Airburst Processes, Evidence from
Small Terrestrial Craters and Numerical Modeling
Authors:
Newsom, H. E.; Boslough, M. B. E.
Publication:
39th Lunar and Planetary Science Conference, (Lunar and Planetary Science
XXXIX), held March 10-14, 2008 in League City, Texas.
LPI Contribution No. 1391., p.1460
Publication Date: 03/2008
Origin: LPI
Bibliographic Code: 2008LPI....39.1460N

http://adsabs.harvard.edu/abs/2008LPI....39.1460N find similar articles in
database


[ Measuring the images shows that at 10 seconds,
the ground width of the burst is 13.4 km,
with round area 140 km**2,
while the central cone of ground excavation is deeper than 1/3 km. ]

Lunar and Planetary Science XXXIX (2008) 1460.pdf

Impact melt formation by low-altitude airburst processes, evidence from
small terrestrial craters and numerical modeling.
H. E. Newsom 1, and M. B. E. Boslough 2,
1 Univ. of New Mexico, Institute of Meteoritics,
MSC03-2050, Albuquerque, NM 87131, USA newsom at unm.edu ,
2 Sandia National Laboratories, PO Box 5800, Albuquerque, NM 87185

Introduction

Airbursts in the lower atmosphere from hypervelocity impacts have been
called upon to explain the nature of the Tunguska event and the existence of
unusual impact-related silicate melts such as the Muong-Nong tektites and
Libyan Desert Glass of western Egypt [1].
Impact melts associated with impact craters, however, have been
traditionally attributed to shock melting of the target material that
experiences strong shock compression and heating.
The characteristics of impact melts from small terrestrial craters (< 4 km
diameter) leads to the possibility that the airburst phenomena may have been
responsible for these melts.
This conclusion is supported by numerical modeling of the airburst phenomena
using super computer class facilities at Sandia National Laboratories [1].

Numerical modeling results

Recent models of the airburst phenomena have revealed several important
insights into the coupling of the airburst with the surface and the possible
nature of the resulting silicate melts.
The center of mass of an exploding projectile is transported downward in the
form of a high-temperature jet of expanding gas (Fig. 1).
The jet descends by a significant fraction of the burst altitude before its
velocity becomes subsonic.
The time scale of this descent is similar to the time scale of the explosion
itself, so the jet simultaneously couples its kinetic energy and its
internal energy to the atmosphere.

Because of this downward flow, larger blast waves and stronger thermal
radiation pulses are felt at the surface than would be predicted by a point
source explosion at the height where the burst was initiated.
For impacts with a kinetic energy above some threshold, the hot jet of
vaporized projectile (the descending "fireball") makes contact with the
Earth's surface, where it expands radially.
During the time of radial expansion, the fireball can maintain temperatures
well above the melting temperature of silicate minerals, and its radial
velocity can exceed the sound speed in air.
Boslough and Crawford [1] suggest that the surface materials can ablate by
radiative/convective melting under these conditions, and then quench rapidly
to form glass after the fireball cools and recedes.
For crater-forming impact events, the atmosphere also plays an important, if
not dominant role.
The iron projectile that formed Meteor Crater (Arizona) deposited more than
2.5 times as much energy directly into the atmosphere than it carried to the
surface [2].
Small crater-forming impacts should therefore exhibit phenomena similar to
those associated with airbursts.

Impact melts from small terrestrial craters

Small craters with impact melt fragments include; Wabar, Aouelloul, Henbury,
and Lonar.
The striking characteristics of the impact melt fragments from these craters
is the presence of thin layers of melt.
These layers are sometimes isolated fragments (e.g. Aouelloul),
sometimes stacked into layered accumulations (Lonar),
and sometimes form coatings around unmelted material or layered melt bodies
(Lonar).
The layered accumulations have much in common with the Muong-Nong type
silicate melt materials.

Fig. 1, Airburst for which the fireball descends to the surface [1].
White = 5800 K; Red = 2000 K.
Bottom image shows wind speeds. Red represents supersonic flow.

Materials from individual craters are described below, as a function of the
diameter of the structure:

Wabar, Saudi Arabia, 0.12 km diameter, sedimentary target

The impact melt samples from this crater are unique in consisting of white
material coated by dark impact melt [e.g. 3] (Fig. 2).

Fig. 2, Samples of impact melt coating matrix from the Wabar impact crater
in Saudi Arabia.
Image 6 cm width.

Henbury, Australia, 0.16 km diameter, sedimentary target

The Henbury impact structure consists of numerous small craters, with the
largest being 0.16 km in diameter, ranging down to depressions only a few
meters in diameter containing iron meteorite fragments [e.g. 4].
The Henbury craters reflect the disruption of the impactor at some
significant altitude.
The Henbury samples in our collection have a distinct coating of melt in
many cases (Fig. 3).
Evidence for high temperature gas flow rupturing vesicle walls in Henbury
melt samples has also been reported [5].

Fig. 3, Impact melt clast (6 cm diameter) surface (left image) and cross
section (right image) from Henbury, Australia.
Note the layered texture.

Aouelloul, Mauritania, 0.39 km diameter, sedimentary target

The Aouelloul impact crater contains impact melt fragments (Fig. 4).
Structure is a distinct impact crater with impact melt fragments [6].

Fig. 4, Impact melt fragments from Aouelloul (image 10 cm across).
The impact melts form layers placed on edge in the image, except for the
sample on the bottom.

Lonar, India, 1.83 km diameter, basaltic target

The impact melt deposits described in this abstract (e.g., Fig. 5) come from
the eastern rim of the impact crater, and are thought to represent the
uppermost layer of ejecta [7-10].
The samples consist of layers of impact melt loosely organized into large
coherent masses.
In some cases (not illustrated) the melt forms ropes and blobs like taffy,
on a scale of a few mm.
The formation of the impact melt by an airburst, as opposed to shock
melting, may be consistent with the limited evidence for hydrothermal
processes in the ejecta blanket at Lonar [8] and the absence of abundant
impact melt in the drill cores from the floor of the crater [9].

A similar sample has recently been found at the 4 km diameter Ramgarh
structure [11].

Fig. 5, Impact melt bomb from the eastern side of the Lonar impact crater.
Note the layered texture of this sample. Width 6 cm.

Conclusions

Numerical modeling suggests that low altitude airbursts due to the
interaction of hypervelocity projectiles with the atmosphere can produce
surface melting forming thin layers as seen in the materials from Aouelloul.
The accompanying supersonic velocity flow field can redistribute the melted
surface layer forming accumulations of layers as seen in the Muong-Nong
tektites and some of the larger Lonar impact melt masses.
The ropy surface textures of some of the Lonar melts could result from
transport of the melted layers.
The impact melt rinds found on samples from Wabar, Henbury and Lonar can be
the result of melting due to incorporation of materials into the hot flow
field, much like the fusion crust on meteorites.

References

[1] Boslough, M.B.E. and Crawford, D.A. (2007)
Int. J. Impact Eng., in press.

[2] Melosh, H.J. and Collins, G.S. (2005)
Nature, 434, 157.

[3] Shoemaker, E. M., and Wynn, J. C., (1997).
Lunar and Planetary Science XXVIII, pp. 1313-1314.

[4] Taylor, S. R. (1967)
Geochimica et Cosmochimica Acta, v. 31, pp. 961-968.

[5] C. Bender Koch (2007)
Geochimica et Cosmochimica Acta, 71, Suppl. 1, A48.

[6] Koeberl, C., Auer, P. (1991)
Lunar and Planetary Science XXII, pp. 731-732.
[7] Osae, S., Misra, S. , Koeberl, C. , Sengupta, D. and Ghosh, S. (2005)
Meteoritics & Planetary Science 40, Nr 9/10, P. 1473 - 1492.

[8] Newsom, H. E.; Misra, S.; Nelson, M. J. (2007)
Lunar and Planetary Sci. XXXVIII, abs. # 2056.

[9] Hagerty, J. J., and Newsom, H. E. (2003)
Meteoritics and Planet. Sci., 38, 365-381.

[10] Misra S. et al. (2006) LPSC 37th, abs. # 2123.

[11] Misra S. et al. (2007) LPSC 39th submitted.

Supported by NASA P.G.&G. NNG 05GJ42G (H. Newsom).
Sandia is operated by Sandia Corporation, a Lockheed Martin Company, for the
United States Dept. of Energy under Contract DE-AC04-94AL85000.
The computational work was funded at Sandia by the LDRD and CSRF programs
(M. Boslough).
___________________________________________


3 times more downward energy from directed force of meteor airburst in 3D
simulations by Mark B. E. Boslough, Sandia Lab 2007.12.17: Rich Murray
2010.08.30
http://rmforall.blogspot.com/2010_08_01_archive.htm
Monday, August 30, 2010
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Boston University Graduate School 1967 psychology,
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Received on Wed 17 Nov 2010 02:06:20 AM PST