[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

Wed Nov 17 02:06:20 EST 2010

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,
BS MIT 1964, history and physics,
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