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Shock effects from a large impact on the moon

O'Keefe, John D. and Ahrens, Thomas J. (1976) Shock effects from a large impact on the moon. In: Lunar Science Conference. Geochimica et cosmochimica acta. Supplement 6. No.3. Pergamon Press , New York, NY, pp. 2831-2844. ISBN 9780080205663.

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The shock and shear deformation induced internal energy distribution is calculated for a major basin-forming hypervelocity meteorite impact on the moon. Using the Hageman and Walsh formulation of the axisymmetric two-dimensional conservation equations in finite difference form, the flow field induced upon impact of an iron meteorite traveling at 15 km/sec with a gabbroic anorthosite lunar crust is calculated for sequential time steps over a grid of hoop-shaped zones fixed in space. A grid of massless tracer particles follows the outline in detail of the deformed meteorite and lunar surf ace in the cratering region. Assuming an energy of 5 x 10^(32) ergs for the projectile energy required to excavate a basin of Imbrium size yields a flow field some 210 km in radius, ~19 sec after impact. At this point the maximum stress level has decayed to pressures less than 200 kbar and left a zone of shock-melted rock immediately surrounding the meteorite at stresses below 100 kbar. However, portions of the meteorite are still moving downward at speeds of 2 km/sec. At this time some 7.2 and 4.2 meteorite masses of lunar surf ace material has been partially or completely melted or vaporized, respectively. Using recent estimates of ejecta volume for Imbrium-sized craters and the present results implies that the ejecta from a single such event will contain only 0.03-8% melt depending upon the scaling relation used. This implies that for impacts in the present velocity range, multiple events are required to produce the (larger) mass fractions of glass seen in the lunar breccias and soils. Comparison with predictions of the Gault and Heitowit formulation indicates that this earlier model is approximately valid. It predicts too rapid a decay of shock pressure with distance and hence, energy densities which are too high in the mass of surf ace material immediately surrounding the impacting meteorite.

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Additional Information:© 1975 Lunar and Planetary Institute. Provided by the NASA Astrophysics Data System. Contribution No. 2616, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125. This research was supported under NASA grant NGL-05-002-105. We appreciate the helpful comments made by Donald E. Gault and Henry J. Moore on our work and the computational assistance of Micheal Lainhart and suggestions of Laura Walsh.
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Caltech Division of Geological and Planetary Sciences2616
Series Name:Geochimica et cosmochimica acta. Supplement 6
Issue or Number:3
Record Number:CaltechAUTHORS:20150709-070308864
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Official Citation:Shock effects from a large impact on the moon Authors:Okeefe, J. D.; Ahrens, T. J. Publication: In: Lunar Science Conference, 6th, Houston, Tex., March 17-21, 1975, Proceedings. Volume 3. (A78-46741 21-91) New York, Pergamon Press, Inc., 1975, p. 2831-2844.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:58817
Deposited By: Ruth Sustaita
Deposited On:22 Jul 2015 19:05
Last Modified:03 Oct 2019 08:40

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