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Impact ejecta on the moon

O'Keefe, John D. and Ahrens, Thomas J. (1976) Impact ejecta on the moon. In: 7th Lunar Science Conference. Geochimica et cosmochimica acta. Supplement. Vol.3. No.7. Pergamon Press , New York, NY, pp. 3007-3025. ISBN 0080217710.

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The partitioning of energy and the distribution of the resultant ejecta on the moon is numerically modeled using a Eulerian finite-difference grid. The impact of an iron meteoroid at 15 km/sec on a gabbroic anorthosite lunar crust is examined in detail. The high-speed impact-induced flow is described over the entire hydrodynamic regime from a time where the peak pressures are 6 Mbar until the stresses everywhere in the flow are linearly elastic, and less than 5 kbar. For 5 cm radius projectile the latter condition is achieved some ~0.5 msec after impact. The effect of taking into account the shock-induced polymorphic phase changes, in the plagioclase and pyroxene structure (in gabbro) to the hollandite and perovskite structures, respectively, and the subsequent reversion to low-pressure phases is demonstrated to enhance shock-wave attenuation. A rate-dependent equation of state, is used for describing the hysteretic effect of the phase change. The ballistic equations for spherical planet taking into account the decrease of gravity with height, are systematically applied to material with net velocity away from the moon. The mass of material escaping the moon corresponds to some 28% of the mass of meteorite, less than previous estimates, and most of the material lost, is lunar crust. Only 0.2% of the meteoroid escapes the moon, all in the vapor phase. In the case of accreting planets, a relatively sharp decrease in energy and mass lost from such impacts occurs when the escape velocities begin to exceed ~ 1 km/sec. This implies that since the fraction of kinetic energy lost is less than 5%, impact heating of lunar-sized planets in the latter stages of accretion is efficient. Most of the impact energy remains on the lunar crust (86.1%), however the bulk of the impact energy (66.8%) resides in crustal impact ejecta which is distributed with a surface density (mass/area) which decays as R^(-2715), where R, is the radius from the impact. At large distances from the impact, the ratio of lunar to meteorite ejecta is ⪞10^2, implying that the higher concentrations of meteorite components observed in the Apollo 17 breccias, resulted from mostly local impacts.

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Additional Information:© 1976 Lunar and Planetary Institute. This research was supported by NASA grant NGL-05-002-105. We appreciate the computational assistance of M. Lainhart. The critical comments made by F. Horz, H. Moore and D. E. Gault on this paper were most helpful. This work is contribution number 2763, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California.
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Caltech Division of Geological and Planetary Sciences 2763
Series Name:Geochimica et cosmochimica acta. Supplement
Issue or Number:7
Record Number:CaltechAUTHORS:20150902-094720006
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Official Citation:Impact ejecta on the moon Okeefe, J. D.; Ahrens, T. J. Lunar Science Conference, 7th, Houston, Tex., March 15-19, 1976, Proceedings. Volume 3. (A77-34651 15-91) New York, Pergamon Press, Inc., 1976, p. 3007-3025.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:60025
Deposited By: Ruth Sustaita
Deposited On:11 Sep 2015 18:36
Last Modified:03 Oct 2019 08:52

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