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Oblique Impact: A Process for Obtaining Meteorite Samples from Other Planets
JOHN D. O'KEEFE 1 and THOMAS J. AHRENS 1

1 Seismological Laboratory 252-21, California Institute of Technology, Pasadena, CA 91125.

Cratering flow calculations for a series of oblique to normal (10° to 90°) impacts of silicate projectiles onto a silicate halfspace were carried out to determine whether or not the gas produced upon shock-vaporizing both projectile and target material would form a downstream jet that could entrain and propel SNC meteorites from the Martian surface. The difficult constraints that the impact origin hypothesis for SNC meteorites has to satisfy are that these meteorites are lightly to moderately shocked and yet have been accelerated to speeds in excess of the Martian escape velocity (more than 5 kilometers per second). Two-dimensional finite difference calculations were performed that show that at highly probable impact velocities (7.5 kilometers per second), vapor plume jets are produced at oblique impact angles of 25° to 60° and have speeds as great as 20 kilometers per second. These plumes flow nearly parallel to the planetary surface. It is shown that upon impact of projectiles having radii of 0.1 to 1 kilometer, the resulting vapor jets have densities of 0.1 to 1 gram per cubic centimeter. These jets can entrain Martian surface rocks and accelerate them to velocities greater than 5 kilometers per second. This mechanism may launch SNC meteorites to earth.

[url=http://www.sciencemag.org/cgi/content/abstract/198/4323/1249-a] Source [url]

Meteorite Impact Ejecta: Dependence of Mass and Energy Lost on Planetary Escape Velocity
JOHN D. O'KEEFE 1 and THOMAS J. AHRENS 1

1 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena 91125

The calculated energy efficiency of mass ejection for iron and anorthosite objects striking an anorthosite planet at speeds of 5 to 45 kilometers per second decreases with increasing impact velocity at low escape velocities. At escape velocities of >105 and >2 x 104 centimeters per second, respectively, the slower impactors produce relatively less ejecta for a given impact energy. The impact velocities at which ejecta losses equal meteorite mass gains are found to be approximately 20, 35, and 45 kilometers per second for anorthosite objects and approximately 25, 35, and 40 kilometers per second for iron objects striking anorthosite surfaces for the gravity fields of the moon, Mercury, and Mars.