Magnetic field and shock effects and remanent magnetization in a hypervelocity impact experiment

Abstract

The impact of aluminum projectiles onto high-alumina terrestrial basalt blocks at 13–15 km s −1 in the presence of a variable magnetic field has been studied. Low-frequency search coil data show that plasma is produced. causing local compression of the ambient field. Although field production is expected, it was not detectable with the existing apparatus. Measurements of the remanence of the shocked basalt show that magnetization was acquired in the material near the craters. The acquired remanence is predominantly soft, but also contains a component not demagnetized by 500 Oe AC field treatment. Material shocked in a 10-Oe vertical field exhibits inverse dependence of magnetization upon distance from the crater center. Examination of the shocked basalt in thin section reveals a general lack of shock metamorphism in the material surrounding the crater, except for the presence of a high-pressure melt glass which was splashed onto the crater walls. Micro-probe analyses show that the glass is a whole-rock melt of fairly uniform composition, and is contaminated with aluminum from the projectile. The mineralogical data support the view that the acquired magnetization is shock remanence, since negligible shock heating occurred in the magnetized material. These results bear on the problem of lunar magnetism, suggesting that shock effects or possibly thermoremanence in ejecta fragments may be responsible for part of the magnetization of the lunar surface.