LABORATORY INVESTIGATIONS OF IMPACT-GENERATED PLASMA

The production of magnetic fields by hypervelocity meteoroid impact has been proposed to explain the presence of paleomagnetic fields in some lunar samples as well as on the lunar surface. Impact-generated magnetic fields also may be significant for the paleomagnetic record on a variety of cratered surfaces in the solar system, such as the Moon, Mercury, Phobos, and asteroids. Previous experiments with the two-stage hydrogen light gas gun at the NASA Ames Vertical Gun Range demonstrated that hypervelocity impacts can produce impact-generated magnetic fields by the expansion of an impact-derived ionized vapor cloud (impact-generated plasma). Here we further characterize impact-generated magnetic fields and plasma with hypervelocity impact experiments at the NASA Ames Vertical Gun. Spherical projectiles (0.32 cm Fe, Cu; 0.64 cm Al, nylon) with velocities from 5.2 to 6.0 km/s were impacted at impact angles of 15-degrees, 30-degrees, 60-degrees, and 90-degrees (measured from horizontal) into powdered dolomite, silica sand, and aluminum plate. Three sets of experiments using search coils as magnetic detectors characterized impact-generated magnetic fields as functions of ambient field strength and orientation, projectile/target composition, and impact angle. Experiments using Langmuir probes indicated a charged particle density (between 10(9) and 10(11) ions/el cm-3) and an electron temperature (approximately 4500 K) of the impact-generated plasma, the inferred source of impact-induced magnetic fields. These new results demonstrate that impact-generated magnetic fields at the laboratory scale exhibit spatial and temporal complexity dependent on impact angle, velocity, and projectile/target composition thereby suggesting that crater-related paleomagnetism associated with this mechanism should exhibit similar complexity with spatial wavelengths on the order of a fraction of the crater radius.