Laboratory observations of impact–generated magnetic fields

Abstract

Radiofrequency emissions have been observed previously during vertical hypervelocity impacts into hard targets, but the dominant magnetic signatures of these events were attributed to amplification of the applied magnetic field^1–3. Here we report laboratory experiments which document spontaneous magnetic fields generated by low-angle, hypervelocity impacts in a low-field environment. Low-angle impacts enhance the production of a partially ionized vapour cloud (a dusty plasma) which expands above the impact point^4,5. The observed spontaneous magnetic fields may result from runaway thermal electrons producing a toroidal field confined to this plasma and by non-aligned thermal and electron density gradients producing a vertical field within the target^6,7. These new observations hold promise both for experimentally probing early-time impact phenomena and for understanding the anomalously high thermal magnetic remanence observed in a young lunar impact melt returned by Apollo 17 (ref. 8). They also may provide clues for understanding the enigmatic record of broad-scale magnetic anomalies on the Moon^9,10 and anticipated palaeomagnetic signatures from future missions such as the Soviet rendezvous with Phobos.