Magnetic field, shock, and crustal magnetization effects of lunar basin-forming impacts

Mathematics – Logic

Scientific paper

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1517 Magnetic Anomalies: Modeling And Interpretation, 1527 Paleomagnetism Applied To Geologic Processes, 1595 Planetary Magnetism: All Frequencies And Wavelengths, 5109 Magnetic And Electrical Properties (0925)

Scientific paper

Orbital mapping has shown that lunar crustal magnetic fields are weak within young large impact basins (especially Imbrium and Orientale) but are unusually strong near the antipodes of the same basins. The weak fields near the basins have been attributed to shock demagnetization of relatively soft crustal magnetization (Helekas et al., MAPS, 2003). The strong antipodal anomalies have been attributed mainly to shock remanent magnetization (SRM) associated with the impact of ejecta in a magnetic field amplified by the converging impact vapor-melt cloud (Hood & Artemieva, Icarus, 2008). Both of these interpretations allow but do not require the existence of a former lunar core dynamo at the times of the impacts. Initial 3D simulations indicate that the time of maximum amplification of an ambient magnetic field (early solar wind or core dynamo) is ~ 1 hour after the impact, coinciding roughly with the period of ejecta convergence and impact near the antipode. During this period, shock stresses are produced within the range of 5-25 GPa where stable SRM of lunar soils has been found experimentally to occur (e.g., Fuller et al., Moon, 1974). Additional shock stresses in this range are produced by converging compressional waves from the impact; however, these waves arrive at a time (400 - 700 s) well before the antipodal field amplification. Seismic surface waves are unlikely to produce significant antipodal shock stresses because of the existence of a highly fractured (and dry) near-surface zone, which produces intense scattering and destruction of coherent surface reflections. Calculated ejecta thicknesses are only marginally sufficient to explain the amplitudes of observed magnetic anomalies if mean magnetization intensities are comparable to those produced experimentally. This suggests that pre-existing ejecta materials, which would also contain abundant metallic iron remanence carriers, may be important anomaly sources. The latter possibility is consistent with enhanced magnetic anomalies observed peripheral to the South Pole-Aitken (SPA) basin, which may have been produced by amplified secondary ejecta impact shock waves in the thick SPA ejecta mantle near the antipodes of the Imbrium and Serenitatis basins.

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